Module evaluator 

Evaluator: turns AST nodes into Swan Values.

Entry point

eval takes a &Node and a &Scope and returns a Value. It is infallible: every error (syntax errors, invalid names, type mismatches) is returned as a Value::Undefined(...), and errors in sub-expressions propagate as ordinary values. It never panics.

What lives where

This file handles literals, identifier lookup, grouping ([], {}), function definition and application, pairing dispatch, mapping, assignment, labelling, subcontexting, promise chaining (~>), logic (&, |), conditional (?, ;), composition (<<, >>) and syntax-error nodes. Operations with non-trivial logic are delegated to the sub-modules declared below (unary operators, pairing, assignment, ~>, arithmetic and comparison).

Tuple semantics

Tuples are flat and normalized (Nothing items are dropped). Operators are lifted over tuples by two internal macros:

• map_values! for unary operations: Nothing gives Nothing, a tuple is mapped item by item, any other value goes straight to the operation.
• map_pairs! for binary operations: tuples are paired element-wise (the shorter side is padded with Nothing) and the result is normalized.

The short-circuit operators &, |, ? and ; are not lifted: they look at their left operand as a whole.

Evaluation order

Operands are evaluated eagerly, left then right. The exceptions are the short-circuit operators, whose right operand B is evaluated only when needed: A & B and A ? B when A is truthy, A | B when A is falsy, and A ; B when A is Nothing. &, | and ? cast A to bool as a whole (a tuple is truthy if any of its items is), so they are not paired over tuples. For ->, = and : the left operand is not evaluated; it is resolved into names by eval_names.

Scopes

• {X} evaluates X in a new child scope and returns it as a Namespace.
• A -> B captures the current scope as a closure; applying it evaluates the body in a child of that closure scope.
• ns.X evaluates X inside the scope of the namespace ns.

Functions

eval

Evaluates an AST node within a given Scope context and returns a Value.

eval_apply

func arg - applies a callable or indexable Swan value to an argument.