[Haskell-cafe] trace function

[Malcolm Wallace]

Alexander Vodomerov <alex at sectorb.msk.ru> wrote:

> main = do
>   putStrLn "xxx"
>   return (trace "yyy" ())
>   putStrLn "zzz"
> 
> only xxx and zzz is displayed. yyy is missing.

This is because you never demanded the value of (trace "yyy" ()), so it
was never computed.  The joys of laziness!  To force its evaluation try

  main = do
    putStrLn "xxx"
    () <- return (trace "yyy" ())
    putStrLn "zzz"

Regards,
    Malcolm