[Haskell-cafe] Lazy Evaluation in Monads

[Artyom Kazak]

Scott Lawrence <bytbox at gmail.com> 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.