[Haskell-beginners] Weird (++) behavior when adding 2 vectors

[Lorenzo Bolla]

Hi,

On Tue, Oct 18, 2011 at 1:19 PM, Alexander Raasch <info at alexraasch.de>wrote:

> Hi,
>
> so I wrote this function to add two vectors represented as lists:
>
> add a b = add' a b [] where
>    add' [] [] s = s
>    add' (a:as) (b:bs) s ++ [a+b]
>
>
I think something mangled your function, as this is not valid Haskell code.

Anyway, I tried to rewrite your function.
The first version works as expected; the second gives reversed output.
Note that there is no need for the accumulator "s".

add a b = add' a b
        where add' [] [] = []
              add' (a:as) (b:bs) = [a+b] ++ (add' as bs)

add a b = add' a b
        where add' [] [] = []
              add' (a:as) (b:bs) = (add' as bs) ++ [a+b] -- reversed output

Obviously, the same function can be written as:
zipWith (+) [1,2,3] [1,2,3]

hth,
L.
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