elipl/todo.md

1. Implement Parsing for (union <type1> <type2> ...) Type Specifiers:

   • Modify Til.Typer.ExpressionTyper.resolve_type_specifier_node to recognize and parse S-expressions like (union integer string).
   • This will involve recursively resolving the inner type specifiers and constructing a raw union type definition. The existing interning and subtyping logic for unions can then be leveraged.
   • Add tests for type checking expressions annotated with these explicit union types, e.g., (the (union integer string) some-expression).

2. Implement Parsing for Basic Function Type Specifiers:

   • Modify Til.Typer.ExpressionTyper.resolve_type_specifier_node to parse function type specifiers, e.g., (function (Arg1Type Arg2Type ...) ReturnType).
   • Add interning support for function types in Til.Typer.Interner.
   • Implement basic subtyping rules for function types in Til.Typer.SubtypeChecker (initially, arity checking; then contravariant arguments, covariant return).

3. Implement Basic Function Definition (e.g., def or lambda):

   • Define syntax (e.g., (def my-fn (param1 param2) body-expr)).
   • Add parser support for this syntax.
   • Typer:
     • For this initial step, infer a basic function type (e.g., based on arity, with any for parameter and return types if not annotated).
     • Add the function name and its inferred type to the environment.

4. Implement Basic Function Calls:

   • Extend Til.Typer.ExpressionTyper.infer_s_expression_type for function calls:
     • When the operator of an S-expression is a symbol, look up its type in the environment.
     • If it's a function type (from step 4), perform an arity check against the provided arguments.
     • The inferred type of the call would be the function's (currently basic) return type.

5. Enhance Function Definitions with Type Annotations:

   • Extend the function definition syntax to support type annotations for parameters and return types (e.g., (def my-fn ((p1 P1Type) (p2 P2Type)) :: ReturnType body-expr)).
   • Update the parser for this extended syntax.
   • Typer:
     • Use these annotations to construct a more precise function type.
     • When typing the function body, use the annotated parameter types in the local environment.
     • Verify that the inferred type of the function body is a subtype of the annotated return type.
   • Update function call typing (from step 5) to use these precise function types for argument type checking and to determine the call's return type.

6. Implement Type Inference for Core Map Operation: (map-get map key):

   • Define the S-expression syntax (map-get map-expr key-expr).
   • In Til.Typer.ExpressionTyper, implement type inference rules for map-get based on the logic outlined in todo.md. This includes:
     • Typing map-expr and key-expr.
     • Handling cases where key-expr's type is a literal (allowing lookup in known_elements).
     • Handling cases where key-expr's type is a general type (using index_signature and potentially unioning types from known_elements).

7. Improve User-Facing Type Error Messages:

   • For common errors like type_annotation_mismatch or function call argument mismatches, enhance the error reporting.
   • Develop a utility to pretty-print type definitions (from their internal map representation or ID) for inclusion in error messages, making them more readable than raw type IDs or structures.
   • Ensure source locations (file, line, column) are clearly associated with type errors.

8. Implement Parsing for (intersection <type1> <type2> ...) Type Specifiers:

   • Similar to union types, update Til.Typer.ExpressionTyper.resolve_type_specifier_node for intersection type S-expressions.
   • Add interning and subtyping rules for intersection types in Til.Typer.Interner and Til.Typer.SubtypeChecker.

9. Implement Simple Type Aliases (e.g., deftype):

   • Define syntax for non-generic type aliases (e.g., (deftype PositiveInteger (refinement integer ...)) or (deftype UserMap (map atom any))).
   • Add parser support.
   • Typer:
     • Store these alias definitions.
     • Modify Til.Typer.ExpressionTyper.resolve_type_specifier_node to recognize and expand these aliases when they are used in type annotations.