Re: [Haskell-cafe] Mission: To take args from a list... generally
Thanks for the link. So many variations on a theme: n-vars to a list : Oleg's solution for polyvariadic functions list + n-adic function -> value : This thread list of (n_i)-adic functions + argument -> list of (n_i-1)-adic functions : The link you gave. The first two had very similar answers, and I have a feeling the third could be answered with yet another application of the same idea. As I see it, the answer given in that link amounts to wrapping the function so that it will fit in a homogeneous list. The user still has to wrap their functions with multiple 'cook's. Joel On 10/5/06, [EMAIL PROTECTED] <[EMAIL PROTECTED]> wrote: This came up a while ago (but with a list of functions of different arities, all being fed one argument). I found Scott Turner's pure Haskell 98 solution very illuminating: http://www.haskell.org/pipermail/haskell-cafe/2000-November/001332.html Regards, Tom ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Mission: To take args from a list... generally
Joel Koerwer wrote: > Let's say I want to evaluate a function of type > (a->a->...->a->a), taking the arguments from a > list. If know the function ahead of time, I can > simply wrap it: > > foo a b c d = ... > wrapFoo (a:b:c:d:_) = foo a b c d > > But, as an exercise, I challenged myself to write > a function, multApply :: (a->a->...->a->a) -> [a] > -> a, that automatically does the wrapping for any > such function. This came up a while ago (but with a list of functions of different arities, all being fed one argument). I found Scott Turner's pure Haskell 98 solution very illuminating: http://www.haskell.org/pipermail/haskell-cafe/2000-November/001332.html Regards, Tom ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Mission: To take args from a list... generally
Thanks! Don pointed me to Oleg's page too. I *was* (90%) convinced that this is fundamentally different, but it sure looks similar. Kind of an inside-out polyvariadic function. Any given f only takes n args, but myApply doesn't know the value of n. In fact, a polyvariadic f would cause an infinite loop. At least a stack overflow. Always nice to know you're in good company though. Cheers, Joel On 10/4/06, David House <[EMAIL PROTECTED]> wrote: Nice work! I haven't tried it out, but you seem to have on the right trick here: typeclass hackery. An interesting related exercise to develop a similar function using Template Haskell [1]. Also of note might be Oleg's writings on true polyvariadic functions [2]. [1]: http://haskell.org/th [2]: http://okmij.org/ftp/Haskell/types.html#polyvar-fn ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Mission: To take args from a list... generally
On 04/10/06, Joel Koerwer <[EMAIL PROTECTED]> wrote: I'm posting my solution in hopes to learn from your comments. This solution uses fundeps, multi-parameter classes, and overlapping instances. Note that I don't actually understand these things! :) Nice work! I haven't tried it out, but you seem to have on the right trick here: typeclass hackery. An interesting related exercise to develop a similar function using Template Haskell [1]. Also of note might be Oleg's writings on true polyvariadic functions [2]. [1]: http://haskell.org/th [2]: http://okmij.org/ftp/Haskell/types.html#polyvar-fn -- -David House, [EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Mission: To take args from a list... generally
Haskellers,
Let's say I want to evaluate a function of type (a->a->...->a->a),
taking the arguments from a list. If know the function ahead of time,
I can simply wrap it:
foo a b c d = ...
wrapFoo (a:b:c:d:_) = foo a b c d
But, as an exercise, I challenged myself to write a function,
multApply :: (a->a->...->a->a) -> [a] -> a, that automatically does
the wrapping for any such function.
On #haskell Don Stewart suggested I look at printf, but I've yet to
put much thought into whether that method will work here.
I'm posting my solution in hopes to learn from your comments. This
solution uses fundeps, multi-parameter classes, and overlapping
instances. Note that I don't actually understand these things! :)
MultApply.hs --
{-# OPTIONS -fglasgow-exts -fallow-overlapping-instances #-}
module MultApply where
class MultApply func arg | func -> arg where
multApply :: func -> [arg] -> arg
instance MultApply (a->a) a where
multApply f (x:xs) = f x
multApply f _ = error "MultApply: one too few args"
instance MultApply cascade a => MultApply (a -> cascade) a where
multApply f (x:xs) = multApply (f x) xs
multApply f _ = error "MultApply: n too few args"
-- some random examples
oneArg = multApply sqrt [25..]
twoArg = multApply (+) [1..]
fiveArg = multApply (\a b c d e -> sqrt ((a+b)^2+(d-e)^2)-5*c) [13..]
---End File
Results in ghci:
*MultApply> oneArg
5.0
*MultApply> fiveArg
-47.981487827787404
To compose your own examples in ghci, you'll need
-fallow-overlapping-instances on the command line.
Cheers,
Joel
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