I'm trying to grok Haskell's laziness. In this example, why does the seq not force youTyped to wait for s to be fully evaluated, before returning a result to putStrLn?

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import System.IO (hSetBuffering,stdin,BufferMode(..))

youTyped :: String -> String

youTyped s = s `seq` "you typed: " ++ s

main = do

hSetBuffering stdin NoBuffering

s <- getContents

let l = map youTyped $ lines $ takeWhile ( /= '\04' ) s

mapM_ putStrLn ("Start Typing (Ctrl-D to exit):":l)

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The output looks like:

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$ runhaskell seqLazy.hs

Start Typing (Ctrl-D to exit):

Hyou typed: Heelllloo wwoorrlldd

fyou typed: faaiill

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Changing seq, to deepseq does the trick though.

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import System.IO (hSetBuffering,stdin,BufferMode(..))

import Control.DeepSeq

youTyped :: String -> String

youTyped s = s `deepseq` "you typed: " ++ s

main = do

hSetBuffering stdin NoBuffering

s <- getContents

let l = map youTyped $ lines $ takeWhile ( /= '\04' ) s

mapM_ putStrLn ("Start Typing (Ctrl-D to exit):":l)

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Output:

$ runhaskell deepSeqLazy.hs

Start Typing (Ctrl-D to exit):

Hello world

you typed: Hello world

success

you typed: success

When does it make sense to use seq then?