[Haskell-cafe] Re: distinguish functions from non-functions in a class/instances

[Luke Palmer]

On Dec 7, 2007 7:41 PM, Dan Weston <westondan at imageworks.com> wrote:
> Luke Palmer wrote:
> > You can project the compile time numbers into runtime ones:
>
> Yes, that works well if I know a priori what the arity of the function
> is. But I want to be able to have the compiler deduce the arity of the
> function (e.g. by applying undefined until it is no longer a function),
> precisely so I don't have to supply it myself.
>
> Function arity is (I think) something already known to GHC, so I don't
> know why we can't get at it too.

No, it is not.  Consider:

compose f g x = f (g x)

What is the arity of f?

Luke