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 <[EMAIL PROTECTED]> 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
_______________________________________________
Haskell-Cafe mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/haskell-cafe
_______________________________________________
Haskell-Cafe mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/haskell-cafe
