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)
It is kind of like polymorphic recursion, I suppose. I get the following exception
[1 of 1] Compiling Testing ( strange.hs, interpreted ) *** Exception: typecheck/TcEnv.lhs:(365,0)-(392,32): Non-exhaustive patterns in function find_thing
Is the example supposed to work? I'm trying to determine the source of a problem with type classes and GADTs and I figured this example using "normal" data types would be a good place to start in understanding what was going wrong.
I was using ghci version 6.5.20060503.