[Haskell-cafe] Re: Referential Transparency and Monads
noteed at gmail.com
Fri Apr 10 02:42:34 EDT 2009
2009/4/10 Heinrich Apfelmus <apfelmus at quantentunnel.de>:
> Brandon S. Allbery wrote:
>> Heinrich Apfelmus wrote:
>>> 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.
> Basically, the problem is that neither computation returns the final
> World because neither one terminates.
> More precisely, the goal of the
> IO a ~ World -> (a, World)
> semantics is to assign each expression of type IO a a pure function
> World -> (a, World) . For instance, the expression
> putChar 'c'
> would be assigned a function
> \world -> ((), world where 'c' has been printed)
> or similar.
> Now, the problem is that both loop and loop' are being assigned the
> same semantic function
> loop ~ \world -> _|_
> loop' ~ \world -> _|_
> We can't distinguish between a function that mutilates the world and
> then doesn't terminate and a function that is harmless but doesn't
> terminate either. After all, both return the same result (namely _|_)
> for the same input worlds.
I'm not sure I follow.
> ones = 1:ones
is similar to loop or loop' but I can 'take 5' from it.
Even if loop or loop' do not terminate, some value is produced, isn't it ?
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