[Haskell-beginners] question about comprehension or array creation

[Daniel Fischer]

On Saturday 15 January 2011 15:19:05, cchang wrote:
> Hi,
>
> I tried to create an array like the following.
>
> "array (1,3) [(1,1), (2,2), (3,3)]"
>
> through code in .hs
> e = [1,2,3]
> array (1,3) [(i,v) | i<-[1..3], v<-e]
>
> but I got
> "array (1,3) [(1,3), (2,3), (3,3)]"
>
> why v is always 3 in this case? Can anyone shed some light on this?
>

Because [(x,y) | x <- list1, y <- list2] gives you the cartesian product of 
the two lists [each element of the first paired with each of the second], 
thus

[(i,v) | i <- [1 .. 3], v <- e]
  = [(1,1),(1,2),(1,3),(2,1),(2,2),(2,3),(3,1),(3,2),(3,3)]

and (as a known deviation from the Haskell Report), GHC overwrites array 
elements with multiple definitions on creation (according to the report, it 
should throw an error, but checking for duplicates would be too 
inefficient). Thus, with the list above, in each slot the three values 1, 2 
and 3 are written (in that order), the last is the one you see afterwards.

What you want is one of

array (1,3) $ zip [1 .. 3] e

{-# LANGUAGE ParallelListComp #-}
-- Note the two `|'
array (1,3) [(i,v) | i <- [1 .. 3] | v <- e]

or equivalent.