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Phase 2: Typechecker + Borrow Implementation Plan

For agentic workers: REQUIRED SUB-SKILL: Use superpowers:subagent-driven-development or superpowers:executing-plans.

Goal: Reject ill-typed programs; attach types to AST; enforce lexical borrow rules and raises effect tracking.

Architecture: li_types owns TypeCtx, unification for int literals, borrow state per scope in Borrowck.

Tech Stack: indexmap, rustc-hash

Depends on: Phase 1
Blocks: Phase 3

Proof gaps (Doc-c): G-vc · G-bnd · G-def · G-math-syn

Type system (v1)

Type Notes
int, uint, wrapping_int, float64, bool, unit, string int default
array[N, T] N const usize
Option[T] no null
enum Name user enums
object Name product types
proc(Args) -> Ret effect !raises set on decl

Rules: - Literal 42 → int; suffix 42u → uint - + on two int → int; int + float → error - Index a[i] requires i: int and bounds proof for literal i - while body requires raises Loop or raises IO on enclosing proc - echo requires raises IO

Task 1: Type representation

Files: - Create: crates/li_types/src/ty.rs - Create: crates/li_types/src/context.rs

  • enum Type { Int, Uint, WrappingInt, Float64, Bool, Unit, String, Array { len: u64, elem: Box<Type> }, Option(Box<Type>), Enum(EnumId), Object(ObjectId), Proc { .. } } (C++: TypeExpr + typecheck.cpp)
  • TypeCtx::define_type, lookup, define_proc (C++: TypecheckState in typecheck.cpp)

Task 2: Typecheck expressions and stmts

Files: - Create: crates/li_types/src/check.rs - Test: crates/li_types/tests/check_fib.rs

  • check_module(&Module) -> Result<TypedModule, Vec<TypeError>> (C++: typecheck_module)
  • Return typed wrapper nodes or side table node_id → Type (C++: TypecheckResult + AST type fields)

Task 3: Borrow checker (lexical)

Files: - Create: crates/li_types/src/borrow.rs - Test: crates/li_types/tests/borrow_errors.rs

  • Track Owned | BorrowImm | BorrowMut per local (borrowck.cpp)
  • Reject: use after move, two mut borrows, mut while imm borrow live (li-tests/borrow/)
  • v1: no references in struct fields yet

Task 4: Scientific error fixtures

Files: - Create: tests/fixtures/bad_array_index.li - Create: tests/fixtures/bad_numeric_mix.li - Create: tests/fixtures/bad_overflow_mode.li

# bad_array_index.li — board[25, 0] when array[20, array[10, int]]
  • Tests assert compile error messages mention dimension / type mismatch (li-tests/typecheck/bad_*.li)

Task 5: CLI lic check

Files: - Modify: crates/lic/src/main.rs

  • lic check file.li — parse + typecheck, exit 1 on errors (compiler/lic/main.cpp)

Phase 2 exit gate

  • fib.li typechecks (li-tests/typecheck/fib.li)
  • All bad_*.li fail with expected errors
  • Borrow double-mut test fails cleanly (li-tests/borrow/double_mut.li)