Monads.hs

{-# OPTIONS_GHC -Wno-missing-signatures #-}
{-# OPTIONS_GHC -Wno-type-defaults #-}
module Spec.Monads.Monads where

newtype Identity a = Identity { runIdentity :: a }

instance Functor Identity where
  fmap f (Identity x) = Identity (f x)

instance Applicative Identity where
  pure = Identity
  Identity f <*> Identity x = Identity (f x)

instance Monad Identity where
  return = pure
  Identity x >>= f = f x

newtype Identity' a = Identity' { runIdentity' :: a}

instance Functor Identity' where
  fmap f (Identity' x) = Identity' (f x)

instance Applicative Identity' where
  pure = Identity'
  Identity' f <*> Identity' x = Identity' (f x)

instance Monad Identity' where
  return = pure
  Identity' x >>= f = f x

foo = do
  _x <- Identity 3
  Identity 4

bar :: Identity' Integer -- oh no (the type signature breaks the evidence variables)
bar = do
  _x <- Identity' 3
  Identity' 4