[Haskell-beginners] Overlapping instances

[Daniel Fischer]

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.