# Haskell Coding Practices

> Use when authoring or reviewing Haskell, 4-space/80-col layout, Haddock, explicit imports, type signatures, avoid partial functions, strict data, IO separation, and stylish-haskell/HLint/cabal test in CI.

- Skill: `ryan-brosas/haskell-coding-practices` (Agent Skill)
- Install (CLI): `npx skillmds@latest add ryan-brosas/haskell-coding-practices`
- Raw SKILL.md: https://api.skillmd.com/api/skills/ryan-brosas/haskell-coding-practices/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: ryan-brosas (https://skillmd.com/u/ryan-brosas)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/ryan-brosas/haskell-coding-practices

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# Haskell Coding Practices

Application skill for Haskell style learning (from the archived `awesome-guidelines` style capsules). For GHC internals or specific linters (fourmolu vs stylish-haskell), follow project config first.

## Core Principle

Haskell quality is **explicit equational modules**, typed total functions in pure core, strict data by default, IO pushed to the rim.

## When to Use / NOT

- Haskell libraries, applications, and GHC-adjacent tools using Cabal/Stack.
- Setting up stylish-haskell/fourmolu, HLint, `-Wall -Werror`, Haddock in CI.

**NOT when:**

- Pure FFI C fragments, use C practice skills for foreign code.
- TH/splice-generated-only modules, validate generators.

## Workflow

1. **Layout**, indent, cols, case (`haskell-style-formatting-layout.md`).
2. **Modules**, names, imports, Haddock (`haskell-style-naming-imports.md`).
3. **Functions**, sigs, totality, guards (`haskell-style-functions-control.md`).
4. **Types/IO**, strict data, boundaries (`haskell-style-types-io.md`).
5. **Verify**, formatter, HLint, `cabal build`/`stack test`, Haddock on changed modules.

## Red Flags

- Tabs or trailing whitespace
- Lines >80 without strong reason
- Missing Haddock on exports
- Unqualified Map/Set imports
- Blanket imports without explicit list
- Missing top-level type signatures on exports
- `head`, `tail`, `fromJust` on untrusted input
- Brace-semicolon case layout
- Type synonyms for domain types
- Lazy record fields by default
- Pure/IO mixed without module seam
- `<- return` instead of `let`
- Lazy read+write same file
- Over-point-free unreadable code
- `-Wall` warnings ignored
- `String` in new public API
- Commented-out dead code
- Debug `trace` as user output

## Verification

- stylish-haskell / fourmolu (project config)
- HLint with project rules
- `cabal build --ghc-options=-Wall` or `-Werror` policy
- Haddock build for libraries
- Capsule checklist on export list + signatures


## References

- `awesome-guidelines/references/haskell-style-learning-note.md`
- `awesome-guidelines/references/haskell-style-formatting-layout.md`
- `awesome-guidelines/references/haskell-style-naming-imports.md`
- `awesome-guidelines/references/haskell-style-functions-control.md`
- `awesome-guidelines/references/haskell-style-types-io.md`