[Haskell-beginners] Inferring functions from parameter types

[Michael Snoyman]

On Tue, Dec 7, 2010 at 8:35 AM, Russ Abbott <russ.abbott at gmail.com> wrote:
> Is there any theoretical reason why GHC (or Haskell) doesn't permit
> functions to be inferred from their parameter types?
> For example, if I write
>
> null Data.Set.empty
>
> I get a diagnostic saying
>
>
> Ambiguous occurrence `null'
>     It could refer to either `List.null', imported from Data.List at ...
>                           or `Map.null', imported from Data.Map at ...
>                           or `IntSet.null', imported from Data.IntSet at ...
>
> Why doesn't GHC figure it out?
> More generally it would be nice to be able to define:
>
> f :: Type1 -> ResultType1
> f :: Type2 -> ResultType2
>
> and expect GHC to select the appropriate f based on the argument and implied
> result types. Even if the type can't always be determined, why not do it
> when possible and issue a complaint when it can't?
>
>
> -- Russ
>

I believe this is covered by the proposal for Type Directed Name
Resolution. You may be interested in a recent thread on the cafe[1]
discussing its merits and problems.

Michael

[1] http://www.mail-archive.com/haskell-cafe@haskell.org/msg84144.html