[Haskell-cafe] varsum, was zip3, zip4 ... -> zipn?
ctm at cs.nott.ac.uk
Sun Aug 12 06:31:42 EDT 2007
I'd have my membership of the one-leg club taken away if I didn't
write in and say that,...
On 12 Aug 2007, at 04:25, Stefan O'Rear wrote:
> On Sun, Aug 12, 2007 at 12:56:31PM +1000, Alexis Hazell wrote:
>> On Sunday 12 August 2007 05:24, Stefan O'Rear wrote:
>>> Currying makes it MUCH harder to implement varargs functions.
...while I wouldn't disagree,...
>> That's interesting - why is that the case?
> varsum 2 3 -- varsum receives 2, and returns a function, which when
> -- passed 3, returns 5
> varsum 2 3 4 -- varsum receives 2, and returns a function, which when
> -- passed 3, returns a function that when passed 4
> -- 9.
...this is one of the more elementary exercises in overloading...
> Because of this, the number of arguments must somehow be passed
> but then the type of the whole function (usually) must
> depend on the control parameter, requiring dependent types.
...of dependent walk you can mimic by hopping in Haskell.
> module VarSum where
> class VarSum t where
> varacc :: Int -> t
> varsum :: VarSum t => t
> varsum = varacc 0
> type Z = Int
> type S = (->) Int
> instance VarSum Z where
> varacc a = a
> instance VarSum t => VarSum (S t) where
> varacc a b = varacc (a + b)
Of course, you have to say stuff like
varsum (2 :: Int) (3 :: Int) :: Int
to determine the type at which the overloading happens. Or perhaps
(varsum :: S (S Z)) 2 3
But there am I, proving your point. In Haskell, this sort of thing
is a stunt. I'd much rather it was boring.
All the best
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