[Haskell-cafe] Re: Referential Transparency and Monads
apfelmus at quantentunnel.de
Fri Apr 10 02:34:05 EDT 2009
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
would be assigned a function
\world -> ((), world where 'c' has been printed)
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.
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