[Haskell-beginners] multi parameter classes and instance declarations

[Markus Barenhoff]

On Mon 16.02 15:28, Markus Barenhoff wrote:
> Good Afternoon,
> 
> i'am trying to express something like this here:
> 
> > -- CI.hs http://pastebin.com/f5b17cf57
> > {-# OPTIONS -XMultiParamTypeClasses #-}
> 
> > module CI(D,f) where
> 
> > class B b
> 
> > class (B b) => A a b where
> >     f :: b
> 
> > data D = D
> 
> > instance B D
> 
> > instance A D D where
> >     f = D
> > -- CI.hs
> 
> > -- MultiClassTest.hs http://pastebin.com/f1c481e8d
> > import CI 
> 
> > a = f
> > -- MultiClassTest.hs

I forgot to mention that constrain a :: D does leed to the same
ghc messages.

> 
> ghc output:
> MultiClassTest.hs:4:4:
>     No instance for (CI.A a D)
>       arising from a use of `f' at MultiClassTest.hs:4:4
>     Possible fix: add an instance declaration for (CI.A a D)
>     In the expression: f
>     In the definition of `a': a = f
> Failed, modules loaded: CI.
> 

Markus

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