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

Luke Palmer wrote:

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

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

Luke Palmer wrote: 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.

No, at compile time. Type is static.

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?

Prelude> :t let compose f g = f . g in compose let compose f g = f . g in compose :: (b -> c) -> (a -> b) -> a -> c Prelude> :t let compose' f g x = f (g x) in compose' let compose' f g x = f (g x) in compose' :: (t -> t1) -> (t2 -> t) -> t2 -> t1 The arity is the number of top-level -> Both are arity 3.