name: Haskell Ecosystem description: This skill should be used when working with Haskell projects, "cabal.project", "stack.yaml", "ghc", "cabal build/test/run", "stack build/test/run", or Haskell language patterns. Provides comprehensive Haskell ecosystem patterns and best practices. version: 0.1.0

Haskell Ecosystem

Core Concepts

• Purity: Functions have no side effects; same input always produces same output
• Laziness: Expressions evaluated only when needed; enables infinite data structures
• Type inference: Hindley-Milner; explicit signatures recommended for top-level definitions
• Monads: Compose computations with effects (IO, Maybe, Either, State, Reader)
• Type classes: Ad-hoc polymorphism (Eq, Ord, Functor, Applicative, Monad)

Common Type Classes

• Functor - fmap, <$>
• Applicative - <*>, pure
• Monad - >>=, return, do-notation
• Foldable - foldr, foldl, toList
• Traversable - traverse, sequenceA
• Semigroup - <>
• Monoid - mempty, mappend

Type-Level Programming

-- GADTs
{-# LANGUAGE GADTs #-}
data Expr a where
  LitInt  :: Int -> Expr Int
  LitBool :: Bool -> Expr Bool
  Add     :: Expr Int -> Expr Int -> Expr Int

-- Type Families
{-# LANGUAGE TypeFamilies #-}
type family Element c where
  Element [a]     = a
  Element (Set a) = a

-- DataKinds
{-# LANGUAGE DataKinds, KindSignatures #-}
data Nat = Zero | Succ Nat
data Vec (n :: Nat) a where
  VNil  :: Vec 'Zero a
  VCons :: a -> Vec n a -> Vec ('Succ n) a

Monad Transformers (mtl)

import Control.Monad.Reader
import Control.Monad.State
import Control.Monad.Except

doSomething :: (MonadReader Config m, MonadState AppState m, MonadError AppError m) => m Result
doSomething = do
  cfg <- ask
  st <- get
  when (invalid cfg) $ throwError InvalidConfig
  pure (compute cfg st)

Common transformers: ReaderT, StateT, ExceptT, WriterT, MaybeT

Lens (optics)

import Control.Lens

data Person = Person { _name :: String, _age :: Int }
makeLenses ''Person

getName p = p ^. name        -- view
setName n p = p & name .~ n  -- set
modifyAge f p = p & age %~ f -- over

Operators: ^. (view), .~ (set), %~ (over), ^? (preview), ^.. (toListOf)

Cabal File

cabal-version: 3.0
name:          my-project
version:       0.1.0.0

common warnings
  ghc-options: -Wall -Wcompat -Wincomplete-record-updates

library
  import:           warnings
  exposed-modules:  MyProject
  build-depends:    base ^>=4.18, text ^>=2.0
  hs-source-dirs:   src
  default-language: GHC2021

Testing

-- HSpec
import Test.Hspec
main = hspec $ do
  describe "Calculator" $ do
    it "adds two numbers" $ add 1 2 `shouldBe` 3

-- QuickCheck property
prop_reverseReverse :: [Int] -> Bool
prop_reverseReverse xs = reverse (reverse xs) == xs

-- With HSpec
it "is its own inverse" $ property $
  \xs -> reverse (reverse xs) == (xs :: [Int])

Anti-Patterns

• Partial functions: Avoid head, tail, fromJust; use safe alternatives
• String for text: Use Text or ByteString for performance
• Lazy IO: Use strict IO or streaming (conduit, pipes) in production
• Orphan instances: Use newtype wrappers or define in appropriate modules

Commands

• cabal build / cabal test / cabal run
• cabal repl - Start GHCi
• stack build / stack test / stack ghci
• hlint src/ - Linter
• fourmolu -i src/**/*.hs - Formatter

Build Tool Choice

• Stack: Reproducible builds with Stackage LTS; simpler for beginners
• Cabal: Native format for Hackage; better for complex dependency overrides
• Nix: Use with haskell.nix or nixpkgs Haskell infrastructure

Context7 Reference

• Servant: /haskell-servant/servant
• HSpec: /websites/hackage_haskell_package_hspec-2_11_12
• Optics: /websites/hackage_haskell_package_optics-0_4_2_1