Module expr_typing 

Fase 3 expression typer.

Walks an ast::Expr tree and assigns a concrete DataType to every node, using:

• Column scope (table → column → DataType) supplied by the caller as a closure so we don’t depend on the schema registry directly — keeps this module trivially testable.
• The cast catalog (schema::cast_catalog) for implicit coercion paths.
• The type-category preferred-member rule for tie breaks (PG func_select_candidate heuristic, simplified).

Output is a TypedExpr mirroring the shape of Expr but with a ty: DataType slot on every node. The runtime evaluator can use that to skip the Value→Number tagging dance the current expr_eval does at every step.

Scope today (Fase 3 starter): handles literals, columns, unary, binary arith / comparison / logical, cast, IsNull, Between, InList, Case, and built-in FunctionCall nodes resolved through the static function catalog. The remaining gap is advanced polymorphic signatures beyond lightweight cases such as COALESCE. Subqueries / parameters / advanced polymorphism are out of scope.

This module is not yet wired into the parser → planner flow. It exists so Fase 3 Week 4+ can plug it in once the parser v2 emits Expr trees as the canonical projection / filter representation.

Structs

TypedExpr

Resolved type for an expression node. Mirrors Expr shape with an added ty slot. Span is preserved so analyzer diagnostics can still point at the original token range.

Enums

TypeError

Errors reported by the expression typer. All variants are recoverable diagnostics — the analyzer can collect several and emit them together rather than fail-fast like the parser.

TypedExprKind

Traits

Scope

Closure-based column scope. The analyzer passes a closure that resolves (table, column) to a DataType, returning None if the column doesn’t exist in the active scope. Callers wire this to the schema registry (production) or a static map (tests).

Functions

type_expr

Type a single expression against the given scope.