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