On Wed, Oct 7, 2015 at 3:35 AM, Herbert Valerio Riedel <hvr@gnu.org> wrote:

--8<---------------cut here---------------start------------->8---
import Control.Applicative as A (Applicative(..))

data Maybe' a = Nothing' | Just' a

instance Functor Maybe' where
fmap f (Just' v) = Just' (f v)
fmap _ Nothing' = Nothing'

instance A.Applicative Maybe' where
pure = Just'
f1 <*> f2 = f1 >>= \v1 -> f2 >>= (pure . v1)

instance Monad Maybe' where
Nothing' >>= _ = Nothing'
Just' x >>= f = f x

return = pure -- "deprecated" since GHC 7.10
--8<---------------cut here---------------end--------------->8---

Alternately,

import Control.Applicative

import Prelude

data Maybe' a = Nothing' | Just' a

instance Functor Maybe' where
fmap f (Just' v) = Just' (f v)
fmap _ Nothing' = Nothing'

instance Applicative Maybe' where

-- hvr

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