explicit signatures and default for integer literals

[Dinko Tenev]

*Main> :type fromList
fromList :: (Ord k) => [(k, a)] -> Map k a
*Main> :type new
new :: (New a b) => a -> b

The type of new probably accounts for the difference (even though it
still makes me wonder what the big deal is :)

Using functional dependencies seems to fix it, i.e.:

class New a b | a -> b where new :: a -> b

   or, perhaps more appropriately:

class New a b | b -> a where new :: a -> b


Cheers,

D. Tenev


On 5/27/05, Mirko Rahn <rahn at ira.uka.de> wrote:
> 
> 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,
> 
> --
> -- Mirko Rahn -- Tel +49-721 608 7504 --
> --- http://liinwww.ira.uka.de/~rahn/ ---
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