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{-# OPTIONS -XRecursiveDo #-}
-- test of user defined instance of MonadFix
module Main (main) where
import Control.Monad
import Control.Monad.Fix
data X a = X a deriving Show
instance Functor X where
fmap f (X a) = X (f a)
instance Applicative X where
pure = X
(<*>) = ap
instance Monad X where
(X a) >>= f = f a
instance MonadFix X where
mfix f = fix (f . unX)
where unX ~(X x) = x
z :: X [Int]
z = mdo x <- return (1:x)
return (take 4 x)
main = print z
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