Enumerating all 3-tuples

[bartc]

[repost as original seems to have gone to email; my newsreader has 
somehow acquired a 'Reply' button where 'Followup' normally goes.]

On 10/03/2018 14:23, Ben Bacarisse wrote:
> Ben Bacarisse <ben.usenet at bsb.me.uk> writes:

> Off topic: I knocked up this Haskell version as a proof-of-concept:
> 
>    import Data.List
> 
>    pn n l = pn' n (map (:[]) l)
>             where pn' n lists | n == 1 = lists
>                               | otherwise = diag (pn' (n-1) lists) lists
>                   diag l1 l2 = zipWith (++) (concat (inits l1))
>                                             (concat (map reverse (inits l2)))
> 
> Notes:
> 
> map (:[]) l turns [1, 2, 3, ...] into [[1], [2], [3], ...]
> 
> inits gives the list of initial segments of l.  I.e. (inits "abc") is
> ["", "a", "ab", "abc"].
> 
> concat joins a list of lists into one list.
> 
> zipWith (++) l1 l2 makes a list by pair-wise appending the elements of
> l1 and l2.
> 

What's the output? (And what's the input; how do you invoke pn, if 
that's how it's done?)


-- 
Bartc