Re: [Haskell-cafe] Lazy Evaluation in Monads

31 May 2011

      Scott Lawrence  писал(а) в своём письме Tue, 31 May 2011  
23:29:49 +0300:
...
On 05/31/2011 04:20 PM, Artyom Kazak wrote:
...
Suppose iRecurse looks like this:
  iRecurse = do
    x <- launchMissiles
    r <- iRecurse
    return 1
As x is never needed, launchMissiles will never execute. It obviously is
not what is needed.
Prelude> let launchMissiles = putStrLn "UH OH" >> return 1
Prelude> let iRecurse = launchMissiles >> return 1
Prelude> iRecurse
UH OH
1
Prelude>
Looks like launchMissiles /does/ execute, even though x is (obviously)
never needed.
Oh, sorry. I was unclear. I have meant "assuming IO is lazy", as Yves  
wrote.

And saying "some hacks" I meant unsafeInterleaveIO, which lies beneath the  
laziness of, for example, getContents.