2009/4/10 Heinrich Apfelmus
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 ? Thu