> The ghc compiler complains about 2 type errors in the following code:
>
> > data SearchResult a = Found a | Fail
> >
> > class (Eq key, Ord key) => Dictionary dict key dat where
> > delete :: key -> dict -> dict
> > search :: key -> dict -> (key,SearchResult dat,dict)
> > searchList :: [key] -> dict -> ([(key,SearchResult dat)],dict)
> >
> > searchList [] d = ([],d)
> > searchList (x:xs) d = let (sresults,d') = searchList xs d
> > (x',sresult,d'') = search x d'
> > new_sres = (x',sresult):sresults
> > in (new_sres,d'')
>
> the first error:
>
> Class type variable `dat' does not appear in method signature
> delete :: key -> dict -> dict
>
> Why does ghc expect that I use all of the type variables?
> Obviously I only need
> the key to remove an entry of a dictionary.
You're right. The restriction is excessive. Thanks for pointing
this out. Probably we should only require that at least one
of the class variables is constrained.
Actually I think you would be better off with a class like
this:
class (Eq key, Ord key) => Dictionary dict key where
delete :: key -> dict dat -> dict dat
search :: key -> dict dat -> (key, SearchResult dat, dict dat)
searchList :: [key] -> dict dat -> ([(key,SearchResult dat)],dict dat)
That is, you don't need the 'dat' type variable the class at all.
> Ambiguous type variable(s)
> `key', `dict'
> in the constraint `Dictionary dict key a10v'
> arising from use of `searchList' at Dtest2.hs:11
> In an equation for function `searchList':
> searchList (x : xs) d
> = let
> (sresults, d') = searchList xs d
> (x', sresult, d'') = search x d'
> new_sres = (x', (sresult)) : sresults
> in (new_sres, (d''))
> In the definition for method `searchList'
You don't say which version of the compiler you are using,
but I think this a palpable bug in 3.01 that is fixed in 3.02.
So you are right to be confused by it!
Simon
