Scope collection

Scope collection gives the identifiers in scope for each expression in an item. Note that at this point, we don't perform name resolution, as resolving call identifiers requires the types of the arguments to perform function overloading resolution, and resolving record field access requires knowledge of the accessed record's type.

It would be possible to only keep track of this for expressions which are identifiers, rather than all expressions (and we could even perform name resolution for variables and non-overloaded function calls). However, having the scopes computed for every expression is useful for providing code completion information in the language server.

Scope collection results

Scope collection results in a data structure containing an arena of Scopes which contain the identifiers defined in each scope in an item. Each Scope keeps track of its parent Scope index, so that the chain of scopes can be followed to resolve an identifier. Each expression index in an item is mapped to a scope index and generation (see below for details on generations).

The db.lookup_item_scope(item_ref) query produces a ScopeResult which allows for easy lookup of functions and variables in scope for an expression. It automatically traverses the item scopes and will bubble up to global scope to find an identifier. Note that the result of looking up identifiers is a PatternRef (or list of them for overloaded functions). These point to the declaring identifier pattern of the identifier (as there may be several identifiers declared in a single item, via destructuring or in a let expression, so it is not sufficient to give only the item).

Scopes and generations

A Scope keeps track of the variables and functions declared in a particular scope, along with the parent scope (if any). For example:

function foo(int: a) =
    let {
        int: x = 3 * a;
        int: y = x + 2;
    } in a + y;

Has two scopes: one created by the function item for its body, and one created by the let item.

The first scope contains a declaration for the identifier a and its parent scope (assuming this is a top-level item) is the global scope. The second scope has declarations for the identifiers x and y, and its parent scope is the first scope.

Then, the scope for the outer let would be computed to be the first scope, while the scope for the RHS of x, the RHS of y, and the in expression a + y would be the second scope (and all of their sub-expressions would also be assigned to be the second scope).

Note that items in let expressions are such that they only enter into scope once they appear, so an item cannot refer to an item later than itself. For example:

let {
    int: y = 10;
    int: z = let {
        int: x = y;
        int: y = 1;
    } in x;
} in z

Will give z = 10 as the let item int: y = 1 is not in scope for the declaration of x.

This is achieved by keeping track of the 'generation' where a let-item identifier has been declared. The generation is incremented every time we visit an identifier pattern which creates a new variable. It should also be noted that the patterns for a declaration in a let are always visited after processing their RHS. Expressions are then assigned both a scope, along with a generation defining the minimum generation an identifier must be to be considered available in the expression. Exiting a scope restores the generation the value it had upon entering the scope.

Following the example:

The declaration int: y = 10; is visited
- The RHS expression (10) is assigned scope 0, gen 0
- The LHS identifier y is assigned scope 0, gen 1
The declaration int: z = let { int: x = y; int: y = 1; } in x is visited
- The RHS let expression is assigned scope 0, gen 1
  - The declaration int: x = y; is visited
    - The RHS expression y is assigned scope 1, gen 1
    - The LHS identifier x is assigned scope 1, gen 2
  - The declaration int: y = 1 is visited
    - The RHS expression 1 is assigned scope 1, gen 2
    - The LHS identifier y is assigned scope 1, gen 3
  - The in expression x is assigned scope 1, gen 3
- The LHS identifier z is assigned scope 0, gen 1 (restored because exited scope 1)
The in expression z is assigned scope 0, gen 1

From this, we can see that while the RHS of int: x = y is scope 1, since its generation is 1, the y does not refer to the y in scope 1 (which is generation 3), and so instead refers to the y in scope 0 (which is generation 1).

It should be noted that the 'generation' values are an implementation detail - what actually matters in the end is to be able to determine whether or not an identifier is in scope or not for a given expression.

Patterns

Patterns used for variable or parameter declarations are only allowed to contain tuples, records and identifiers. Otherwise, the pattern is refutable and may not match all values. Identifiers in variable declarations always create new variables, and do not refer to enumeration/annotation atoms (instead they get shadowed).

enum Foo = {A, B};

% OK: creates variables `x`, `y` and `z`.
any: (x, (foo: y, bar: z)) = (1, (foo: 2, bar: 3));

% OK: shadows enum values `A` and `B`
any: z = let {
    int: A = 1;
    int: B = 2;
} in A + B;

% ERROR: refutable pattern may not match all cases.
any: (m, 1) = (3, 2);

In comprehension generators and case expressions, all patterns are allowed, and identifiers can either refer to enumeration/annotation atoms, or if none match, they create new variables. A problem with this approach is that since we don't have namespacing, a user could create an atom with the same name as a variable binding in a case expression or generator in another part of the code, and then that variable binding would instead match that atom, changing the behaviour (likely causing the model to fail type checking).

The Shackle Almanac

Scope collection