Implement map destructuring in let bindings

[ecto]

Summary

Add support for map destructuring patterns in let bindings, allowing users to extract multiple values from a map in a single binding. This feature is implemented in both the type checker and EIR lowering, with comprehensive test coverage.

Changes

• Type Checker (crates/loon-lang/src/check/mod.rs)

  • Added bind_map_destructure method to handle type inference for map destructuring patterns
  • Each key symbol in the pattern is bound to the map's value type
  • Default expressions (when provided) are type-checked and unified against the element type
  • Supports shorthand syntax {name name} (no default) and explicit defaults {name default_expr}

• EIR Lowering (crates/loon-lang/src/eir/lower.rs)

  • Refactored lower_let_binding to use pattern matching instead of simple symbol check
  • Added lower_map_destructure method to generate efficient IR for map destructuring
  • Shorthand patterns (no default) use a single Op::Field that returns unit for missing keys
  • Patterns with defaults branch on key presence using Op::Builtin(Contains) to avoid evaluating defaults unnecessarily
  • Missing keys without defaults bind unit, matching the interpreter's behavior

• Tests (crates/loon-lang/tests/backend_parity.rs)

  • Added three test cases covering:
    • Shorthand destructuring: {name name age age} binds keys to their values
    • Default expressions: defaults are only evaluated when keys are absent
    • Missing keys without defaults: bind unit and print as ()

Implementation Details

The lowering strategy uses conditional branching for patterns with defaults to ensure side effects in default expressions only occur when needed. For shorthand patterns without defaults, a simpler single-operation approach is used since Op::Field naturally returns unit for missing keys, matching the legacy interpreter's semantics.

https://claude.ai/code/session_01JzaPzShDQzi6KqNoWqCbE5

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Project	Deployment	Actions	Updated (UTC)
loon	Ready	Preview, Comment	Jul 7, 2026 6:57am

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[chojiai]

What shipped

You can now extract multiple values from a map in a single binding step. For example, binding {name age} to a map pulls out both keys at once, with optional fallback values for keys that may be absent. Fallback expressions are only evaluated when a key is actually missing, so any side effects in a default stay contained to that case.

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