WTF

из Control.Applicative (с) Conor McBride and Ross Paterson 2005

newtype Const a b = Const { getConst :: a }

instance Functor (Const m) where
    fmap _ (Const v) = Const v

instance Monoid m => Applicative (Const m) where
    pure _ = Const mempty
    Const f <*> Const v = Const (f `mappend` v)

из Data.Functor.Constant (c) Ross Paterson 2010
(transformers package)

import Control.Applicative -- Sic!

...

newtype Constant a b = Constant { getConstant :: a }

instance Functor (Constant a) where
    fmap f (Constant x) = Constant x

...

instance (Monoid a) => Applicative (Constant a) where
    pure _ = Constant mempty
    Constant x <*> Constant y = Constant (x `mappend` y)

UPD. Росс ответил в кафе, что it was an accident