Time cost of Haskell `seq` operator

Question

This FAQ says that

The seq operator is

seq :: a -> b -> b

x seq y will evaluate x, enough to chec

Answer 1

You can always check with unsafePerformIO or trace…

import System.IO.Unsafe (unsafePerformIO)

main = print (x `seq` f (x + x))
  where
    f = (+4)
    x = unsafePerformIO $ print "Batman!" >> return 3