[Haskell-cafe] Re: Referential Transparency and Monads

[Brandon S. Allbery KF8NH]

On 2009 Apr 10, at 0:33, Heinrich Apfelmus wrote:
> Luke Palmer wrote:
>> Miguel Mitrofanov wrote:
>>
>>> I'm not sure what you mean by that, but semantically IO is  
>>> definitely
>>>> *not* a state monad.  Under any circumstances or any set of  
>>>> assumptions.
>>>>
>>> Ehm? Why not?
>>
>> Mainly forkIO.  There may be other reasons.
>  loop' :: IO ()
>  loop' = putStr "o" >> loop'
>
> are indistinguishable in the
>
>  IO a  ~  World -> (a, World)


I still don't understand this; we are passing a World and getting a  
World back, *conceptually* the returned World is modified by putStr.   
It's not in reality, but we get the same effects if we write to a  
buffer and observe that buffer with a debugger --- state threading  
constrains the program to the rules that must be followed for ordered  
I/O, which is what matters.

-- 
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery at kf8nh.com
system administrator [openafs,heimdal,too many hats] allbery at ece.cmu.edu
electrical and computer engineering, carnegie mellon university    KF8NH


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