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From: Paul Rubin <no.email@nospam.invalid>
Newsgroups: comp.lang.ada
Subject: Re: If not Ada, what else...
Date: Wed, 29 Jul 2015 22:17:37 -0700
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Date: 2015-07-29T22:17:37-07:00
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Georg Bauhaus <bauhaus@futureapps.invalid> writes:
> picksum :: [Int] -> Int -> Int
> picksum [] m = 0
> picksum xs m
>            | m <= 0 = 0
>            | m  > 0 = mth xs
>   where mth l@(y:ys) = y + mth (drop m l)
>         mth []       = 0
>
> Then, try this with the justly famed easy list notation, passing
> [1 .. 12345678], or larger. (I have GHC 7.8.3, maybe that's too old?)
> Of course, the language war soldier of Haskell will argue that the
> enemy writer of the above algorithm was stupid,

Nah, there's a trick that's not obvious to beginners but it's one of the
first things you learn in functional programming, and it's not a big
deal once you know it.  Basically the recursive call to "mth" has to
return a value that gets added to y, which means the stack frame can't
get released til the addition has happened.  You instead want to write
it tail recursively with an accumulator, something like:

> ...        | m  > 0 = mth xs 0
>   where mth l@(y:ys) !a = mth (drop m l) (y+a)
>         mth [] a       = a

Note the !a in the pattern-- that's a strictness marker to stop
unevaluated sums from piling up on the stack.  Again it takes some
getting used to but you learn to spot these things.

I might have written the function like this:

p2 :: [Int] -> Int -> Int
p2 xs m = sum . map head . takeWhile (not . null) . iterate (drop m) $ xs

but the sum might have to be replaced with "foldl1' (+)" for strictness.