[Haskell-cafe] Maybe, maybe not.

[Tony Morris]

Ivan Miljenovic wrote:
> 2010/1/27 Tony Morris <tonymorris at gmail.com>:
>   
>> It might be more obvious by giving:
>>
>> fromMaybe :: a -> (a -> x, x) -> x
>>     
>
> I actually found this more confusing, and am not sure of its validity:
> should that be "Maybe a" there at the beginning?
>
>   

Sorry a mistake. Correction: fromMaybe :: a -> ((a -> x, x) -> x) -> x

{-# LANGUAGE RankNTypes #-}

data Maybe' a = M (forall x. (a -> x, x) -> x)

to :: Maybe' t -> Maybe t
to (M f) = f (Just, Nothing)

from :: Maybe a -> Maybe' a
from (Just a) = M (flip fst a)
from Nothing  = M snd


-- 
Tony Morris
http://tmorris.net/