Simon Peyton-Jones wrote:
Right. The crash was definitely a bug, but it seems to have been fixed.
The error message about ambiguity is just what you'd expect.
I must admit I can't understand where the ambiguity actually is. bar has been defined as a -> Int, so surely anything on the rhs of an equation for an instance of the bar method in Foo is therefore an Int also, so having to explicitly write 1::Int seems superfluous. bar([x]) = 1 -- why is ::Int needed when we know that bar:: a->Int ??? Also, the error message suggests that the ambiguity is caused by Foo t and Num t, but what's the problem - surely this just means t belongs to both Foo and Num? Thanks, Brian.
So it seems that this is all fine.
Simon
-----Original Message----- From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users- bounces@haskell.org] On Behalf Of Geoffrey Alan Washburn Sent: 04 June 2006 18:53 To: glasgow-haskell-users@haskell.org Cc: Stephanie C Weirich Subject: Re: Bug? Or at least a better error message?
I just upgraded to ghc 6.5.20060603 and now I get the following error
Prelude> :load strange.hs [1 of 1] Compiling Testing ( strange.hs, interpreted )
strange.hs:9:13: Ambiguous type variable `t' in the constraints: `Foo t' arising from use of `bar' at strange.hs:9:13-19 `Num t' arising from the literal `1' at strange.hs:9:18 Probable fix: add a type signature that fixes these type variable(s) Failed, modules loaded: none.
If I wrap "1" with ( :: Int) it seems to be accepted.
Geoffrey Alan Washburn wrote:
While trying out the following example, in an attempt to learn something about the fiddly case where a type class instance tries to use an instance that is more specific than itself
module Testing where
class Foo a where { bar :: a -> Int }
instance Foo Int where bar i = i
instance Foo a => Foo [a] where bar [] = bar [1] bar ([x]) = 1 bar (x:xs) = (bar x) + (bar xs)
-- Logic empowers us and Love gives us purpose. Yet still phantoms restless for eras long past, congealed in the present in unthought forms, strive mightily unseen to destroy us. http://www.metamilk.com