# Type Checker Tests

> Add integration tests for type checker inference and checking functions

- Skill: `majiayu000/type-checker-tests-2` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds@latest add majiayu000/type-checker-tests-2`
- Raw SKILL.md: https://api.skillmd.com/api/skills/majiayu000/type-checker-tests-2/raw
- Safety review: pending (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: AI & ML
- Author: majiayu000 (https://skillmd.com/u/majiayu000)
- Updated: 2026-09-22
- Page: https://skillmd.com/skills/majiayu000/type-checker-tests-2

---


# Type Checker Integration Tests

Use this skill when adding new type checker functions or expanding behavior.

**Language:** Test fixtures use PureScript syntax, not Haskell.

## Quick Reference

| Action | Command |
|--------|---------|
| Find next test number | `ls tests-integration/fixtures/checking/ \| tail -5` |
| Run a test or multiple tests | `just tc NNN` or `just tc 101 102` |
| Run all checking tests | `just tc` |
| Accept all pending snapshots | `cargo insta accept` |

Use `just tc --help` for all options.

## Creating a Test

### 1. Create fixture directory

```bash
mkdir tests-integration/fixtures/checking/{NNN_descriptive_name}
```

Tests are auto-discovered by `build.rs` - no manual registration needed.

### 2. Write Main.purs

**Standard pattern** - pair typed (checking) and untyped (inference) variants:

```purescript
module Main where

-- Checking mode: explicit signature constrains type checker
test :: Array Int -> Int
test [x] = x

-- Inference mode: type checker infers unconstrained
test' [x] = x
```

**Guidelines:**
- Test ONE specific behavior per fixture
- Name tests descriptively: `test`, `test'`, `test2`, `test2'`, etc.
- Include edge cases relevant to the behavior being tested

### 3. Generate and review snapshot

```bash
just tc NNN
```

This outputs:
- `CREATED path` (green) with numbered lines showing full content
- `UPDATED path` (yellow) with chunked diff (2 lines context, line numbers)

## Multi-File Tests

For testing imports, re-exports, or cross-module behavior, add multiple `.purs` files
to the same fixture directory. The type checker loads all `.purs` files in the folder.

**Example structure:**
```
tests-integration/fixtures/checking/NNN_import_test/
├── Main.purs    # The test file (snapshot generated for Main)
├── Lib.purs     # Supporting module
└── Main.snap    # Generated snapshot
```

**Lib.purs:**
```purescript
module Lib where

life :: Int
life = 42

data Maybe a = Just a | Nothing
```

**Main.purs:**
```purescript
module Main where

import Lib (life, Maybe(..))

test :: Maybe Int
test = Just life
```

**Key points:**
- Module name must match filename (`Lib.purs` -> `module Lib where`)
- Only `Main.purs` generates a snapshot (the test runs against `Main`)
- Use standard PureScript import syntax

## Reviewing Snapshots

Snapshots have this structure:

```
Terms
functionName :: InferredOrCheckedType
...

Types
TypeName :: Kind
...

Errors
ErrorKind { details } at [location]
```

### Acceptance Criteria

**Before accepting, verify:**

1. **Types are correct** - Check that inferred types match expectations
   - `test :: Array Int -> Int` - explicit signature preserved
   - `test' :: forall t. Array t -> t` - polymorphism inferred correctly

2. **No unexpected `???`** - This indicates inference failure
   - `test :: ???` - STOP: the term failed to type check
   - `CannotUnify { ??? -> ???, Int }` - OK in error tests, shows unresolved unification variables

3. **Errors appear where expected** - For tests validating error behavior
   - Confirm error kind matches expectations (e.g., `NoInstanceFound`, `CannotUnify`)
   - Verify error location points to the correct declaration

4. **Polymorphism is appropriate**
   - Check type variable names (`t6`, `a`, etc.) are scoped correctly
   - Verify constraints propagate as expected

### Common Issues

| Symptom | Likely Cause |
|---------|--------------|
| `test :: ???` | Test code has syntax error or uses undefined names |
| Unexpected monomorphism | Missing polymorphic context or over-constrained signature |
| Wrong error location | Check binder/expression placement in source |
| Missing types in snapshot | Module header or imports incorrect |

## Accept and Verify

```bash
# Accept only after thorough review
cargo insta accept

# Verify all checking tests pass
just tc
```

## Debugging

When investigating a potential compiler bug:

```bash
# Focus on single test to reduce noise
just tc NNN
```

You should run `just tc` to check for regressions.