[Haskell-cafe] Implicit concatenation in list comprehensions

[porges at porg.es]

2009/7/21 Roel van Dijk <vandijk.roel at gmail.com>:
>>> I am also wondering what the following would/should mean:
>>>
>>>   (1, , ( , 2), ) 'a' 'b' 'c'
>>
>> I would expect it to be a type error, since I think the following is the
>> only sane type the tuple can have (assuming numeric literals :: Int):
>>
>> (1, , ( , 2), ) :: a -> b -> (Int, a, c -> (c, Int), b)
>>
>> Ganesh
>
> Ah you're completely right! I would be applying a tuple to a Char
> which is nonsense.

You can kind of do it if you indicate which 'field' is to be applied:

  import Control.Applicative

  newtype Fst b a = Fst (a,b) deriving (Show)
  newtype Snd a b = Snd (a,b) deriving (Show)

  instance Functor (Fst b) where
  	fmap f (Fst (a,b)) = Fst (f a,b)

  instance Functor (Snd a) where
  	fmap f (Snd (a,b)) = Snd (a,f b)

  f <$$> x = ($x) <$> f

...

> Fst ((,)1,3) <$$> 2 -- no section, don't have the patch ;)
Fst ((1,2),3)
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