Parallel list comprehensions

[Jan-Willem Maessen]

On Feb 4, 2006, at 1:31 PM, Jon Fairbairn wrote:
> ...
> There ought to be a list_product somewhere (I mean [1..]
> `list_product` [4..] ==
> [(1,4),(2,4),(1,5),(3,4),(2,5),(1,6),...]). Is there?

Not that I know of, but here's one which handles finite lists  
correctly; it'd be a nice addition to Data.List:

dzip :: [a] -> [b] -> [(a,b)]
dzip =  dzipWith (,)

dzipWith :: (a -> b -> c) -> [a] -> [b] -> [c]
dzipWith f [] ys = []
dzipWith f as [] = []
dzipWith f as (y:ys) = dzipK ys [y]
   where dzipK (b:bs) rbs =
             zipWith f as rbs ++ dzipK bs (b : rbs)
         dzipK []     rbs = dzipT as
           where dzipT ys@(_:yt) = zipWith f ys rbs ++ dzipT yt
                 dzipT []     = []

-Jan-Willem Maessen

>
>  Jón
>
> -- 
> Jón Fairbairn                              Jon.Fairbairn at  
> cl.cam.ac.uk
>
>
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