Re: [Haskell-cafe] Referential Transparency and Monads

On Thu, 2009-04-09 at 21:57 -0400, Brandon S. Allbery KF8NH wrote:

On 2009 Apr 9, at 16:09, Luke Palmer wrote:

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On Thu, Apr 9, 2009 at 1:33 PM, 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.

I thought I had excluded that stuff to simplify the question; the fact that IO is Haskell's toxic waste dump is more or less irrelevant to the core concept.

Well, the `core concept' of IO includes the concept of a user who's watching and interacting with your program as it runs, no? Say I know that a file named `/my_file' exists and is readable and writeable and etc. Now consider the program do s <- readFile "/my_file" writeFile "/my_file" "Hello, world!\n" threadDelay 10000 -- If you don't like threadDelay, just substitute forcing -- an expensive thunk here writeFile "/my_file" s As a function from initial state to final state, this program is just the identity; but surely this program should be considered different than just threadDelay 10000 . To give a meaningful semantics to IO, you have to consider the intermediate state(s) the system goes through, where a state monad denotation for IO would discard them. jcc