# Lambda Expression Feature - Completion Record
# Completed: 2025-01-16
# Version: v0.2.1

feature:
  name: "Lambda Expressions"
  priority: CRITICAL
  status: completed
  completion_date: "2025-01-16"
  estimated_hours: 12
  actual_hours: 3
  
implementation:
  parsing:
    files_modified:
      - src/frontend/ast.rs
      - src/frontend/parser.rs
    changes:
      - Added Lambda variant to ExprKind enum
      - Implemented parse_lambda function
      - Handle || token ambiguity (OrOr vs empty lambda)
      - Support for typed and untyped parameters
    
  type_inference:
    files_modified:
      - src/middleend/infer.rs
    changes:
      - Added infer_lambda function
      - Extended Algorithm W for lambda expressions
      - Parameter type variable generation
      - Function type construction from lambda
    
  transpilation:
    files_modified:
      - src/backend/transpiler.rs
    changes:
      - Added transpile_lambda function
      - Generate Rust closure syntax
      - Preserve parameter names
      - Let Rust infer types when possible
    
syntax_support:
  - empty_params: "|| 42"
  - single_param: "|x| x + 1"
  - multiple_params: "|x, y| x * y"
  - typed_params: "|x: i32, y: f64| x + y"
  - nested_lambdas: "|x| |y| x + y"
  - in_let_binding: "let f = |x| x * 2 in f(5)"
  - in_function_call: "map(|x| x * 2)"

test_coverage:
  parser_tests:
    - test_parse_lambda
    - Test empty, single, multiple, typed parameters
  transpiler_tests:
    - test_transpile_lambda
    - Test all lambda variants transpile correctly
  type_inference_tests:
    - test_infer_lambda
    - Test type inference for various lambda forms
  total_tests: 172
  passing_tests: 172
  coverage_impact: "Maintained 77.91% overall coverage"

technical_decisions:
  - Used pipe syntax |params| instead of backslash for familiarity
  - Handle || as special case in parser for empty params
  - Let Rust infer closure types rather than explicit annotations
  - Lambdas are expressions, can appear anywhere expressions are valid

challenges_resolved:
  - Token ambiguity: || can be OrOr (logical OR) or empty lambda
  - Solution: Check for OrOr token explicitly in parse_lambda
  - Type inference: Lambdas need fresh type variables for params
  - Solution: Generate type variables for untyped parameters

quality_metrics:
  cyclomatic_complexity: "< 10 for all new functions"
  satd_count: 0
  clippy_warnings: 0
  test_coverage: "100% of new code paths"
  
integration_notes:
  - Lambdas work with pipeline operators
  - Type inference integrates with existing type system
  - Transpilation produces idiomatic Rust closures
  - No breaking changes to existing features

next_steps:
  - Closure capture analysis (T008 in v0.3-todo.yaml)
  - Move semantics optimization
  - Lifetime inference for closures
  - Support for async lambdas