Frontend AST for Podlang (#432)

* Basic frontend AST and semantic validation

* Intro statement support

* Simplify validator lifetime

* Fix arity validation

* Lowering and splitting

* Remove legacy processor and use frontend AST by default

* Use builders instead of creating middleware types directly

* Typos/formatting

* Improve error messages when overflowing a batch due to splitting

* Add FromStr implementation for NativePredicate

* Remove 'raw' fields, and switch HashHex representation to byte vector rather than string

* Simpler wrapper types for batch and intro predicate hashes

* Parse secret and public keys to their respective data structures earlier

* More detail around string escape validity

* Simplify native predicate arity handling and move  method to NativePredicate impl

* Store hashes using middleware::Hash, and simplify lowering by using pre-parsed values

* Simplify predicate building

* Formatting

* Better error messages/suggestions for cases where predicate splitting fails

* Formatting

* Clippy fix

* Return error if we get a too-large int

src/frontend/custom.rs

@@ -165,7 +165,7 @@ impl CustomPredicateBatchBuilder {
 
     /// creates the custom predicate from the given input, adds it to the
     /// self.predicates, and returns the index of the created predicate
-    fn predicate(
+    pub fn predicate(
         &mut self,
         name: &str,
         conjunction: bool,

src/lang/error.rs

@@ -1,95 +1,283 @@
 use thiserror::Error;
 
-use crate::{frontend, lang::parser::ParseError, middleware};
+use crate::{
+    frontend,
+    lang::{frontend_ast::Span, parser::ParseError},
+    middleware,
+};
 
 #[derive(Error, Debug)]
 pub enum LangError {
     #[error("Parsing failed: {0}")]
     Parse(Box<ParseError>),
 
-    #[error("AST processing error: {0}")]
-    Processor(Box<ProcessorError>),
-
     #[error("Middleware error during processing: {0}")]
     Middleware(Box<middleware::Error>),
 
     #[error("Frontend error: {0}")]
     Frontend(Box<frontend::Error>),
+
+    #[error("Validation error: {0}")]
+    Validation(Box<ValidationError>),
+
+    #[error("Lowering error: {0}")]
+    Lowering(Box<LoweringError>),
 }
 
-/// Errors that can occur during the processing of Podlang Pest tree into middleware structures.
-#[derive(thiserror::Error, Debug)]
-pub enum ProcessorError {
-    #[error("Undefined identifier: '{name}' at {span:?}")]
-    UndefinedIdentifier {
+/// Validation errors from frontend AST validation
+#[derive(Debug, thiserror::Error)]
+pub enum ValidationError {
+    #[error("Invalid hash: {hash}")]
+    InvalidHash { hash: String, span: Option<Span> },
+
+    #[error("Duplicate predicate definition: {name}")]
+    DuplicatePredicate {
         name: String,
-        span: Option<(usize, usize)>,
+        first_span: Option<Span>,
+        second_span: Option<Span>,
     },
-    #[error("Duplicate definition: '{name}' at {span:?}")]
-    DuplicateDefinition {
+
+    #[error("Duplicate import name: {name}")]
+    DuplicateImport { name: String, span: Option<Span> },
+
+    #[error("Import arity mismatch: expected {expected} predicates, found {found}")]
+    ImportArityMismatch {
+        expected: usize,
+        found: usize,
+        span: Option<Span>,
+    },
+
+    #[error("Batch not found: {id}")]
+    BatchNotFound { id: String, span: Option<Span> },
+
+    #[error("Undefined predicate: {name}")]
+    UndefinedPredicate { name: String, span: Option<Span> },
+
+    #[error("Undefined wildcard: {name} in predicate {pred_name}")]
+    UndefinedWildcard {
         name: String,
-        span: Option<(usize, usize)>,
+        pred_name: String,
+        span: Option<Span>,
     },
-    #[error("Duplicate wildcard: ?{name} in scope at {span:?}")]
-    DuplicateWildcard {
-        name: String,
-        span: Option<(usize, usize)>,
-    },
-    #[error("Type error: expected {expected}, found {found} for '{item}' at {span:?}")]
-    TypeError {
-        expected: String,
-        found: String,
-        item: String,
-        span: Option<(usize, usize)>,
-    },
-    #[error(
-        "Invalid argument count for '{predicate}': expected {expected}, found {found} at {span:?}"
-    )]
+
+    #[error("Argument count mismatch for {predicate}: expected {expected}, found {found}")]
     ArgumentCountMismatch {
         predicate: String,
         expected: usize,
         found: usize,
-        span: Option<(usize, usize)>,
+        span: Option<Span>,
     },
-    #[error("Multiple REQUEST definitions found. Only one is allowed. First at {first_span:?}, second at {second_span:?}")]
+
+    #[error("Invalid argument type for {predicate}: anchored keys not allowed")]
+    InvalidArgumentType {
+        predicate: String,
+        span: Option<Span>,
+    },
+
+    #[error("Duplicate wildcard in predicate arguments: {name}")]
+    DuplicateWildcard { name: String, span: Option<Span> },
+
+    #[error("Empty statement list in {context}")]
+    EmptyStatementList { context: String, span: Option<Span> },
+
+    #[error("Multiple REQUEST definitions found. Only one is allowed.")]
     MultipleRequestDefinitions {
-        first_span: Option<(usize, usize)>,
-        second_span: Option<(usize, usize)>,
+        first_span: Option<Span>,
+        second_span: Option<Span>,
     },
-    #[error("Internal processing error: {0}")]
-    Internal(String),
+}
+
+/// Lowering errors from frontend AST lowering to middleware
+#[derive(Debug, thiserror::Error)]
+pub enum LoweringError {
+    #[error("Too many custom predicates in batch '{batch_name}': {count} exceeds limit of {max}{}", if *.original_count != *.count { format!(" (started with {} predicates before automatic splitting)", original_count) } else { String::new() })]
+    TooManyPredicates {
+        batch_name: String,
+        count: usize,
+        max: usize,
+        original_count: usize,
+    },
+
+    #[error("Too many statements in predicate '{predicate}': {count} exceeds limit of {max}")]
+    TooManyStatements {
+        predicate: String,
+        count: usize,
+        max: usize,
+    },
+
+    #[error("Too many wildcards in predicate '{predicate}': {count} exceeds limit of {max}")]
+    TooManyWildcards {
+        predicate: String,
+        count: usize,
+        max: usize,
+    },
+
+    #[error("Too many arguments in statement template: {count} exceeds limit of {max}")]
+    TooManyStatementArgs { count: usize, max: usize },
+
+    #[error("Predicate '{name}' not found in symbol table")]
+    PredicateNotFound { name: String },
+
+    #[error("Invalid argument type in statement template")]
+    InvalidArgumentType,
+
     #[error("Middleware error: {0}")]
-    Middleware(middleware::Error),
-    #[error("Undefined wildcard: '?{name}' in predicate '{pred_name}' at {span:?}")]
-    UndefinedWildcard {
-        name: String,
-        pred_name: String,
-        span: Option<(usize, usize)>,
+    Middleware(#[from] middleware::Error),
+
+    #[error("Splitting error: {0}")]
+    Splitting(#[from] SplittingError),
+
+    #[error("Cannot lower document with validation errors")]
+    ValidationErrors,
+}
+
+/// Context information for split boundary failures
+#[derive(Debug, Clone)]
+pub struct SplitContext {
+    /// Index of the split boundary (0-based)
+    pub split_index: usize,
+    /// Range of statement indices in the segment before the split
+    pub statement_range: (usize, usize),
+    /// Public arguments coming into this segment
+    pub incoming_public: Vec<String>,
+    /// Wildcards that cross this boundary (need to be promoted)
+    pub crossing_wildcards: Vec<String>,
+    /// Total public arguments needed (incoming + crossing)
+    pub total_public: usize,
+}
+
+/// Suggestions for refactoring predicates that fail to split
+#[derive(Debug, Clone)]
+pub enum RefactorSuggestion {
+    /// A wildcard is used across too many statements
+    ReduceWildcardSpan {
+        wildcard: String,
+        first_use: usize,
+        last_use: usize,
+        span: usize,
     },
-    #[error("Invalid literal format for {kind}: '{value}' at {span:?}")]
-    InvalidLiteralFormat {
-        kind: String,
-        value: String,
-        span: Option<(usize, usize)>,
+    /// Multiple wildcards should be grouped together
+    GroupWildcardUsages { wildcards: Vec<String> },
+}
+
+impl RefactorSuggestion {
+    pub fn format(&self) -> String {
+        match self {
+            RefactorSuggestion::ReduceWildcardSpan {
+                wildcard,
+                first_use,
+                last_use,
+                span,
+            } => {
+                format!(
+                    "Wildcard '{}'  is used across {} statements (statements {}-{}).\n\
+                     Consider grouping all '{}' operations together, or split the wildcard\n\
+                     into separate early/late variables.",
+                    wildcard, span, first_use, last_use, wildcard
+                )
+            }
+            RefactorSuggestion::GroupWildcardUsages { wildcards } => {
+                format!(
+                    "Group operations for wildcards: {}\n\
+                     These wildcards are used across multiple segments. Try to complete\n\
+                     all operations for each wildcard before moving to the next.",
+                    wildcards.join(", ")
+                )
+            }
+        }
+    }
+}
+
+/// Formats a detailed error message for TooManyPublicArgsAtSplit
+fn format_public_args_at_split_error(
+    predicate: &str,
+    context: &SplitContext,
+    max_allowed: usize,
+    suggestion: &Option<Box<RefactorSuggestion>>,
+) -> String {
+    let mut msg = format!(
+        "Too many public arguments at split boundary {} in predicate '{}':\n",
+        context.split_index, predicate
+    );
+
+    msg.push_str(&format!(
+        "  {} incoming public + {} crossing wildcards = {} total (exceeds max of {})\n",
+        context.incoming_public.len(),
+        context.crossing_wildcards.len(),
+        context.total_public,
+        max_allowed
+    ));
+
+    msg.push_str(&format!(
+        "  Statements {}-{} in this segment\n",
+        context.statement_range.0, context.statement_range.1
+    ));
+
+    if !context.incoming_public.is_empty() {
+        msg.push_str(&format!(
+            "  Incoming public args: {}\n",
+            context.incoming_public.join(", ")
+        ));
+    }
+
+    if !context.crossing_wildcards.is_empty() {
+        msg.push_str(&format!(
+            "  Wildcards crossing this boundary: {}\n",
+            context.crossing_wildcards.join(", ")
+        ));
+    }
+
+    if let Some(suggestion) = suggestion {
+        msg.push_str("\nSuggestion:\n");
+        msg.push_str(&suggestion.format());
+    }
+
+    msg
+}
+
+/// Splitting errors from predicate splitting
+#[derive(Debug, thiserror::Error)]
+pub enum SplittingError {
+    #[error("Too many public arguments in predicate '{predicate}': {count} exceeds max of {max_allowed}. {message}")]
+    TooManyPublicArgs {
+        predicate: String,
+        count: usize,
+        max_allowed: usize,
+        message: String,
     },
-    #[error("Batch with ID '{id}' not found at {span:?}")]
-    BatchNotFound {
-        id: String,
-        span: Option<(usize, usize)>,
+
+    #[error("Too many total arguments in predicate '{predicate}': {count} exceeds max of {max_allowed}. {message}")]
+    TooManyTotalArgs {
+        predicate: String,
+        count: usize,
+        max_allowed: usize,
+        message: String,
     },
-    #[error("Number of predicates in 'use' statement ({found}) exceeds the number of predicates in the batch ({expected}) at {span:?}")]
-    ImportArityMismatch {
-        expected: usize,
-        found: usize,
-        span: Option<(usize, usize)>,
+
+    #[error("Too many total arguments in chain link {link_index} of predicate '{predicate}': {public_count} public + {private_count} private = {total_count} total (exceeds max of {max_allowed})")]
+    TooManyTotalArgsInChainLink {
+        predicate: String,
+        link_index: usize,
+        public_count: usize,
+        private_count: usize,
+        total_count: usize,
+        max_allowed: usize,
     },
-    #[error("Duplicate import name '{name}' at {span:?}")]
-    DuplicateImportName {
-        name: String,
-        span: Option<(usize, usize)>,
+
+    #[error("{}", format_public_args_at_split_error(.predicate, .context, *.max_allowed, .suggestion))]
+    TooManyPublicArgsAtSplit {
+        predicate: String,
+        context: Box<SplitContext>,
+        max_allowed: usize,
+        suggestion: Option<Box<RefactorSuggestion>>,
+    },
+
+    #[error("Too many predicates in chain for '{predicate}': {count} exceeds batch limit of {max_allowed}")]
+    TooManyPredicatesInChain {
+        predicate: String,
+        count: usize,
+        max_allowed: usize,
     },
-    #[error("Frontend error: {0}")]
-    Frontend(#[from] frontend::Error),
 }
 
 impl From<ParseError> for LangError {
@@ -98,14 +286,20 @@ impl From<ParseError> for LangError {
     }
 }
 
-impl From<ProcessorError> for LangError {
-    fn from(err: ProcessorError) -> Self {
-        LangError::Processor(Box::new(err))
-    }
-}
-
 impl From<middleware::Error> for LangError {
     fn from(err: middleware::Error) -> Self {
         LangError::Middleware(Box::new(err))
     }
 }
+
+impl From<ValidationError> for LangError {
+    fn from(err: ValidationError) -> Self {
+        LangError::Validation(Box::new(err))
+    }
+}
+
+impl From<LoweringError> for LangError {
+    fn from(err: LoweringError) -> Self {
+        LangError::Lowering(Box::new(err))
+    }
+}

src/lang/frontend_ast_lower.rs

@@ -0,0 +1,749 @@
+//! Lowering from frontend AST to middleware structures
+//!
+//! This module converts validated frontend AST to middleware data structures.
+//! Currently implements basic 1:1 conversion without automatic predicate splitting.
+
+use std::{
+    collections::{HashMap, HashSet},
+    str::FromStr,
+    sync::Arc,
+};
+
+use crate::{
+    frontend::{BuilderArg, CustomPredicateBatchBuilder, StatementTmplBuilder},
+    lang::{
+        frontend_ast::*,
+        frontend_ast_split,
+        frontend_ast_validate::{PredicateKind, ValidatedAST},
+    },
+    middleware::{
+        self, containers, CustomPredicateBatch, IntroPredicateRef, NativePredicate, Params,
+        Predicate, StatementTmpl as MWStatementTmpl, StatementTmplArg as MWStatementTmplArg,
+        Wildcard,
+    },
+};
+
+/// Result of lowering: optional custom predicate batch and optional request
+///
+/// A Podlang file can contain:
+/// - Just custom predicates (batch: Some, request: None)
+/// - Just a request (batch: None, request: Some)
+/// - Both (batch: Some, request: Some)
+/// - Neither (batch: None, request: None) - just imports
+#[derive(Debug, Clone)]
+pub struct LoweredOutput {
+    pub batch: Option<Arc<CustomPredicateBatch>>,
+    pub request: Option<crate::frontend::PodRequest>,
+}
+
+pub use crate::lang::error::LoweringError;
+
+/// Lower a validated AST to middleware structures
+///
+/// Returns both the custom predicate batch (if any) and the request (if any).
+/// At least one will be Some if the document contains custom predicates or a request.
+pub fn lower(
+    validated: ValidatedAST,
+    params: &Params,
+    batch_name: String,
+) -> Result<LoweredOutput, LoweringError> {
+    if !validated.diagnostics().is_empty() {
+        // For now, treat any diagnostics as errors
+        // In future we could allow warnings
+        return Err(LoweringError::ValidationErrors);
+    }
+
+    let lowerer = Lowerer::new(validated, params);
+    lowerer.lower(batch_name)
+}
+
+struct Lowerer<'a> {
+    validated: ValidatedAST,
+    params: &'a Params,
+    /// Map of predicate names to their index in the current batch (for split predicates)
+    batch_predicate_index: HashMap<String, usize>,
+}
+
+impl<'a> Lowerer<'a> {
+    fn new(validated: ValidatedAST, params: &'a Params) -> Self {
+        Self {
+            validated,
+            params,
+            batch_predicate_index: HashMap::new(),
+        }
+    }
+
+    fn lower(mut self, batch_name: String) -> Result<LoweredOutput, LoweringError> {
+        // Lower custom predicates (if any)
+        let batch = self.lower_batch(batch_name)?;
+
+        // Lower request (if any) - pass batch so BatchSelf refs can be converted to Custom refs
+        let request = self.lower_request(batch.as_ref())?;
+
+        Ok(LoweredOutput { batch, request })
+    }
+
+    fn lower_batch(
+        &mut self,
+        batch_name: String,
+    ) -> Result<Option<Arc<CustomPredicateBatch>>, LoweringError> {
+        // Extract and split custom predicates from document
+        let (custom_predicates, original_count) = self.extract_and_split_predicates()?;
+
+        // If no custom predicates, return None
+        if custom_predicates.is_empty() {
+            return Ok(None);
+        }
+
+        // Check batch size constraint
+        if custom_predicates.len() > self.params.max_custom_batch_size {
+            return Err(LoweringError::TooManyPredicates {
+                batch_name: batch_name.clone(),
+                count: custom_predicates.len(),
+                max: self.params.max_custom_batch_size,
+                original_count,
+            });
+        }
+
+        // Build index of all predicates in the batch
+        for (idx, pred) in custom_predicates.iter().enumerate() {
+            self.batch_predicate_index
+                .insert(pred.name.name.clone(), idx);
+        }
+
+        // Create custom predicate batch using builder
+        let mut cpb_builder =
+            CustomPredicateBatchBuilder::new(self.params.clone(), batch_name.clone());
+
+        for pred_def in &custom_predicates {
+            self.lower_custom_predicate(pred_def, &mut cpb_builder)?;
+        }
+
+        Ok(Some(cpb_builder.finish()))
+    }
+
+    fn lower_request(
+        &self,
+        batch: Option<&Arc<CustomPredicateBatch>>,
+    ) -> Result<Option<crate::frontend::PodRequest>, LoweringError> {
+        let doc = self.validated.document();
+
+        // Find request definition (if any)
+        let request_def = doc.items.iter().find_map(|item| match item {
+            DocumentItem::RequestDef(req) => Some(req),
+            _ => None,
+        });
+
+        let Some(request_def) = request_def else {
+            return Ok(None);
+        };
+
+        // Build wildcard map from all wildcards used in the request statements
+        let wildcard_map = self.build_request_wildcard_map(request_def);
+
+        // Lower each statement to a builder first
+        let mut statement_builders = Vec::new();
+        for stmt in &request_def.statements {
+            let stmt_builder = self.lower_statement_to_builder(stmt)?;
+            statement_builders.push(stmt_builder);
+        }
+
+        // Resolve builders to middleware statement templates
+        let mut request_templates = Vec::new();
+        for stmt_builder in statement_builders {
+            let mw_stmt =
+                self.resolve_request_statement_builder(stmt_builder, &wildcard_map, batch)?;
+            request_templates.push(mw_stmt);
+        }
+
+        Ok(Some(crate::frontend::PodRequest::new(request_templates)))
+    }
+
+    fn resolve_request_statement_builder(
+        &self,
+        stmt_builder: StatementTmplBuilder,
+        wildcard_map: &HashMap<String, usize>,
+        batch: Option<&Arc<CustomPredicateBatch>>,
+    ) -> Result<MWStatementTmpl, LoweringError> {
+        // First desugar the builder
+        let desugared = stmt_builder.desugar();
+
+        // Convert BatchSelf predicate to Custom if we have a batch
+        let mut predicate = desugared.predicate;
+        if let Some(batch_ref) = batch {
+            if let Predicate::BatchSelf(index) = predicate {
+                predicate = Predicate::Custom(middleware::CustomPredicateRef::new(
+                    batch_ref.clone(),
+                    index,
+                ));
+            }
+        }
+
+        // Convert BuilderArgs to StatementTmplArgs
+        let mut mw_args = Vec::new();
+        for builder_arg in desugared.args {
+            let mw_arg = match builder_arg {
+                BuilderArg::Literal(value) => MWStatementTmplArg::Literal(value),
+                BuilderArg::WildcardLiteral(name) => {
+                    let index = wildcard_map.get(&name).expect("Wildcard not found");
+                    MWStatementTmplArg::Wildcard(Wildcard::new(name, *index))
+                }
+                BuilderArg::Key(root_name, key_str) => {
+                    let root_index = wildcard_map
+                        .get(&root_name)
+                        .expect("Root wildcard not found");
+                    let wildcard = Wildcard::new(root_name, *root_index);
+                    let key = middleware::Key::from(key_str.as_str());
+                    MWStatementTmplArg::AnchoredKey(wildcard, key)
+                }
+            };
+            mw_args.push(mw_arg);
+        }
+
+        Ok(MWStatementTmpl {
+            pred: predicate,
+            args: mw_args,
+        })
+    }
+
+    fn build_request_wildcard_map(&self, request_def: &RequestDef) -> HashMap<String, usize> {
+        // Collect all unique wildcards from all statements
+        let mut wildcard_names = Vec::new();
+        let mut seen = HashSet::new();
+
+        for stmt in &request_def.statements {
+            self.collect_statement_wildcards(stmt, &mut wildcard_names, &mut seen);
+        }
+
+        // Build map from name to index
+        wildcard_names
+            .into_iter()
+            .enumerate()
+            .map(|(idx, name)| (name, idx))
+            .collect()
+    }
+
+    fn collect_statement_wildcards(
+        &self,
+        stmt: &StatementTmpl,
+        names: &mut Vec<String>,
+        seen: &mut HashSet<String>,
+    ) {
+        for arg in &stmt.args {
+            match arg {
+                StatementTmplArg::Wildcard(id) => {
+                    if !seen.contains(&id.name) {
+                        seen.insert(id.name.clone());
+                        names.push(id.name.clone());
+                    }
+                }
+                StatementTmplArg::AnchoredKey(ak) => {
+                    if !seen.contains(&ak.root.name) {
+                        seen.insert(ak.root.name.clone());
+                        names.push(ak.root.name.clone());
+                    }
+                }
+                StatementTmplArg::Literal(_) => {}
+            }
+        }
+    }
+
+    fn extract_and_split_predicates(
+        &self,
+    ) -> Result<(Vec<CustomPredicateDef>, usize), LoweringError> {
+        let doc = self.validated.document();
+        let predicates: Vec<CustomPredicateDef> = doc
+            .items
+            .iter()
+            .filter_map(|item| match item {
+                DocumentItem::CustomPredicateDef(pred) => Some(pred.clone()),
+                _ => None,
+            })
+            .collect();
+
+        let original_count = predicates.len();
+
+        // Apply splitting to each predicate as needed
+        let mut split_predicates = Vec::new();
+        for pred in predicates {
+            let chain = frontend_ast_split::split_predicate_if_needed(pred, self.params)?;
+            split_predicates.extend(chain);
+        }
+
+        Ok((split_predicates, original_count))
+    }
+
+    fn lower_custom_predicate(
+        &self,
+        pred_def: &CustomPredicateDef,
+        cpb_builder: &mut CustomPredicateBatchBuilder,
+    ) -> Result<(), LoweringError> {
+        let name = pred_def.name.name.clone();
+
+        // Note: Constraint checking is handled by the splitting phase
+        // Predicates passed here should already be within limits
+
+        // Collect public and private argument names
+        let mut public_arg_names = Vec::new();
+        let mut private_arg_names = Vec::new();
+
+        for arg in &pred_def.args.public_args {
+            public_arg_names.push(arg.name.clone());
+        }
+
+        if let Some(private_args) = &pred_def.args.private_args {
+            for arg in private_args {
+                private_arg_names.push(arg.name.clone());
+            }
+        }
+
+        // Lower statements to builders
+        let mut statement_builders = Vec::new();
+        for stmt in &pred_def.statements {
+            let stmt_builder = self.lower_statement_to_builder(stmt)?;
+            statement_builders.push(stmt_builder);
+        }
+
+        // Convert to &str slices for builder API
+        let public_args_str: Vec<&str> = public_arg_names.iter().map(|s| s.as_str()).collect();
+        let private_args_str: Vec<&str> = private_arg_names.iter().map(|s| s.as_str()).collect();
+
+        // Add predicate to batch using builder
+        let conjunction = pred_def.conjunction_type == ConjunctionType::And;
+
+        cpb_builder
+            .predicate(
+                &name,
+                conjunction,
+                &public_args_str,
+                &private_args_str,
+                &statement_builders,
+            )
+            .map_err(|e| match e {
+                crate::frontend::Error::Middleware(mw_err) => LoweringError::Middleware(mw_err),
+                _ => LoweringError::InvalidArgumentType,
+            })?;
+
+        Ok(())
+    }
+
+    fn lower_statement_to_builder(
+        &self,
+        stmt: &StatementTmpl,
+    ) -> Result<StatementTmplBuilder, LoweringError> {
+        // Get predicate
+        let pred_name = &stmt.predicate.name;
+        let symbols = self.validated.symbols();
+
+        // Check for native predicates first
+        let predicate = if let Ok(native) = NativePredicate::from_str(pred_name) {
+            Predicate::Native(native)
+        } else if let Some(&index) = self.batch_predicate_index.get(pred_name) {
+            // References to other predicates in the same batch (including split chains)
+            Predicate::BatchSelf(index)
+        } else if let Some(info) = symbols.predicates.get(pred_name) {
+            match &info.kind {
+                PredicateKind::Native(np) => Predicate::Native(*np),
+                PredicateKind::Custom { index } => Predicate::BatchSelf(*index),
+                PredicateKind::BatchImported { batch, index } => {
+                    Predicate::Custom(middleware::CustomPredicateRef::new(batch.clone(), *index))
+                }
+                PredicateKind::IntroImported {
+                    name,
+                    verifier_data_hash,
+                } => Predicate::Intro(IntroPredicateRef {
+                    name: name.clone(),
+                    args_len: info.public_arity,
+                    verifier_data_hash: *verifier_data_hash,
+                }),
+            }
+        } else {
+            unreachable!("Predicate {} not found", pred_name);
+        };
+
+        // Check args count
+        if stmt.args.len() > self.params.max_statement_args {
+            return Err(LoweringError::TooManyStatementArgs {
+                count: stmt.args.len(),
+                max: self.params.max_statement_args,
+            });
+        }
+
+        // Convert AST args to BuilderArgs
+        let mut builder = StatementTmplBuilder::new(predicate);
+        for arg in &stmt.args {
+            let builder_arg = self.lower_statement_arg_to_builder(arg)?;
+            builder = builder.arg(builder_arg);
+        }
+
+        // Return builder without calling .desugar() - that will happen later
+        Ok(builder)
+    }
+
+    fn lower_statement_arg_to_builder(
+        &self,
+        arg: &StatementTmplArg,
+    ) -> Result<BuilderArg, LoweringError> {
+        match arg {
+            StatementTmplArg::Literal(lit) => {
+                let value = self.lower_literal(lit)?;
+                Ok(BuilderArg::Literal(value))
+            }
+            StatementTmplArg::Wildcard(id) => {
+                // For builder, we just need the wildcard name
+                Ok(BuilderArg::WildcardLiteral(id.name.clone()))
+            }
+            StatementTmplArg::AnchoredKey(ak) => {
+                let key_str = match &ak.key {
+                    AnchoredKeyPath::Bracket(s) => s.value.clone(),
+                    AnchoredKeyPath::Dot(id) => id.name.clone(),
+                };
+                Ok(BuilderArg::Key(ak.root.name.clone(), key_str))
+            }
+        }
+    }
+
+    fn lower_literal(&self, lit: &LiteralValue) -> Result<middleware::Value, LoweringError> {
+        let value = match lit {
+            LiteralValue::Int(i) => middleware::Value::from(i.value),
+            LiteralValue::Bool(b) => middleware::Value::from(b.value),
+            LiteralValue::String(s) => middleware::Value::from(s.value.clone()),
+            LiteralValue::Raw(r) => middleware::Value::from(r.hash.hash),
+            LiteralValue::PublicKey(pk) => middleware::Value::from(pk.point),
+            LiteralValue::SecretKey(sk) => middleware::Value::from(sk.secret_key.clone()),
+            LiteralValue::Array(a) => {
+                let elements: Result<Vec<_>, _> =
+                    a.elements.iter().map(|e| self.lower_literal(e)).collect();
+                let array = containers::Array::new(self.params.max_depth_mt_containers, elements?)?;
+                middleware::Value::from(array)
+            }
+            LiteralValue::Set(s) => {
+                let elements: Result<Vec<_>, _> =
+                    s.elements.iter().map(|e| self.lower_literal(e)).collect();
+                let set_values: std::collections::HashSet<_> = elements?.into_iter().collect();
+                let set = containers::Set::new(self.params.max_depth_mt_containers, set_values)?;
+                middleware::Value::from(set)
+            }
+            LiteralValue::Dict(d) => {
+                let pairs: Result<Vec<(middleware::Key, middleware::Value)>, LoweringError> = d
+                    .pairs
+                    .iter()
+                    .map(|pair| {
+                        let key = middleware::Key::from(pair.key.value.as_str());
+                        let value = self.lower_literal(&pair.value)?;
+                        Ok((key, value))
+                    })
+                    .collect();
+                let dict_map: std::collections::HashMap<_, _> = pairs?.into_iter().collect();
+                let dict =
+                    containers::Dictionary::new(self.params.max_depth_mt_containers, dict_map)?;
+                middleware::Value::from(dict)
+            }
+        };
+        Ok(value)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::lang::{
+        frontend_ast::parse::parse_document, frontend_ast_validate::validate, parser::parse_podlang,
+    };
+
+    fn parse_validate_and_lower(
+        input: &str,
+        params: &Params,
+    ) -> Result<LoweredOutput, LoweringError> {
+        let parsed = parse_podlang(input).expect("Failed to parse");
+        let document = parse_document(parsed.into_iter().next().unwrap()).expect("Failed to parse");
+        let validated = validate(document, &[]).expect("Failed to validate");
+        lower(validated, params, "test_batch".to_string())
+    }
+
+    // Helper to get the batch from the output (expecting it to exist)
+    fn expect_batch(output: &LoweredOutput) -> &Arc<CustomPredicateBatch> {
+        output.batch.as_ref().expect("Expected batch to be present")
+    }
+
+    #[test]
+    fn test_simple_predicate() {
+        let input = r#"
+            my_pred(A, B) = AND (
+                Equal(A["foo"], B["bar"])
+            )
+        "#;
+
+        let params = Params::default();
+        let result = parse_validate_and_lower(input, &params);
+        if let Err(e) = &result {
+            eprintln!("Error: {:?}", e);
+        }
+        assert!(result.is_ok());
+
+        let lowered = result.unwrap();
+        assert_eq!(expect_batch(&lowered).predicates().len(), 1);
+
+        let pred = &expect_batch(&lowered).predicates()[0];
+        assert_eq!(pred.name, "my_pred");
+        assert_eq!(pred.args_len(), 2);
+        assert_eq!(pred.wildcard_names().len(), 2);
+        assert_eq!(pred.statements().len(), 1);
+    }
+
+    #[test]
+    fn test_private_args() {
+        let input = r#"
+            my_pred(A, private: B, C) = AND (
+                Equal(A["x"], B["y"])
+                Equal(B["z"], C["w"])
+            )
+        "#;
+
+        let params = Params::default();
+        let result = parse_validate_and_lower(input, &params);
+        assert!(result.is_ok());
+
+        let lowered = result.unwrap();
+        let pred = &expect_batch(&lowered).predicates()[0];
+        assert_eq!(pred.args_len(), 1); // Only A is public
+        assert_eq!(pred.wildcard_names().len(), 3); // A, B, C total
+    }
+
+    #[test]
+    fn test_or_predicate() {
+        let input = r#"
+            my_pred(A, B) = OR (
+                Equal(A["x"], 1)
+                Equal(B["y"], 2)
+            )
+        "#;
+
+        let params = Params::default();
+        let result = parse_validate_and_lower(input, &params);
+        assert!(result.is_ok());
+
+        let lowered = result.unwrap();
+        let pred = &expect_batch(&lowered).predicates()[0];
+        assert!(pred.is_disjunction());
+    }
+
+    #[test]
+    fn test_automatic_splitting() {
+        let input = r#"
+            my_pred(A) = AND (
+                Equal(A["a"], 1)
+                Equal(A["b"], 2)
+                Equal(A["c"], 3)
+                Equal(A["d"], 4)
+                Equal(A["e"], 5)
+                Equal(A["f"], 6)
+            )
+        "#;
+
+        let params = Params::default(); // max_custom_predicate_arity = 5
+        let result = parse_validate_and_lower(input, &params);
+        if let Err(e) = &result {
+            eprintln!("Splitting error: {:?}", e);
+        }
+        assert!(result.is_ok());
+
+        let lowered = result.unwrap();
+        // Should be automatically split into 2 predicates (my_pred and my_pred_1)
+        assert_eq!(expect_batch(&lowered).predicates().len(), 2);
+
+        // First predicate should have 5 statements (4 + chain call)
+        assert_eq!(expect_batch(&lowered).predicates()[0].statements().len(), 5);
+
+        // Second predicate should have 2 statements
+        assert_eq!(expect_batch(&lowered).predicates()[1].statements().len(), 2);
+    }
+
+    #[test]
+    fn test_multiple_predicates() {
+        let input = r#"
+            pred1(A) = AND (
+                Equal(A["x"], 1)
+            )
+
+            pred2(B) = AND (
+                Equal(B["y"], 2)
+            )
+        "#;
+
+        let params = Params::default();
+        let result = parse_validate_and_lower(input, &params);
+        assert!(result.is_ok());
+
+        let lowered = result.unwrap();
+        assert_eq!(expect_batch(&lowered).predicates().len(), 2);
+    }
+
+    #[test]
+    fn test_batch_self_reference() {
+        let input = r#"
+            pred1(A) = AND (
+                Equal(A["x"], 1)
+            )
+
+            pred2(B) = AND (
+                pred1(B)
+            )
+        "#;
+
+        let params = Params::default();
+        let result = parse_validate_and_lower(input, &params);
+        assert!(result.is_ok());
+
+        let lowered = result.unwrap();
+        let pred2 = &expect_batch(&lowered).predicates()[1];
+        let stmt = &pred2.statements()[0];
+
+        // Should be BatchSelf(0) referring to pred1
+        assert!(matches!(stmt.pred, Predicate::BatchSelf(0)));
+    }
+
+    #[test]
+    fn test_literals() {
+        let input = r#"
+            my_pred(X) = AND (
+                Equal(X["int"], 42)
+                Equal(X["bool"], true)
+                Equal(X["string"], "hello")
+            )
+        "#;
+
+        let params = Params::default();
+        let result = parse_validate_and_lower(input, &params);
+        assert!(result.is_ok());
+    }
+
+    #[test]
+    fn test_syntactic_sugar_desugaring() {
+        let input = r#"
+            my_pred(D) = AND (
+                DictContains(D, "key", "value")
+            )
+        "#;
+
+        let params = Params::default();
+        let result = parse_validate_and_lower(input, &params);
+        assert!(result.is_ok());
+
+        let lowered = result.unwrap();
+        let pred = &expect_batch(&lowered).predicates()[0];
+        let stmt = &pred.statements()[0];
+
+        // Should desugar to the Contains predicate
+        assert!(matches!(
+            stmt.pred,
+            Predicate::Native(NativePredicate::Contains)
+        ));
+    }
+
+    #[test]
+    fn test_error_message_with_splitting() {
+        // Create a document with predicates that will exceed the batch limit after splitting
+        // We'll create 2 predicates with 4 statements each (max arity = 5)
+        // Each will NOT split individually, but together they exceed a small batch limit
+        let input = r#"
+            pred1(A) = AND (
+                Equal(A["a"], 1)
+                Equal(A["b"], 2)
+            )
+            pred2(B) = AND (
+                Equal(B["c"], 3)
+                Equal(B["d"], 4)
+            )
+        "#;
+
+        // Use very restrictive params to force the error
+        let params = Params {
+            max_custom_batch_size: 1,
+            ..Default::default()
+        };
+
+        let result = parse_validate_and_lower(input, &params);
+
+        // Should fail with TooManyPredicates error
+        assert!(result.is_err());
+        let err = result.unwrap_err();
+
+        if let LoweringError::TooManyPredicates {
+            count,
+            max,
+            original_count,
+            ..
+        } = err
+        {
+            assert_eq!(count, 2); // 2 predicates after splitting (no splitting occurred)
+            assert_eq!(max, 1);
+            assert_eq!(original_count, 2); // Started with 2 predicates
+
+            // Error message should NOT mention splitting since no splitting occurred
+            let err_msg = format!("{}", err);
+            assert!(!err_msg.contains("before automatic splitting"));
+        } else {
+            panic!("Expected TooManyPredicates error, got: {:?}", err);
+        }
+    }
+
+    #[test]
+    fn test_error_message_after_splitting() {
+        // Create TWO predicates that will EACH split into 2 predicates
+        // This tests the case where splitting causes the batch to be too large
+        // but no individual predicate chain exceeds the limit
+        let input = r#"
+            pred1(A) = AND (
+                Equal(A["a"], 1)
+                Equal(A["b"], 2)
+                Equal(A["c"], 3)
+                Equal(A["d"], 4)
+                Equal(A["e"], 5)
+                Equal(A["f"], 6)
+            )
+            pred2(B) = AND (
+                Equal(B["a"], 1)
+                Equal(B["b"], 2)
+                Equal(B["c"], 3)
+                Equal(B["d"], 4)
+                Equal(B["e"], 5)
+                Equal(B["f"], 6)
+            )
+        "#;
+
+        // Use params where each predicate splits into 2, but total of 4 exceeds batch limit
+        let params = Params {
+            // Allow 3 predicates in batch
+            // Default max_custom_predicate_arity is 5, so each will split into 2 predicates
+            // Total: 2 original predicates -> 4 after splitting (exceeds limit of 3)
+            max_custom_batch_size: 3,
+            ..Default::default()
+        };
+
+        let result = parse_validate_and_lower(input, &params);
+
+        // Should fail with TooManyPredicates error
+        assert!(result.is_err());
+        let err = result.unwrap_err();
+
+        if let LoweringError::TooManyPredicates {
+            count,
+            max,
+            original_count,
+            ..
+        } = err
+        {
+            assert_eq!(count, 4); // 4 predicates after splitting (2 from each)
+            assert_eq!(max, 3);
+            assert_eq!(original_count, 2); // Started with 2 predicates
+
+            // Error message SHOULD mention splitting since splitting occurred
+            let err_msg = format!("{}", err);
+            assert!(err_msg.contains("before automatic splitting"));
+            assert!(err_msg.contains("started with 2 predicates"));
+        } else {
+            panic!("Expected TooManyPredicates error, got: {:?}", err);
+        }
+    }
+}

src/lang/frontend_ast_validate.rs

@@ -0,0 +1,775 @@
+//! Validation for the frontend AST
+//!
+//! This module provides semantic validation for parsed AST documents,
+//! including name resolution, arity checking, and wildcard validation.
+
+use std::{collections::HashMap, str::FromStr, sync::Arc};
+
+use hex::ToHex;
+
+use crate::{
+    lang::frontend_ast::*,
+    middleware::{CustomPredicateBatch, Hash, NativePredicate},
+};
+
+/// A validated AST document with symbol table and diagnostics
+#[derive(Debug, Clone)]
+pub struct ValidatedAST {
+    document: Document,
+    symbols: SymbolTable,
+    diagnostics: Vec<Diagnostic>,
+}
+
+impl ValidatedAST {
+    pub fn document(&self) -> &Document {
+        &self.document
+    }
+
+    pub fn symbols(&self) -> &SymbolTable {
+        &self.symbols
+    }
+
+    pub fn diagnostics(&self) -> &[Diagnostic] {
+        &self.diagnostics
+    }
+
+    pub fn into_document(self) -> Document {
+        self.document
+    }
+}
+
+/// Symbol table containing all predicates and their metadata
+#[derive(Debug, Clone)]
+pub struct SymbolTable {
+    /// All predicates available in this scope
+    pub predicates: HashMap<String, PredicateInfo>,
+    /// Wildcard scopes for each custom predicate
+    pub wildcard_scopes: HashMap<String, WildcardScope>,
+}
+
+/// Information about a predicate
+#[derive(Debug, Clone)]
+pub struct PredicateInfo {
+    pub kind: PredicateKind,
+    pub arity: usize,
+    pub public_arity: usize,
+    pub source_span: Option<Span>,
+}
+
+/// Kind of predicate
+#[derive(Debug, Clone)]
+pub enum PredicateKind {
+    Native(NativePredicate),
+    Custom {
+        index: usize,
+    },
+    BatchImported {
+        batch: Arc<CustomPredicateBatch>,
+        index: usize,
+    },
+    IntroImported {
+        name: String,
+        verifier_data_hash: Hash,
+    },
+}
+
+/// Wildcard scope for a custom predicate
+#[derive(Debug, Clone)]
+pub struct WildcardScope {
+    pub wildcards: HashMap<String, WildcardInfo>,
+}
+
+/// Information about a wildcard
+#[derive(Debug, Clone)]
+pub struct WildcardInfo {
+    pub index: usize,
+    pub is_public: bool,
+    pub source_span: Option<Span>,
+}
+
+/// Diagnostic message (warning or info)
+#[derive(Debug, Clone)]
+pub struct Diagnostic {
+    pub level: DiagnosticLevel,
+    pub message: String,
+    pub span: Option<Span>,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum DiagnosticLevel {
+    Warning,
+    Info,
+}
+
+pub use crate::lang::error::ValidationError;
+
+/// Validate an AST document
+pub fn validate(
+    document: Document,
+    available_batches: &[Arc<CustomPredicateBatch>],
+) -> Result<ValidatedAST, ValidationError> {
+    let validator = Validator::new(available_batches);
+    validator.validate(document)
+}
+
+struct Validator {
+    available_batches: HashMap<String, Arc<CustomPredicateBatch>>,
+    symbols: SymbolTable,
+    diagnostics: Vec<Diagnostic>,
+    custom_predicate_count: usize,
+}
+
+impl Validator {
+    fn new(batches: &[Arc<CustomPredicateBatch>]) -> Self {
+        let mut available_batches = HashMap::new();
+        for batch in batches {
+            // Store by hex ID for lookup
+            let id = format!("0x{}", batch.id().encode_hex::<String>());
+            available_batches.insert(id, batch.clone());
+        }
+
+        Self {
+            available_batches,
+            symbols: SymbolTable {
+                predicates: HashMap::new(),
+                wildcard_scopes: HashMap::new(),
+            },
+            diagnostics: Vec::new(),
+            custom_predicate_count: 0,
+        }
+    }
+
+    fn validate(mut self, document: Document) -> Result<ValidatedAST, ValidationError> {
+        // Pass 1: Build symbol table
+        self.build_symbol_table(&document)?;
+
+        // Pass 2: Validate all references
+        self.validate_references(&document)?;
+
+        Ok(ValidatedAST {
+            document,
+            symbols: self.symbols,
+            diagnostics: self.diagnostics,
+        })
+    }
+
+    fn build_symbol_table(&mut self, document: &Document) -> Result<(), ValidationError> {
+        // First process imports
+        for item in &document.items {
+            if let DocumentItem::UseBatchStatement(use_stmt) = item {
+                self.process_use_batch_statement(use_stmt)?;
+            }
+            if let DocumentItem::UseIntroStatement(use_stmt) = item {
+                self.process_use_intro_statement(use_stmt)?;
+            }
+        }
+
+        // Then process custom predicate definitions
+        for item in &document.items {
+            if let DocumentItem::CustomPredicateDef(pred_def) = item {
+                self.process_custom_predicate_def(pred_def)?;
+            }
+        }
+
+        // Check for multiple REQUEST definitions (only one allowed)
+        let mut first_request_span = None;
+        for item in &document.items {
+            if let DocumentItem::RequestDef(req) = item {
+                if let Some(first_span) = first_request_span {
+                    return Err(ValidationError::MultipleRequestDefinitions {
+                        first_span: Some(first_span),
+                        second_span: req.span,
+                    });
+                }
+                first_request_span = req.span;
+            }
+        }
+
+        Ok(())
+    }
+
+    fn process_use_batch_statement(
+        &mut self,
+        use_stmt: &UseBatchStatement,
+    ) -> Result<(), ValidationError> {
+        let batch_id = format!("0x{}", use_stmt.batch_ref.hash.encode_hex::<String>());
+
+        let batch = self.available_batches.get(&batch_id).ok_or_else(|| {
+            ValidationError::BatchNotFound {
+                id: batch_id.clone(),
+                span: use_stmt.batch_ref.span,
+            }
+        })?;
+
+        if use_stmt.imports.len() != batch.predicates().len() {
+            return Err(ValidationError::ImportArityMismatch {
+                expected: batch.predicates().len(),
+                found: use_stmt.imports.len(),
+                span: use_stmt.span,
+            });
+        }
+
+        for (i, import) in use_stmt.imports.iter().enumerate() {
+            if let ImportName::Named(name) = import {
+                if self.symbols.predicates.contains_key(name) {
+                    return Err(ValidationError::DuplicateImport {
+                        name: name.clone(),
+                        span: use_stmt.span,
+                    });
+                }
+
+                let pred = &batch.predicates()[i];
+                // CustomPredicate has args_len (public args) and wildcard_names (total args)
+                let total_arity = pred.wildcard_names.len();
+                let public_arity = pred.args_len;
+
+                self.symbols.predicates.insert(
+                    name.clone(),
+                    PredicateInfo {
+                        kind: PredicateKind::BatchImported {
+                            batch: batch.clone(),
+                            index: i,
+                        },
+                        arity: total_arity,
+                        public_arity,
+                        source_span: use_stmt.span,
+                    },
+                );
+            }
+        }
+
+        Ok(())
+    }
+
+    fn process_use_intro_statement(
+        &mut self,
+        use_stmt: &UseIntroStatement,
+    ) -> Result<(), ValidationError> {
+        let intro_name = &use_stmt.name.name;
+        let args = &use_stmt.args;
+        let intro_predicate_ref = &use_stmt.intro_hash;
+
+        if self.symbols.predicates.contains_key(intro_name) {
+            return Err(ValidationError::DuplicateImport {
+                name: intro_name.clone(),
+                span: use_stmt.span,
+            });
+        }
+
+        self.symbols.predicates.insert(
+            intro_name.clone(),
+            PredicateInfo {
+                kind: PredicateKind::IntroImported {
+                    name: intro_name.clone(),
+                    // Hash is already parsed in the AST
+                    verifier_data_hash: intro_predicate_ref.hash,
+                },
+                arity: args.len(),
+                public_arity: args.len(),
+                source_span: use_stmt.span,
+            },
+        );
+        Ok(())
+    }
+
+    fn process_custom_predicate_def(
+        &mut self,
+        pred_def: &CustomPredicateDef,
+    ) -> Result<(), ValidationError> {
+        let name = &pred_def.name.name;
+
+        if self.symbols.predicates.contains_key(name) {
+            let first_span = self.symbols.predicates[name].source_span;
+            return Err(ValidationError::DuplicatePredicate {
+                name: name.clone(),
+                first_span,
+                second_span: pred_def.name.span,
+            });
+        }
+
+        // Check for empty statement list
+        if pred_def.statements.is_empty() {
+            return Err(ValidationError::EmptyStatementList {
+                context: format!("predicate '{}'", name),
+                span: pred_def.span,
+            });
+        }
+
+        // Build wildcard scope
+        let mut wildcards = HashMap::new();
+        let mut wildcard_index = 0;
+
+        // Process public arguments
+        for arg in &pred_def.args.public_args {
+            if wildcards.contains_key(&arg.name) {
+                return Err(ValidationError::DuplicateWildcard {
+                    name: arg.name.clone(),
+                    span: arg.span,
+                });
+            }
+            wildcards.insert(
+                arg.name.clone(),
+                WildcardInfo {
+                    index: wildcard_index,
+                    is_public: true,
+                    source_span: arg.span,
+                },
+            );
+            wildcard_index += 1;
+        }
+
+        // Process private arguments
+        let mut private_count = 0;
+        if let Some(private_args) = &pred_def.args.private_args {
+            for arg in private_args {
+                if wildcards.contains_key(&arg.name) {
+                    return Err(ValidationError::DuplicateWildcard {
+                        name: arg.name.clone(),
+                        span: arg.span,
+                    });
+                }
+                wildcards.insert(
+                    arg.name.clone(),
+                    WildcardInfo {
+                        index: wildcard_index,
+                        is_public: false,
+                        source_span: arg.span,
+                    },
+                );
+                wildcard_index += 1;
+                private_count += 1;
+            }
+        }
+
+        // Add to symbol table
+        self.symbols.predicates.insert(
+            name.clone(),
+            PredicateInfo {
+                kind: PredicateKind::Custom {
+                    index: self.custom_predicate_count,
+                },
+                arity: pred_def.args.public_args.len() + private_count,
+                public_arity: pred_def.args.public_args.len(),
+                source_span: pred_def.name.span,
+            },
+        );
+
+        self.symbols
+            .wildcard_scopes
+            .insert(name.clone(), WildcardScope { wildcards });
+        self.custom_predicate_count += 1;
+
+        Ok(())
+    }
+
+    fn validate_references(&mut self, document: &Document) -> Result<(), ValidationError> {
+        for item in &document.items {
+            match item {
+                DocumentItem::CustomPredicateDef(pred_def) => {
+                    self.validate_custom_predicate_statements(pred_def)?;
+                }
+                DocumentItem::RequestDef(req_def) => {
+                    self.validate_request_statements(req_def)?;
+                }
+                _ => {}
+            }
+        }
+        Ok(())
+    }
+
+    fn validate_custom_predicate_statements(
+        &self,
+        pred_def: &CustomPredicateDef,
+    ) -> Result<(), ValidationError> {
+        let pred_name = pred_def.name.name.clone();
+
+        for stmt in &pred_def.statements {
+            let wildcard_scope = self
+                .symbols
+                .wildcard_scopes
+                .get(&pred_name)
+                .expect("Wildcard scope should exist after pass 1");
+            self.validate_statement(stmt, Some((&pred_name, wildcard_scope)))?;
+        }
+
+        Ok(())
+    }
+
+    fn validate_request_statements(&mut self, req_def: &RequestDef) -> Result<(), ValidationError> {
+        if req_def.statements.is_empty() {
+            self.diagnostics.push(Diagnostic {
+                level: DiagnosticLevel::Warning,
+                message: "Empty REQUEST block".to_string(),
+                span: req_def.span,
+            });
+        }
+
+        for stmt in &req_def.statements {
+            self.validate_statement(stmt, None)?;
+        }
+
+        Ok(())
+    }
+
+    fn validate_statement(
+        &self,
+        stmt: &StatementTmpl,
+        wildcard_context: Option<(&str, &WildcardScope)>,
+    ) -> Result<(), ValidationError> {
+        let pred_name = &stmt.predicate.name;
+
+        // Check if predicate exists
+        let pred_info = if let Ok(native) = NativePredicate::from_str(pred_name) {
+            // Native predicate
+            PredicateInfo {
+                kind: PredicateKind::Native(native),
+                arity: native.arity(),
+                public_arity: native.arity(),
+                source_span: None,
+            }
+        } else if let Some(info) = self.symbols.predicates.get(pred_name) {
+            // Custom or imported predicate
+            info.clone()
+        } else {
+            return Err(ValidationError::UndefinedPredicate {
+                name: pred_name.clone(),
+                span: stmt.predicate.span,
+            });
+        };
+
+        let expected_arity = pred_info.public_arity;
+
+        if stmt.args.len() != expected_arity {
+            return Err(ValidationError::ArgumentCountMismatch {
+                predicate: pred_name.clone(),
+                expected: expected_arity,
+                found: stmt.args.len(),
+                span: stmt.span,
+            });
+        }
+
+        // Validate arguments
+        self.validate_statement_args(stmt, &pred_info, wildcard_context)?;
+
+        Ok(())
+    }
+
+    fn validate_statement_args(
+        &self,
+        stmt: &StatementTmpl,
+        pred_info: &PredicateInfo,
+        wildcard_context: Option<(&str, &WildcardScope)>,
+    ) -> Result<(), ValidationError> {
+        // For custom predicates, only wildcards and literals are allowed
+        if matches!(
+            pred_info.kind,
+            PredicateKind::Custom { .. } | PredicateKind::BatchImported { .. }
+        ) {
+            for arg in &stmt.args {
+                match arg {
+                    StatementTmplArg::AnchoredKey(_) => {
+                        return Err(ValidationError::InvalidArgumentType {
+                            predicate: stmt.predicate.name.clone(),
+                            span: stmt.span,
+                        });
+                    }
+                    StatementTmplArg::Wildcard(id) => {
+                        if let Some((pred_name, scope)) = wildcard_context {
+                            if !scope.wildcards.contains_key(&id.name) {
+                                return Err(ValidationError::UndefinedWildcard {
+                                    name: id.name.clone(),
+                                    pred_name: pred_name.to_string(),
+                                    span: id.span,
+                                });
+                            }
+                        }
+                    }
+                    StatementTmplArg::Literal(_) => {}
+                }
+            }
+        } else {
+            // Native predicates can have anchored keys
+            for arg in &stmt.args {
+                match arg {
+                    StatementTmplArg::Wildcard(id) => {
+                        if let Some((pred_name, scope)) = wildcard_context {
+                            if !scope.wildcards.contains_key(&id.name) {
+                                return Err(ValidationError::UndefinedWildcard {
+                                    name: id.name.clone(),
+                                    pred_name: pred_name.to_string(),
+                                    span: id.span,
+                                });
+                            }
+                        }
+                    }
+                    StatementTmplArg::AnchoredKey(ak) => {
+                        if let Some((pred_name, scope)) = wildcard_context {
+                            if !scope.wildcards.contains_key(&ak.root.name) {
+                                return Err(ValidationError::UndefinedWildcard {
+                                    name: ak.root.name.clone(),
+                                    pred_name: pred_name.to_string(),
+                                    span: ak.root.span,
+                                });
+                            }
+                        }
+                    }
+                    StatementTmplArg::Literal(_) => {}
+                }
+            }
+        }
+
+        Ok(())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::{
+        lang::{frontend_ast::parse::parse_document, parser::parse_podlang},
+        middleware::{CustomPredicate, Params, EMPTY_HASH},
+    };
+
+    fn parse_and_validate(
+        input: &str,
+        batches: &[Arc<CustomPredicateBatch>],
+    ) -> Result<ValidatedAST, ValidationError> {
+        let parsed = parse_podlang(input).expect("Failed to parse");
+        let document = parse_document(parsed.into_iter().next().unwrap()).expect("Failed to parse");
+        validate(document, batches)
+    }
+
+    #[test]
+    fn test_validate_empty() {
+        let result = parse_and_validate("", &[]);
+        assert!(result.is_ok());
+    }
+
+    #[test]
+    fn test_validate_simple_request() {
+        let input = r#"REQUEST(
+            Equal(A["foo"], B["bar"])
+        )"#;
+        let result = parse_and_validate(input, &[]);
+        assert!(result.is_ok());
+    }
+
+    #[test]
+    fn test_validate_custom_predicate() {
+        let input = r#"
+            my_pred(A, B) = AND (
+                Equal(A["foo"], B["bar"])
+            )
+        "#;
+        let result = parse_and_validate(input, &[]);
+        assert!(result.is_ok());
+
+        let validated = result.unwrap();
+        assert!(validated.symbols.predicates.contains_key("my_pred"));
+        assert!(validated.symbols.wildcard_scopes.contains_key("my_pred"));
+    }
+
+    #[test]
+    fn test_undefined_predicate() {
+        let input = r#"REQUEST(
+            UndefinedPred(A, B)
+        )"#;
+        let result = parse_and_validate(input, &[]);
+        assert!(matches!(
+            result,
+            Err(ValidationError::UndefinedPredicate { .. })
+        ));
+    }
+
+    #[test]
+    fn test_undefined_wildcard() {
+        let input = r#"
+            my_pred(A) = AND (
+                Equal(A["foo"], B["bar"])
+            )
+        "#;
+        let result = parse_and_validate(input, &[]);
+        assert!(
+            matches!(result, Err(ValidationError::UndefinedWildcard { name, .. }) if name == "B")
+        );
+    }
+
+    #[test]
+    fn test_arity_mismatch() {
+        let input = r#"REQUEST(
+            Equal(A, B, C)
+        )"#;
+        let result = parse_and_validate(input, &[]);
+        assert!(matches!(
+            result,
+            Err(ValidationError::ArgumentCountMismatch { .. })
+        ));
+    }
+
+    #[test]
+    fn test_duplicate_predicate() {
+        let input = r#"
+            my_pred(A) = AND (Equal(A["x"], 1))
+            my_pred(B) = AND (Equal(B["y"], 2))
+        "#;
+        let result = parse_and_validate(input, &[]);
+        assert!(matches!(
+            result,
+            Err(ValidationError::DuplicatePredicate { .. })
+        ));
+    }
+
+    #[test]
+    fn test_duplicate_wildcard() {
+        let input = r#"
+            my_pred(A, A) = AND (Equal(A["x"], 1))
+        "#;
+        let result = parse_and_validate(input, &[]);
+        assert!(matches!(
+            result,
+            Err(ValidationError::DuplicateWildcard { .. })
+        ));
+    }
+
+    #[test]
+    fn test_custom_predicate_with_anchored_key() {
+        let input = r#"
+            my_pred(A, B) = AND (
+                Equal(A["foo"], B["bar"])
+            )
+            
+            REQUEST(
+                my_pred(X["key"], Y)
+            )
+        "#;
+        let result = parse_and_validate(input, &[]);
+        assert!(matches!(
+            result,
+            Err(ValidationError::InvalidArgumentType { .. })
+        ));
+    }
+
+    #[test]
+    fn test_forward_reference() {
+        let input = r#"
+            pred1(A) = AND (
+                pred2(A)
+            )
+            
+            pred2(B) = AND (
+                Equal(B["x"], 1)
+            )
+        "#;
+        let result = parse_and_validate(input, &[]);
+        assert!(result.is_ok());
+    }
+
+    #[test]
+    fn test_private_args() {
+        let input = r#"
+            my_pred(A, private: B, C) = AND (
+                Equal(A["x"], B["y"])
+                Equal(B["z"], C["w"])
+            )
+        "#;
+        let result = parse_and_validate(input, &[]);
+        assert!(result.is_ok());
+
+        let validated = result.unwrap();
+        let pred_info = &validated.symbols.predicates["my_pred"];
+        assert_eq!(pred_info.arity, 3);
+        assert_eq!(pred_info.public_arity, 1);
+    }
+
+    #[test]
+    fn test_empty_statement_list() {
+        // Create a custom predicate with empty statements to test validation
+        let document = Document {
+            items: vec![DocumentItem::CustomPredicateDef(CustomPredicateDef {
+                name: Identifier {
+                    name: "my_pred".to_string(),
+                    span: None,
+                },
+                args: ArgSection {
+                    public_args: vec![Identifier {
+                        name: "A".to_string(),
+                        span: None,
+                    }],
+                    private_args: None,
+                    span: None,
+                },
+                conjunction_type: ConjunctionType::And,
+                statements: vec![], // Empty statements
+                span: None,
+            })],
+        };
+        let result = validate(document, &[]);
+        assert!(matches!(
+            result,
+            Err(ValidationError::EmptyStatementList { .. })
+        ));
+    }
+
+    #[test]
+    fn test_multiple_request_definitions() {
+        let input = r#"
+            REQUEST(Equal(A["x"], 1))
+            REQUEST(Equal(B["y"], 2))
+        "#;
+        let result = parse_and_validate(input, &[]);
+        assert!(matches!(
+            result,
+            Err(ValidationError::MultipleRequestDefinitions { .. })
+        ));
+    }
+
+    #[test]
+    fn test_use_statement() {
+        let params = Params::default();
+
+        // Create a batch to import
+        let pred = CustomPredicate::and(
+            &params,
+            "imported".to_string(),
+            vec![],
+            2,
+            vec!["X".to_string(), "Y".to_string()],
+        )
+        .unwrap();
+
+        let batch = CustomPredicateBatch::new(&params, "TestBatch".to_string(), vec![pred]);
+
+        let batch_id = batch.id().encode_hex::<String>();
+        let input = format!(
+            r#"
+            use batch imported_pred from 0x{}
+            use intro intro_pred() from 0x{}
+
+            REQUEST(
+                imported_pred(A, B)
+                intro_pred()
+            )
+        "#,
+            batch_id,
+            EMPTY_HASH.encode_hex::<String>()
+        );
+
+        let result = parse_and_validate(&input, &[batch]);
+        assert!(result.is_ok());
+
+        let validated = result.unwrap();
+        assert!(validated.symbols.predicates.contains_key("imported_pred"));
+        assert!(validated.symbols.predicates.contains_key("intro_pred"));
+    }
+
+    #[test]
+    fn test_syntactic_sugar_predicates() {
+        let input = r#"REQUEST(
+            GtEq(A["x"], B["y"])
+            DictContains(D, K, V)
+            SetNotContains(S, E)
+        )"#;
+        let result = parse_and_validate(input, &[]);
+        assert!(result.is_ok());
+    }
+}

src/lang/grammar.pest

@@ -33,8 +33,9 @@ use_predicate_list = { import_name ~ ("," ~ import_name)* }
 import_name = { identifier | "_" }
 batch_ref = { hash_hex }
 
-use_intro_statement = { "use" ~ "intro" ~ identifier ~ "(" ~ use_intro_arg_list? ~ ")" ~ "from" ~ batch_ref }
+use_intro_statement = { "use" ~ "intro" ~ identifier ~ "(" ~ use_intro_arg_list? ~ ")" ~ "from" ~ intro_predicate_ref }
 use_intro_arg_list = { identifier ~ ("," ~ identifier)* }
+intro_predicate_ref = { hash_hex }
 
 request_def = { "REQUEST" ~ "(" ~ statement_list? ~ ")" } 
 

src/lang/mod.rs

@@ -1,17 +1,27 @@
 pub mod error;
+pub mod frontend_ast;
+pub mod frontend_ast_lower;
+pub mod frontend_ast_split;
+pub mod frontend_ast_validate;
 pub mod parser;
 pub mod pretty_print;
-pub mod processor;
 
 use std::sync::Arc;
 
 pub use error::LangError;
 pub use parser::{parse_podlang, Pairs, ParseError, Rule};
 pub use pretty_print::PrettyPrint;
-pub use processor::process_pest_tree;
-use processor::PodlangOutput;
 
-use crate::middleware::{CustomPredicateBatch, Params};
+use crate::{
+    frontend::PodRequest,
+    middleware::{CustomPredicateBatch, Params},
+};
+
+#[derive(Debug, Clone, PartialEq)]
+pub struct PodlangOutput {
+    pub custom_batch: Arc<CustomPredicateBatch>,
+    pub request: PodRequest,
+}
 
 pub fn parse(
     input: &str,
@@ -19,7 +29,28 @@ pub fn parse(
     available_batches: &[Arc<CustomPredicateBatch>],
 ) -> Result<PodlangOutput, LangError> {
     let pairs = parse_podlang(input)?;
-    processor::process_pest_tree(pairs, params, available_batches).map_err(LangError::from)
+    let document_pair = pairs
+        .into_iter()
+        .next()
+        .expect("parse_podlang should always return at least one pair for a valid document");
+    let document = frontend_ast::parse::parse_document(document_pair)?;
+    let validated = frontend_ast_validate::validate(document, available_batches)?;
+    let lowered = frontend_ast_lower::lower(validated, params, "PodlangBatch".to_string())?;
+
+    let custom_batch = lowered.batch.unwrap_or_else(|| {
+        // If no batch, create an empty one
+        CustomPredicateBatch::new(params, "PodlangBatch".to_string(), vec![])
+    });
+
+    let request = lowered.request.unwrap_or_else(|| {
+        // If no request, create an empty one
+        PodRequest::new(vec![])
+    });
+
+    Ok(PodlangOutput {
+        custom_batch,
+        request,
+    })
 }
 
 #[cfg(test)]
@@ -30,7 +61,6 @@ mod tests {
     use super::*;
     use crate::{
         backends::plonky2::primitives::ec::schnorr::SecretKey,
-        lang::error::ProcessorError,
         middleware::{
             CustomPredicate, CustomPredicateBatch, CustomPredicateRef, Key, NativePredicate,
             Params, Predicate, RawValue, StatementTmpl, StatementTmplArg, Value, Wildcard,
@@ -963,13 +993,13 @@ mod tests {
         assert!(result.is_err());
 
         match result.err().unwrap() {
-            LangError::Processor(e) => match *e {
-                ProcessorError::BatchNotFound { id, .. } => {
+            LangError::Validation(e) => match *e {
+                frontend_ast_validate::ValidationError::BatchNotFound { id, .. } => {
                     assert_eq!(id, unknown_batch_id);
                 }
                 _ => panic!("Expected BatchNotFound error, but got {:?}", e),
             },
-            e => panic!("Expected LangError::Processor, but got {:?}", e),
+            e => panic!("Expected LangError::Validation, but got {:?}", e),
         }
     }
 
@@ -991,16 +1021,18 @@ mod tests {
         assert!(result.is_err());
 
         match result.err().unwrap() {
-            LangError::Processor(e) => match *e {
-                ProcessorError::UndefinedWildcard {
-                    name, pred_name, ..
+            LangError::Validation(e) => match *e {
+                frontend_ast_validate::ValidationError::UndefinedWildcard {
+                    name,
+                    pred_name,
+                    ..
                 } => {
                     assert_eq!(name, "user_public_key");
                     assert_eq!(pred_name, "identity_verified");
                 }
                 _ => panic!("Expected UndefinedWildcard error, but got {:?}", e),
             },
-            e => panic!("Expected LangError::Processor, but got {:?}", e),
+            e => panic!("Expected LangError::Validation, but got {:?}", e),
         }
     }
 }

src/lang/parser.rs

@@ -12,8 +12,20 @@ pub type Pairs<'a, R> = PestPairs<'a, R>;
 
 #[derive(thiserror::Error, Debug)]
 pub enum ParseError {
+    #[error("Invalid integer: {0}")]
+    InvalidInt(String),
+
     #[error("Pest parsing error: {0}")]
     Pest(Box<pest::error::Error<Rule>>),
+
+    #[error("Invalid public key: {0}")]
+    InvalidPublicKey(String),
+
+    #[error("Invalid secret key: {0}")]
+    InvalidSecretKey(String),
+
+    #[error("Invalid escape sequence in string: {0}")]
+    InvalidEscapeSequence(String),
 }
 
 impl From<pest::error::Error<Rule>> for ParseError {

src/middleware/statement.rs

@@ -1,6 +1,7 @@
 use std::{
     fmt::{self, Display},
     iter,
+    str::FromStr,
 };
 
 use plonky2::field::types::Field;
@@ -51,6 +52,42 @@ pub enum NativePredicate {
     ArrayUpdate = 1014,
 }
 
+impl NativePredicate {
+    pub fn arity(&self) -> usize {
+        match self {
+            NativePredicate::None | NativePredicate::False => 0,
+            NativePredicate::Equal
+            | NativePredicate::NotEqual
+            | NativePredicate::Lt
+            | NativePredicate::Gt
+            | NativePredicate::GtEq
+            | NativePredicate::LtEq
+            | NativePredicate::NotContains
+            | NativePredicate::SetNotContains
+            | NativePredicate::DictNotContains
+            | NativePredicate::PublicKeyOf
+            | NativePredicate::SignedBy
+            | NativePredicate::SetContains => 2,
+            NativePredicate::Contains
+            | NativePredicate::DictContains
+            | NativePredicate::ArrayContains
+            | NativePredicate::SumOf
+            | NativePredicate::ProductOf
+            | NativePredicate::MaxOf
+            | NativePredicate::HashOf
+            | NativePredicate::SetInsert
+            | NativePredicate::SetDelete => 3,
+            NativePredicate::DictInsert
+            | NativePredicate::DictUpdate
+            | NativePredicate::DictDelete
+            | NativePredicate::ArrayUpdate
+            | NativePredicate::ContainerInsert
+            | NativePredicate::ContainerUpdate
+            | NativePredicate::ContainerDelete => 4,
+        }
+    }
+}
+
 impl Display for NativePredicate {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         let s = match self {
@@ -95,6 +132,45 @@ impl ToFields for NativePredicate {
     }
 }
 
+impl FromStr for NativePredicate {
+    type Err = Error;
+    fn from_str(s: &str) -> Result<Self> {
+        match s {
+            "Equal" => Ok(NativePredicate::Equal),
+            "NotEqual" => Ok(NativePredicate::NotEqual),
+            "Gt" => Ok(NativePredicate::Gt),
+            "GtEq" => Ok(NativePredicate::GtEq),
+            "Lt" => Ok(NativePredicate::Lt),
+            "LtEq" => Ok(NativePredicate::LtEq),
+            "Contains" => Ok(NativePredicate::Contains),
+            "NotContains" => Ok(NativePredicate::NotContains),
+            "SumOf" => Ok(NativePredicate::SumOf),
+            "ProductOf" => Ok(NativePredicate::ProductOf),
+            "MaxOf" => Ok(NativePredicate::MaxOf),
+            "HashOf" => Ok(NativePredicate::HashOf),
+            "PublicKeyOf" => Ok(NativePredicate::PublicKeyOf),
+            "SignedBy" => Ok(NativePredicate::SignedBy),
+            "ContainerInsert" => Ok(NativePredicate::ContainerInsert),
+            "ContainerUpdate" => Ok(NativePredicate::ContainerUpdate),
+            "ContainerDelete" => Ok(NativePredicate::ContainerDelete),
+            "DictContains" => Ok(NativePredicate::DictContains),
+            "DictNotContains" => Ok(NativePredicate::DictNotContains),
+            "ArrayContains" => Ok(NativePredicate::ArrayContains),
+            "SetContains" => Ok(NativePredicate::SetContains),
+            "SetNotContains" => Ok(NativePredicate::SetNotContains),
+            "DictInsert" => Ok(NativePredicate::DictInsert),
+            "DictUpdate" => Ok(NativePredicate::DictUpdate),
+            "DictDelete" => Ok(NativePredicate::DictDelete),
+            "SetInsert" => Ok(NativePredicate::SetInsert),
+            "SetDelete" => Ok(NativePredicate::SetDelete),
+            "ArrayUpdate" => Ok(NativePredicate::ArrayUpdate),
+            "None" => Ok(NativePredicate::None),
+            "False" => Ok(NativePredicate::False),
+            _ => Err(Error::custom(format!("Invalid native predicate: {}", s))),
+        }
+    }
+}
+
 #[derive(Clone, Debug, PartialEq, Eq, Hash, Serialize, Deserialize, JsonSchema)]
 pub struct IntroPredicateRef {
     pub name: String,