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

On Dec 7, 2007 7:57 PM, Luke Palmer wrote:

On Dec 7, 2007 7:41 PM, Dan Weston wrote:

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Luke Palmer wrote:

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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?

Oh, you're saying at run-time, given an object. Still no, by some definition. compose f g = f . g compose' f g x = f (g x) Are you saying that these two exactly equivalent functions should have different arity? If not, then is the arity 2 or 3? Luke