[Haskell-beginners] Newbie performance question

[Daniel Fischer]

On Saturday 16 October 2010 08:18:39, Jordan Ellis wrote:
>  I have a quick follow-up question: With optimization enabled, will a
> value be shared between recursive function calls? For example, in the
> following:
> myRecursiveFunction 0 m = []
> myRecursiveFunction n m = m : myRecursiveFunction (n - 1) m
> ...will 'm' be calculated 'n' times, or just once? Thanks.

Here, you don't have two equal expressions, the m refers to the same entity 
in both appearances on the RHS. It's not guaranteed by the language 
definition, but every decent implementation will evaluate m only once (all 
occurrences are pointers to the same heap object, so you get n pointers, 
but only one pointed-to value).
With or without optimisation.

That's of course bad if m is large and you need a large part of it once at 
the beginning but only small parts afterwards, e.g.

mapM_ (print . length) $ zipWith take (iterate (`quot` 2) 20000000) $
                        myRec n [1 .. k]

To prevent that, if you don't want it, you have to jump through a couple of 
hoops.