[Haskell-beginners] Re: Is this overkill?

Heinrich Apfelmus apfelmus at quantentunnel.de
Fri Apr 3 22:30:12 EDT 2009

Zachary Turner wrote:
> I thought I would try to see if it were possible to write, in point-free
> form using no lambda functions and no intermediate functions, a function
> that takes 2 lists of Booleans, computes the pairwise logical AND of the two
> lists, and returns a list containing the 0 based indices of the elements
> where the logical and of the two was true.  I know that at some point it
> becomes overkill and for the sake of readability one should know when to
> draw the line.  So I want to see if someone with more experience than me can
> comment on whether or not this is over the line :P
> trueIndices = curry $ map fst . filter snd . zip [0..] . map (uncurry (&&))
> .. (uncurry zip)
> So do all the uncurries and curries make it too hard to understand or is it
> pretty easy to read this?  For me it takes me a while to figure out by
> looking at it because it's hard to trace all the currying and uncurrying.
> And is there a more elegant solution?

Looks very readable to me, though I'd write it as

  trueIndices = (map fst . filter snd . zip [0..] .) . zipWith (&&)

or even simply as

  trueIndices xs ys =
      map fst . filter snd . zip [0..] $ zipWith (&&) xs ys

because composing functions with more than one argument tends to be a
bit messy.

With Conal's semantic editor combinators


it would be written as

  trueIndices =
    (result . result) (map fst . filter snd . zip [0..]) (zipWith (&&))



More information about the Beginners mailing list