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?
"seq" forces its argument to "Weak Head Normal Form". In plain English, that means that it insists that the topmost constructor is specified (here whether x is [] or (:)), but nothing more. If you want to force the entire data structure, you have to walk over it, forcing every part of it. Often people use a DeepSeq class to achieve this. But if you are just learning Haskell, you almost certainly don't need to do this. Just make use of the laziness, and learn to love it! Come back to "seq" when you are writing a large application, and profiling tells you you have a space leak. --KW 8-)