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

[Dan Weston]

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.

> On Dec 7, 2007 6:21 PM, Dan Weston <westondan at imageworks.com> wrote:
>> This is great! Two questions:
>>
>> 1) I want to make sure the function arity matches the list length (as a
>> runtime check). I think I can do this with an arity function using
>> Data.Typeable. I came up with:
>>
>> arity f = a (typeOf f) where
>>    a tr | typeRepTyCon tr /= mkTyCon "->" = 0
>>         | otherwise = 1 + (a . fromJust . funResultTy tr . head
>>                              . typeRepArgs $ tr)
>>
>> This looks awful. Is there a better way to get the function arity?
>>
>> 2) Once I have say arity (+) == 2 at runtime, how can I get it reified
>> into Succ (Succ Zero)) at compile time to be able to use it as the first
>> argument in your nary function? Can/should I use Template Haskell for this?
> 
> You can project the compile time numbers into runtime ones:
> 
>> class ProjectN n where
>>     projectN :: n -> Int
>>
>> instance ProjectN Zero where
>>     projectN _ = 0
>>
>> instance (ProjectN n) => ProjectN (Succ n) where
>>     projectN _ = 1 + projectN (undefined :: n)
> 
> And then make sure the length matches the projected number of
> arguments.  Other disagreements will be resolved at compile time.
> 
> Luke
> 
>> Dan
>>
>> Victor Nazarov wrote:
>>>> {-# OPTIONS -fglasgow-exts #-}
>>>> {-# OPTIONS -fallow-undecidable-instances #-}
>>> data Zero
>>> data Succ a
>>>
>>> class Nary n x y | n x -> y where
>>>   nary :: n -> x -> [String] -> y
>>>
>>> instance Nary Zero x x where
>>>   nary _ x [] = x
>>>
>>> instance (Nary n y z, Read x) => Nary (Succ n) (x->y) z where
>>>   nary _ f (x:xs) = nary (undefined::n) (f $ read x) xs
>>>
>>
>>
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> 
>