[Haskell-cafe] repeat confusion

[Lyndon Maydwell]

Think about it like this:

[image: image.png]

The current elements are zipped with the tails (which are created with
transpose). This has to bottom out with empty tails (repeat []).

On Sat, Apr 24, 2021 at 2:32 AM Galaxy Being <borgauf at gmail.com> wrote:

> I'm in Bird's *Thinking Functionally with Haskell *and he has this code
> to transpose a matrix based on a list of row lists
>
> transpose :: [[a]] -> [[a]]
> transpose [xs] = [[x] | x <- xs]
> transpose (xs:xss) = zipWith (:) xs (transpose xss)
>
> then he says transpose can be rewritten with this pattern
>
> transpose [] = ...
>
> what could be the rest of it? The answer he gives is
>
> transpose2 :: [[a]] -> [[a]]
> transpose2 [] = repeat []
> transpose2 (xs:xss) = zipWith (:) xs (transpose2 xss)
>
> where repeat [] gives an infinite list of repetitions. And, he says, note
> that
>
> transpose [xs] =  zipWith (;) xs (repeat []) = [[x] | x <- xs]
>
> I suppose I get this last equation, but I don't understand repeat in
> transpose2. Can someone explain this to me?
>
> LB
>
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