[Haskell] Ambiguity problem

[Mads Lindstrøm]

Hi Wolfgang

Thank you very much for the reply. I added an extra parameter to the
function, so the code now looks like:

  foo :: (Data a) => a -> Constr -> Int
  foo idType = (numChildren idType) . fromConstr

  numChildren :: (Data a) => a -> a -> Int
  numChildren _ x = sum $ gmapQ (\_ -> 1) x

and it works. However, it does seem ugly to have an extra argument, just
to please the type checker.

This extra argument can fortunately be limited to a few functions, in
the program I am working on. 

/Mads Lindstrøm

Wolfgang Jeltsch:
> Am Freitag, 22. Juli 2005 14:58 schrieb Mads Lindstrøm:
> > Hi
> >
> > I am struggling with an ambiguity problem. I have the following code:
> > > foo :: Constr -> Int
> > > foo = numChildren . fromConstr
> > >
> > > numChildren :: (Data a) => a -> Int
> > > numChildren x = sum $ gmapQ (\_ -> 1) x
> >
> > which I thought would work, but the GHC-6.4 says:
> > > Ambiguous type variable `a' in the constraint:
> > > `Data a' arising from use of `numChildren' at AutoForm.hs:87:6-16
> > > Probable fix: add a type signature that fixes these type variable(s)
> >
> > I cannot see why I need to add (Data a),
> 
> (Data a) is a context, not a type signature.
> 
> I think, the problem is that fromConstr converts to a value of an unknown type 
> which is converted by numChildren afterwards.  The compiler cannot know which 
> type it has to use for the intermediate value, i.e., which concrete 
> implementation of numChildren and fromConstr it has to use.
> 
> > [...]
> 
> Best wishes,
> Wolfgang
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