pod2/book/src/custom2.md

Custom operations (or: how to define a custom predicate): VERSION 2

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SAVING IN THE GITHUB JUST SO WE HAVE A COPY

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The local variable requirement

This spec differs from the main spec in that there are no anchored keys. However, there are still types Origin and Key.

An Origin refers to an input POD; in-circuit, the Origin is the pod ID of the pod.

A Key refers to a value within a POD.

With the exception of the special statement ValueFromPodKey, a key always refers to a value within the POD _self. In other words, a statement (except for ValueFromPodKey) cannot refer to a value from a previous POD, only to a value in the "namespace" of the current POD.

Roughly speaking, the statement

ValueFromPodKey(local_key, origin_id, key)

means that the value of local_key on the current POD (_self) is the same as the value of key on the POD origin_id -- in other words, it is basically the same as

Equals(AnchoredKey(_SELF, local_key), AnchoredKey(origin_id, key)).

I say "basically the same" because, in this spec, it is possible to refer to both keys and origin IDs by reference.

Referencing

Recall that in the front-end, a Key is a string that functions as an identifier (like a variable name in other languages), and a Value is the value of that variable -- an Integer, String, or compound value.

In the back-end, a Key is four field elements (computed as a hash of the front-end key); and a Value is again four field elements. Again, each Key has a unique Value.

A Reference statement allows a key to be reinterpreted as a value; it is analogous to a pointer in C.

The statement

Reference(reference_key, key)

means that reference_key is a key, whose associated value is the same as the key key.

ValueFromPodKey, precisely this time

ValueFromPodKey(local_key: KeyOrLiteral::String, origin_id: KeyOrLiteral::OriginID, key: KeyOrLiteral::String).

means that the values of local_key and key are keys, the value of origin_id is an origin ID, and the value assigned to the key local_key on the present POD is the same as the value assigned to the key key on the pod origin_ID.

An example with literals:

ValueFromPodKey("local_ssn", 0x4030, "ssn")

means that the pod 0x4030 has a key called ssn, the local pod has a key local_ssn, and they have the same value.

An example with keys, that expresses the same semantic meaning:

ValueOf(local_varname, "local_ssn")
ValueOf(remote_varname, "ssn")
ValueOf(gov_id_pod_id, 0x4030)
ValueFromPodKey(local_varname, gov_id_pod_id, remote_varname)

Summary of additional statements in this spec

ValueFromPodKey(local_key: KeyOrLiteral::String, origin_id: KeyOrLiteral::OriginID, key: KeyOrLiteral::String).

In addition to the built-in statements in the main spec:

There is one additional front-end type: OriginID. As the name suggests, it contains the "origin ID" of a POD.

There are two additional built-in statements:

Reference(reference_key: Key::String, key: Key)

ValueFromPodKey(local_key: KeyOrLiteral::String, origin_id: KeyOrLiteral::OriginID, key: KeyOrLiteral::String).

Reference(reference_key, key)

means that the value of reference key is the key name of key.

ValueFromPodKey(local_key, origin_id, key)

means that the key local_key in the local scope has the same value as the key key in the scope of the pod origin_id.

How to work with the local variable requirement

To make a statement about an inherited value (a value introduced in an ancestor POD), the value must be copied to a local value:

The statements below assert that "name" on pod1 and "friend" on pod2 are assigned the same value.

ValueFromPodKey(name_from_pod1, pod1, "name")
ValueFromPodKey(friend_from_pod2, pod2, "friend")
Equal(name_from_pod1, friend_from_pod2)

How to inherit local variables from a previous POD

In this design, an additional complication arises when you carry a value from one POD to another, and you want to keep track of the origin POD on which it originated.

To allow this operation, we introduce an additional deduction rule

InheritValueFromPodKey,

which works as follows.

Suppose "self" is the current POD and "parent_id" is the POD id of one of the input PODs to "self".

Suppose "parent" has, among its public statements, the statement

ValueFromPodKey(parent_name, origin, original_name)

and "self" has the statement (public or private)

ValueFromPodKey(self_name, parent_id, parent_name).

Then InheritValueFromPodKey allows you to generate the following statement on "self":

ValueFromPodKey(self_name, origin, original_name).