[Haskell-cafe] Strict evaluation not working?

Jon Fairbairn Jon.Fairbairn at cl.cam.ac.uk
Tue Oct 12 12:20:09 EDT 2004

On 2004-10-12 at 18:07+0200 Christian Hofer wrote:
> Hi,
> having found a bit of time to play with Haskell, I am trying to figure 
> out how to enforce strict evaluation.
> I wrote the following program:
> main =
> 	let x = zipWith (+) [5..] [6..]
> 	in putStrLn $ show $ x `seq` head x
> I expected this program not to terminate - because of the seq-Operator, 
> but it happily returns 11 in ghc as well as in ghci. What do I make 
> wrong?

head is strict in its first argument, so x `seq` head x is
equivalent to head x.  seq only evaluates to (w?)hnf. To do
more you would need deepSeq.

Jón Fairbairn                                 Jon.Fairbairn at cl.cam.ac.uk

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