Re: [Haskell-cafe] Composition and type magic

On 24.02.2015 06:57, Sumit Sahrawat, Maths & Computing, IIT (BHU) wrote:

Thanks. You've given me a good exercise in equational reasoning.

I'm studying such things because I might start studying combinatory logic (Smullyan's mockingbird book) soon. It's a good exercise for the mind, and is very enjoyable with pencil & paper. Can you give me some more patterns like transducers (foldl foldl) and the dot-dot-dot ( boobies, in your words :) ) example?

Thanks once again.

On 24 February 2015 at 08:57, Michael Orlitzky wrote:

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On 02/23/2015 10:11 PM, Sumit Sahrawat, Maths & Computing, IIT (BHU) wrote:

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Hi everyone,

I defined a function for discrete convolution, i.e

conv xs = sum . zipWith (*) xs . reverse

And I was wondering, can it be done without explicitly passing in the xs (eta-reduction)?

Ehhhhhhhhhh this will work:

conv = (sum .) . (. reverse) . zipWith (*)

But it's so much easier to understand without all the boobies operators:

conv xs ys = sum $ zipWith (*) xs (reverse ys)

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You might be interested in the pointfree [1] tool, which can convert lambda expressions to pointfree versions automatically. Allthough I also find the more complex ones, as in this case, rather obscure. I like you initial version best: conv xs = sum . zipWith (*) xs . reverse [1] http://hackage.haskell.org/package/pointfree