[Haskell-beginners] Seq question

[Tim Perry]

seq evaluates to Weak Head Normal Form (WHNF). WHNF is the first
contructor. So your use of seq only evaluates the first number and the
cons. I.E., it evaluates to:
s:(Thunk)

You are expecting seq to work like deepseq and evaluate the entire list.

Also, mod is slower than rem and the meaning doesn't matter in a sieve so
change mod to rem for faster code.

Tim



On Tue, Apr 17, 2012 at 1:56 PM, Lee Short <blackcat at pro-ns.net> wrote:

> I've got the following code, and I'm not sure why the seq seems to be
> doing nothing.  The code should never terminate if the seq were forcing the
> full evaluation of y.  Instead, it runs just fine (though stack overflows
> if I ask for too many elements of primes).  I didn't see anything in the
> tutorial section of haskell.org that seems to explain this -- pointers to
> a useful source would be welcome.
>
>
> sieve :: [Int] -> [Int]
> sieve xs =
>   let ys = filter (\z -> 0 /= z `mod` head xs) xs
>   in seq ys $ head xs:sieve ys
>
> primes = sieve [2..]
>
> thanks
> Lee
>
>
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