purescript-0.7.2.0: examples/passing/TypeSynonyms.purs

module Main where

import Prelude

type Lens a b =
  { get :: a -> b
  , set :: a -> b -> a
  }

composeLenses :: forall a b c. Lens a b -> Lens b c -> Lens a c
composeLenses = \l1 -> \l2 ->
  { get: \a -> l2.get (l1.get a)
  , set: \a c -> l1.set a (l2.set (l1.get a) c)
  }

type Pair a b = { fst :: a, snd :: b }

fst :: forall a b. Lens (Pair a b) a
fst =
  { get: \p -> p.fst
  , set: \p a -> { fst: a, snd: p.snd }
  }

test1 :: forall a b c. Lens (Pair (Pair a b) c) a
test1 = composeLenses fst fst

main = Control.Monad.Eff.Console.log "Done"