On Dienstag, 25. Dezember 2012, 16:52:00, Nathan Hüsken wrote:

Hi,

I have a test file:

module Main where

class A a where doSomething :: a -> IO () class B b where doMore :: b -> IO ()

instance B b => A b where doSomething = doMore

instance A String where doSomething = putStrLn

main = doSomething "Hello, World!

So there is a class A, and a class B. If something is part of B it is automatically part of A (so A is kind of a superclass to B). But String is just part of A. I try to compile it with:

ghc Test.hs -XFlexibleInstances -XUndecidableInstances

I get:

Test.hs:14:8: Overlapping instances for A [Char] arising from a use of `doSomething' Matching instances: instance B b => A b -- Defined at Test.hs:8:10 instance A String -- Defined at Test.hs:11:10 In the expression: doSomething "Hello, World!" In an equation for `main': main = doSomething "Hello, World!"

Why are the instances overlapping? String is not part of B???

For instance resolution, only the instance head is taken into account, that does not include the constraints. So the instance head of instance B b => A b where is only the `b' type variable, and that matches every type, in particular String, so the instances overlap (with an instance of the form instance context => Class a where any further instance declaration overlaps, since that instance effectively says every type is an instance of Class, but if `context' isn't satisfied, that is a static error).

How can I do this?

Allow overlapping instances if you must. But maybe you can restructure your code to not use overlapping instances.