Laziness

[Jon Fairbairn]

On 2003-08-02 at 14:36PDT "Dominic Steinitz" wrote:
> Could someone explain to me why this doesn't work
> 
> test l =
>    hs
>       where
>          hs = map (\x -> [x]) [0..abs(l `div` hLen)]
>          hLen   = length $ head hs
> 
> whereas this does
> 
> test l =
>    hs
>       where
>          hs = map (\x -> [x]) (0:[1..abs(l `div` hLen)])
>          hLen   = length $ head hs
> 
> I would have thought laziness would allow the compiler to
> know that hs would contain at least one element and
> therefore calculate hLen.

Laziness isn't enough to tell it that. It would also have to
know that abs never returns an answer less than zero 
([0 .. -1] == []).

All the compiler knows is that abs returns an integer.

  Jón

PS I don't know the general policy, but I for one dislike
getting emails in HTML unless it's /absolutely/ necessary for
the content. Multipart/alternative doesn't help much either.


-- 
Jón Fairbairn                                 Jon.Fairbairn@cl.cam.ac.uk