Re: [Haskell-cafe] Execution order in IO

15 Apr 2015

      On Wed, 15 Apr 2015 17:34:50 +0200, Marcin Mrotek  wrote:
...
Changing it to, let's say
main = do
 a <- getLine
 b <- getLine
 c <- foo a b
makes it obvious there's no way to evaluate c before a and b, whatever
monad that would be, as foo may c can change the shape of the monad
anyway it pleases.
import Debug.Trace

foo _ _ = return ()

main = do
  a <- trace "a" <$> getLine
  b <- trace "b" <$> getLine
  c <- trace "c" <$> foo a b
  print c

Running this program:

first input
second input
c
()

So as you can see, ‘a’ and ‘b’ never get evaluated.

Re: [Haskell-cafe] Execution order in IO

Niklas Haas