PureScript Cheat Sheet

module Main whereimport Prelude-- Type signature + bindinggreet :: String -> Stringgreet name = "Hello, " <> name-- Recordstype User = { name :: String, age :: Int }alice :: Useralice = { name: "Alice", age: 30 }older :: User -> Userolder u = u { age = u.age + 1 }-- Algebraic data typesdata Status = Loading | Success String | Failure Stringdescribe :: Status -> Stringdescribe = case _ of  Loading -> "loading..."  Success msg -> "ok: " <> msg  Failure err -> "error: " <> err

class Describable a where  describe :: a -> Stringinstance describableInt :: Describable Int where  describe n = "Int: " <> show n-- Functor / Applicative / Monad for a custom typedata Box a = Box ainstance functorBox :: Functor Box where  map f (Box a) = Box (f a)instance applyBox :: Apply Box where  apply (Box f) (Box a) = Box (f a)instance applicativeBox :: Applicative Box where  pure = Box

import Effect (Effect)import Effect.Console (log)import Effect.Aff (Aff, launchAff_, delay)import Effect.Class (liftEffect)import Data.Time.Duration (Milliseconds(..))main :: Effect Unitmain = log "hello"asyncTask :: Aff UnitasyncTask = do  liftEffect (log "starting")  delay (Milliseconds 1000.0)  liftEffect (log "done after 1s")runIt :: Effect UnitrunIt = launchAff_ asyncTask

component :: forall q i o m. H.Component q i o mcomponent =  H.mkComponent    { initialState: \_ -> 0    , render    , eval: H.mkEval H.defaultEval { handleAction = handleAction }    }  where  render state =    HH.div_      [ HH.button [ HE.onClick \_ -> Decrement ] [ HH.text "-" ]      , HH.text (show state)      , HH.button [ HE.onClick \_ -> Increment ] [ HH.text "+" ]      ]  handleAction = case _ of    Increment -> H.modify_ (_ + 1)    Decrement -> H.modify_ (_ - 1)

-- "r" is a row-type variable: any additional fields are allowedgreet :: forall r. { name :: String | r } -> Stringgreet person = "Hello, " <> person.namegreet { name: "Alice", age: 30 }     -- fine, extra 'age' field ignoredgreet { name: "Bob" }                -- fine, no extra fields-- Record update/delete with row operations (via Record module)import Record (delete, insert)import Data.Symbol (SProxy(..))withoutAge :: forall r. { name :: String, age :: Int | r } -> { name :: String | r }withoutAge = delete (SProxy :: SProxy "age")

import Control.Monad.Reader (ReaderT, runReaderT, ask)import Control.Monad.Except (ExceptT, runExceptT, throwError)import Effect.Aff (Aff)import Effect.Class (liftEffect)type AppConfig = { apiUrl :: String }-- App = "has access to config, can fail with String, runs in Aff"type App a = ReaderT AppConfig (ExceptT String Aff) afetchUser :: Int -> App UserfetchUser userId = do  config <- ask  when (userId < 0) (throwError "invalid id")  -- ... use config.apiUrl to make the request  pure defaultUserrunApp :: AppConfig -> App a -> Aff (Either String a)runApp config app = runExceptT (runReaderT app config)

-- Phantom type parameter "validated" never appears in the runtime valuenewtype Email (validated :: Boolean) = Email Stringunvalidated :: String -> Email falseunvalidated s = Email svalidate :: Email false -> Maybe (Email true)validate (Email s) =  if contains (Pattern "@") s    then Just (Email s)    else Nothing-- Only an Email true can be sent -- the compiler rejects unvalidated onessendEmail :: Email true -> Effect UnitsendEmail (Email s) = log ("sending to " <> s)-- Kind signature restricting a type parameter to a specific setdata Direction = Up | Down | Left | Right

import Data.Foldable (foldMap, sum)import Data.Traversable (traverse)import Data.Maybe (Maybe)-- Monoid: combine values with an identity elementnewtype Sum = Sum Intinstance semigroupSum :: Semigroup Sum where  append (Sum a) (Sum b) = Sum (a + b)instance monoidSum :: Monoid Sum where  mempty = Sum 0total :: Array Int -> Inttotal xs = case foldMap Sum xs of Sum n -> n-- Traversable: run an effect over a structure, short-circuit on failurevalidateAll :: Array String -> Maybe (Array Email)validateAll names = traverse parseEmail names-- Foldable works uniformly over Array, Maybe, Map, custom trees, etc.