[Haskell-cafe] Two-iteration optimisation

Andrew Coppin andrewcoppin at btinternet.com
Wed May 14 14:38:09 EDT 2008

Don Stewart wrote:
> You'd want a general fusion framework for this, and other accumulators,
> that's let's you treat 'f' as a zip of some kind that pulls items from
> its two arguments. And then require only a single rewrite rule for all
> your loop forms.
> Stream fusion (see "Lists to Streams to Nothing at All") at least does this for zips,
>     zip (foldl .. )
>         (foldl .. )
> Will end up with a single loop, but for an arbitrary 'f' instead of zip,
> seems harder.

It seems the thing to do would be to expose a top-level function for 
explicitly specifying that this is what you want to do:

  foo :: (x' -> y' -> z') -> ([x] -> x') -> ([y] -> y') -> [x] -> [y] -> z'

You could then say

  mean xs = foo (/) sum length

which is pretty declarative, and gives the compiler an easy time with 

Of course, if nobody chooses to *use* this function... it won't help you 

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