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

On 7 Dec 2007, at 12:39 PM, Dan Weston wrote:

Luke Palmer wrote:

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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.

What about a type that contains lexical type variables? For that matter, what about a type that ends in a type variable, e.g. head :: [a] -> a Is the arity of head (x:xn) = x Different from that of head' :: [a -> b] -> a -> b head' (x:xn) = x ? jcc