Re: [Haskell-cafe] A function that makes list of Bool lists from list of Int lists

Am Samstag 03 April 2010 19:44:51 schrieb Alexandru Scvortov:

Look at it from the inside, out.

What does const do?

Const is a function that takes two parameters and always returns the first one. For instance, const True x is True, for all x.

What's \x -> map (const x) then? (or map.const in my case)

It's a function that takes something and maps all the elements in a list onto that something. i.e. you say 1 [2, 3, 4]; it replies [1, 1, 1].

What's cycle [True, False]?

It's simply the list [True, False, True, False, ...].

What's (map (\x -> map (const x)) (cycle [True, False]))?

It's a list of functions that map their lists to lists of True or lists of False (alternatively).

And what does the zipWith ($) do?

I dislike the ($) part here, I find it more obvious to write it listbool = zipWith (map . const) (cycle [True,False]) zipWith f (map g xs) ys can always be written as zipWith (f . g) xs ys and usually the latter is much clearer to me. When f is ($), the difference is enormous.

It applies the functions we've just built onto the lists you give it.

Cheers, Alex

On Saturday 03 April 2010 16:34:21 Maur Toter wrote:

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Thanks again!

The last part I cant understand: So I give it for example zipWith ($) (map (\x -> map (const x)) (cycle [True, False])) [[1,2],[3]]

Okay, because of ($) it takes the first element of the last part which is [1,2] and apply the function on it But how makes this: map (\x -> map (const x)) (cycle [True, False]) [True,True] from [1,2]? Then he takes [3] but how makes [False] from that?

It easily can be that my functional programming knowledge isn't enough to understand this and maybe I misunderstood it already. But I hope I'll be able to do so..

Thanks: Maur Toter

so

On Sat, Apr 3, 2010 at 4:31 PM, Edward Z. Yang wrote:

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Excerpts from Maur Toter's message of Sat Apr 03 10:29:34 -0400 2010:

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What does the ($) at zipWith?

($) is function application

Prelude> :t ($) ($) :: (a -> b) -> a -> b

Cheers, Edward