Haskell Conventions

haskellfunctionalbackend

Conventions for writing idiomatic Haskell covering module structure, type class usage, monadic code, error handling with Either/Maybe, and lens patterns.

Целевые файлы

Файл	Формат
.cursorrules	plaintext
CLAUDE.md	markdown

Содержимое

# Haskell Conventions ## Module Structure - Use explicit export lists in every module: `module MyApp.Users (getUser, createUser, UserError(..)) where`. - Organize modules hierarchically by domain: `MyApp.Domain.User`, `MyApp.Persistence.UserRepo`, `MyApp.API.UserHandler`. - Group imports: standard library first, then external packages, then internal modules. Separate groups with blank lines. - Use qualified imports for modules with common names: `import qualified Data.Map.Strict as Map`, `import qualified Data.Text as T`. - Prefer `Data.Map.Strict` over lazy `Data.Map` for most use cases — avoid unexpected memory retention. ## Type System - Prefer `newtype` over `type` aliases for domain types: `newtype UserId = UserId UUID` prevents mixing up IDs. - Derive `Show`, `Eq`, `Ord` via `deriving` for data types. Use `deriving stock` when using `DerivingStrategies`. - Enable `DerivingStrategies` — always annotate: `deriving stock (Show, Eq)`, `deriving newtype (FromJSON)`, `deriving anyclass (NFData)`. - Use `data` for sum types; `newtype` for single-constructor single-field wrappers; `type` only for short synonyms. - Avoid `String` — use `Text` (strict) for all user-facing strings. Use `ByteString` for binary data. ## Type Classes - Implement type class instances in the module that defines the data type — avoid orphan instances. - When an orphan instance is unavoidable, place it in a dedicated `Orphans` module with a `{-# OPTIONS_GHC -Wno-orphans #-}` pragma and a comment explaining why. - Do not write partial type class instances — implement all required methods. - Use `class (Superclass a) => MyClass a where` — be explicit about superclass constraints. ## Monadic Code - Use `do`-notation for sequences of monadic operations. Avoid deeply nested `>>=` chains. - Use `let` bindings in `do` blocks for pure computations, `<-` for effectful binds. - Prefer `when`/`unless` over `if ... then ... else pure ()`. - Use `forM_`, `mapM_`, `traverse_` for effectful iteration — be explicit about whether you need results. - Use `liftA2`, `<*>`, `<$>` for applicative-style composition when sequencing doesn't matter. ## Error Handling - Use `Either AppError a` for recoverable errors in pure and IO code. Define a sum type for `AppError`. - Use `ExceptT AppError IO a` (or `MonadError`) for effectful code with structured errors. - Use `Maybe` only when the absence of a value is not an error. For errors, prefer `Either`. - Avoid `error` and `undefined` in production code — use `throwIO` for IO exceptions or `Left` for pure errors. - Pattern match exhaustively. Enable `-Wincomplete-patterns` and treat warnings as errors in CI. ## Lens & Optics - Use `lens` or `optics` library for record field access and update in complex nested types. - Define lenses with Template Haskell: `makeLenses ''User` or `makeFieldLabels ''User` (with optics). - Prefix lens names with `_` when using `makeLenses`: `_userId`, `_userName`. - Use `view`, `over`, `set` for reading, modifying, and setting. Avoid manual record update syntax for nested fields. - Do not use lens in simple single-level record access — regular record syntax is clearer there. ## Performance - Use `Data.Map.Strict` and `Data.HashMap.Strict` (from unordered-containers) over lazy variants. - Add `{-# UNPACK #-}` pragmas to strict fields in hot data types: `!{-# UNPACK #-} Int`. - Use `Text` with `{-# LANGUAGE OverloadedStrings #-}` — avoid `pack`/`unpack` in hot paths. - Profile before optimizing. Use `-prof -fprof-auto` and `hp2ps` to find real bottlenecks. ## Formatting & Style - Use `fourmolu` or `ormolu` for formatting. Run it in CI and enforce via pre-commit hook. - Maximum line length: 100 characters. - Align `where` clauses consistently. Use 2-space indentation inside `where`. ## Anti-Patterns - Do not use `head`, `tail`, `fromJust`, `read` without explicit guards — they are partial functions. - Do not overuse `IORef` — prefer pure state passing or `State` monad. - Do not ignore `-Wall` warnings — treat them as errors in CI.

CLI

npx mindaxis apply haskell-rules --target cursor --scope project

Используется в паках

Не используется ни в одном паке

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