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Initial term checker and constraint solver (#75)
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‎.claude/skills/type-checker-tests.md‎

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---
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name: type-checker-tests
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description: Add integration tests for type checker inference and checking functions
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---
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# Type Checker Integration Tests
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Use this skill when adding new type checker functions or expanding behavior.
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**Language:** Test fixtures use PureScript syntax, not Haskell.
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## Quick Reference
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| Action | Command |
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|--------|---------|
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| Find next test number | `ls tests-integration/fixtures/checking/ \| tail -5` |
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| Run specific test | `just tc NNN` |
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| Run all checking tests | `just tc` |
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| Accept all pending snapshots | `cargo insta accept` |
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Use `just tc --help` for all options.
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## Creating a Test
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### 1. Create fixture directory
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```bash
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mkdir tests-integration/fixtures/checking/{NNN_descriptive_name}
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```
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Tests are auto-discovered by `build.rs` - no manual registration needed.
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### 2. Write Main.purs
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**Standard pattern** - pair typed (checking) and untyped (inference) variants:
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```purescript
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module Main where
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-- Checking mode: explicit signature constrains type checker
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test :: Array Int -> Int
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test [x] = x
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-- Inference mode: type checker infers unconstrained
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test' [x] = x
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```
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**Guidelines:**
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- Test ONE specific behavior per fixture
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- Name tests descriptively: `test`, `test'`, `test2`, `test2'`, etc.
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- Include edge cases relevant to the behavior being tested
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### 3. Generate and review snapshot
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```bash
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just tc NNN
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```
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This outputs:
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- `CREATED path` (green) with numbered lines showing full content
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- `UPDATED path` (yellow) with chunked diff (2 lines context, line numbers)
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## Multi-File Tests
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For testing imports, re-exports, or cross-module behavior, add multiple `.purs` files
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to the same fixture directory. The type checker loads all `.purs` files in the folder.
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**Example structure:**
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```
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tests-integration/fixtures/checking/NNN_import_test/
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├── Main.purs # The test file (snapshot generated for Main)
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├── Lib.purs # Supporting module
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└── Main.snap # Generated snapshot
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```
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**Lib.purs:**
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```purescript
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module Lib where
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life :: Int
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life = 42
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data Maybe a = Just a | Nothing
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```
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**Main.purs:**
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```purescript
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module Main where
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import Lib (life, Maybe(..))
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test :: Maybe Int
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test = Just life
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```
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**Key points:**
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- Module name must match filename (`Lib.purs` -> `module Lib where`)
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- Only `Main.purs` generates a snapshot (the test runs against `Main`)
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- Use standard PureScript import syntax
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## Reviewing Snapshots
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Snapshots have this structure:
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```
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Terms
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functionName :: InferredOrCheckedType
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...
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Types
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TypeName :: Kind
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...
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Errors
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ErrorKind { details } at [location]
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```
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### Acceptance Criteria
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**Before accepting, verify:**
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1. **Types are correct** - Check that inferred types match expectations
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- `test :: Array Int -> Int` - explicit signature preserved
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- `test' :: forall t. Array t -> t` - polymorphism inferred correctly
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2. **No unexpected `???`** - This indicates inference failure
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- `test :: ???` - STOP: the term failed to type check
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- `CannotUnify { ??? -> ???, Int }` - OK in error tests, shows unresolved unification variables
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3. **Errors appear where expected** - For tests validating error behavior
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- Confirm error kind matches expectations (e.g., `NoInstanceFound`, `CannotUnify`)
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- Verify error location points to the correct declaration
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4. **Polymorphism is appropriate**
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- Check type variable names (`t6`, `a`, etc.) are scoped correctly
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- Verify constraints propagate as expected
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### Common Issues
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| Symptom | Likely Cause |
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|---------|--------------|
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| `test :: ???` | Test code has syntax error or uses undefined names |
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| Unexpected monomorphism | Missing polymorphic context or over-constrained signature |
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| Wrong error location | Check binder/expression placement in source |
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| Missing types in snapshot | Module header or imports incorrect |
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## Accept and Verify
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```bash
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# Accept only after thorough review
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cargo insta accept
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# Verify full suite passes
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just tc
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```
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## Debugging
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When investigating a potential compiler bug:
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```bash
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# Focus on single test to reduce noise
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just tc NNN
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```
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Once the issue is resolved, run the full test suite to check for regressions.

‎.github/dependabot.yml‎

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version: 2
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updates:
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- package-ecosystem: "github-actions"
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directory: "/"
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schedule:
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interval: "monthly"

‎.github/workflows/checks.yml‎

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run: rustup update ${{ matrix.toolchain }} && rustup default ${{ matrix.toolchain }}
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- name: Cache build artifacts
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uses: Swatinem/rust-cache@v2.8.0
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uses: Swatinem/rust-cache@v2.8.2
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with:
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cache-bin: "false"
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prefix-key: "v5-maho-${{ matrix.os }}-${{ matrix.toolchain }}"
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prefix-key: "v6-maho-${{ matrix.os }}-${{ matrix.toolchain }}"
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- name: Install cargo-binstall
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uses: cargo-bins/cargo-binstall@v1.15.3
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- name: Install cargo-nextest
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run: cargo binstall cargo-nextest just
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env:
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GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
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- name: Install tools
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uses: taiki-e/install-action@v2
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with:
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tool: cargo-nextest,just
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- name: Build and Test
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run: |
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run: rustup update ${{ matrix.toolchain }} && rustup default ${{ matrix.toolchain }}
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- name: Cache build artifacts
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uses: Swatinem/rust-cache@v2.8.0
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uses: Swatinem/rust-cache@v2.8.2
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with:
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cache-bin: "false"
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prefix-key: "v5-maho-${{ matrix.toolchain }}"
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prefix-key: "v6-maho-${{ matrix.toolchain }}"
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- name: Install cargo-binstall
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uses: cargo-bins/cargo-binstall@v1.15.3
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- name: Install cargo-nextest
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run: cargo binstall cargo-nextest cargo-llvm-cov just
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env:
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GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
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- name: Install tools
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uses: taiki-e/install-action@v2
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with:
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tool: cargo-nextest,cargo-llvm-cov,just
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- name: Build and Test
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run: |

‎AGENTS.md‎

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- uses techniques like interning and arena allocation to enable better caching patterns
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- for instance, whitespace does not immediately invalidate type checking results
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## Niche Guides
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## Skills
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Load these guides from `agents/guides/` when working on related tasks:
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- `build-system.md` - working with the build system
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- `integration-tests.md` - working with fixtures and snapshots
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Agent skills are specialized instruction sets for common tasks. They're stored in `.claude/skills/`.
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## Communication Guidelines
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- **type-checker-tests**: Adding integration tests for type checker inference and checking
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When working on this project, prefer succinct and direct responses. Keep output tokens
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minimal to reduce costs while using the default thinking budget to maintain reasoning.
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## Commands
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**Key principles:**
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- Provide brief, direct answers to queries
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- Only elaborate when explicitly asked
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- Keep output focused on what's necessary
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- Avoid verbose explanations unless requested
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- Do not write or edit code until permitted to do so
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This is your bread and butter to verify that code compiles, including test code.
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If updating the test code can be deferred, you may skip adding the `--tests flag`
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to speed up the task at hand.
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## Development Commands
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`cargo check` to verify what you've written is correct
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```bash
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# Check all crates
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cargo check
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# Check a crate
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cargo check -p indexing
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```
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`cargo nextest` to run tests blazingly fast
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```bash
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# Run unit tests for a specific crate
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cargo nextest run -p indexing
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# Run integration tests
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just integration
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# Run integration tests with filters
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just integration --test resolving 001
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cargo check -p <crate-name> --tests
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```
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`cargo clippy` and `cargo fmt` for code quality
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```bash
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# Get diagnostics from clippy
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cargo clippy --workspace
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When working on the type checker `./compiler-core/checking/` you must run
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the full type checker snapshot test suite using the following command to
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verify if there's any change in behaviour. Use the `type-checker-tests`
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skill when asked to develop more type checker tests.
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# Format with stable and nightly
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cargo fmt && just format-imports
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```
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just tc
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```
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### Testing Loop
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You can also specify a test fixture number to reduce noise. Once you've
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identified and fixed the bug, you must run the full test suite again to
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verify that the fix generalises.
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This project uses `cargo-insta` for snapshot tests. Fresh snapshots are prefixed with `.snap.new`,
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make sure to review these if a test run produces them. Use `cargo insta accept` once you're satisfied
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with the output.
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```
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just tc 101
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```

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