[Haskell-cafe] Re: explicit signatures and default for integer literals

[Daniel Fischer]

Am Freitag, 27. Mai 2005 15:00 schrieb Mirko Rahn:
> Hi all,
>
> an explicit given signature causes ghc to choose the right types for
> integer literals as in
>
> {-# OPTIONS -fglasgow-exts #-}
>
> import Data.Map
>
> f :: Ord a => [a] -> Map a Int
> f xs = fromList $ zip xs [0..]
>
> Here the Literal 0 is threated as (0::Int).
>
> But the setting
>
> {-# OPTIONS -fglasgow-exts #-}
>
> import Data.Map
>
> class New a b where new :: a -> b
>
> instance Ord a => New [(a,b)] (Map a b) where new = fromList
>
> g :: Ord a => [a] -> Map a Int
> g xs = new $ zip xs [0..]
>
> causes the error message
>
>     Could not deduce (New [(a, b)] (Map a Int)) from the context (Ord a)
>        arising from use of `new' at Why.hs:10:7-9
>
> ghc seems to be unable to threat the Literal 0 as (0::Int) this time but
> I do not understand why :-(
>
> Can anyone explain it?
>
> Thanks,

The problem is that ghc can't know that 0 ought to be Int, as it is, nothing 
prevents you declaring an 

instance Ord a  =>  New [(a, Double)] (Map a Int) where new = ?

and then it's clear that ghc wouldn't know which instance to choose.

To solve the problem, I have three suggestions
- write [0 :: Int .. ], so ghc knows that 
  New [(a,Int)] (Map a Int) is the required instance

- add a fundep:
  class New a b | [a -> b,] b -> a where ...,
  and ghc can again deduce the instance

- add an expression type signature in 'g'
  g xs = (new :: Ord a => [(a,Int)] -> Map a Int) $ zip xs [0 .. ]

any of these will do, depending on what you want,I recommend one of the first 
two.

Cheers,
Daniel