Parallel list comprehensions (was: lists:zip / zip_with / unzip etc)

[Heinz Eriksson]

Vladimir Sekissov wrote:
> 
> From: Heinz Eriksson <heinz.eriksson@REDACTED>
> Subject: lists:zip / zip_with / unzip etc
> Date: Thu, 04 Jul 2002 15:56:29 +0200
> Message-ID: <3D24540D.BD1BB816@REDACTED>
> 
> heinz.eriksson>
> heinz.eriksson> ziph([H1|L1],[H2|L2], Z) ->
> heinz.eriksson>     ziph(L1,L2,[{H1,H2}|Z]);
> heinz.eriksson> ziph([],L2, Z) ->
> heinz.eriksson>     Z;
> heinz.eriksson> ziph(L1,[], Z) ->
> heinz.eriksson>     Z.
> heinz.eriksson>
> heinz.eriksson> zip(A,B) ->
> heinz.eriksson>     lists:reverse(ziph(A,B,[])).
> 
> Slightly generalized version:
> 
> noname@REDACTED> listn:zipn([[1,2,3], [4,5,6], [7,8,9]]) ->
> [{1,4,7},{2,5,8},{3,6,9}]
> 
> Code:
> 
> zipn(Ls) ->
>     [list_to_tuple(L) || L <- listn(Ls)].
> 
> listn(Ls) ->
>   [lists:reverse(L)
>    || L <- foldn(fun (A, Acc) -> [A|Acc] end, [], Ls)].
> 
> foldn(_, _, []) ->
>   [];
> foldn(Fun, Acc0, Ls) ->
>   foldn(Fun, Acc0, Ls, []).
> 
> foldn(_, _, [[]|_], Ret) ->
>   lists:reverse(Ret);
> foldn(Fun, Acc0, Ls, Ret) ->
>   foldn(Fun, Acc0,
>         [tl(L) || L <- Ls],
>         [lists:foldl(Fun, Acc0, [hd(L) || L <- Ls])|Ret]
>        ).
> 
> Best Regards,
> Vladimir Sekissov

Thank you, but what would you think of 'parallel list 
comprehensions'? such as is evidently in (one) Haskell:
http://www.haskell.org/ghc/docs/latest/set/parallel-list-comprehensions.html

There would have to be either a convention to
'comprehend' to the shortest list length or
crash on different length lists. (Just as
with zip/zip_with/zipn).

/hz