Re: [Haskell-cafe] Re: Can we come out of a monad?

On 11 Aug 2010, at 12:39, Ertugrul Soeylemez wrote:

Martijn van Steenbergen wrote:

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On 8/2/10 7:09, Ertugrul Soeylemez wrote:

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Given the definition of a Haskell function, Haskell is a pure language. The notion of a function in other languages is not:

int randomNumber();

The result of this function is an integer. You can't replace the function call by its result without changing the meaning of the program.

I'm not sure this is fair. It's perfectly okay to replace a call "randomNumber()" by that method's *body* (1), which is what you argue is okay in Haskell.

This is not the same. In Haskell you can replace the function call by its /result/, not its body. You can always do that. But the result of an IO-based random number generator is an IO computation, not a value. It's not source code either, and it's not a function body. It's a computation, something abstract without a particular representation.

It's still rather papering over the cracks to call this pure though. The IO based computation itself still has a result that you *can't* replace the IO based computation with. The fact that it's evaluated by the runtime and not strictly in haskell may give us a warm fuzzy feeling inside, but it still means we have to watch out for a lot of things we don't normally have to in a "very pure"[1] computation. Bob [1] Bob's arbitrary definition 1 – very pure computations are ones which can be replaced with their result without changing the behavior of the program *even* if said result is computed in the runtime and not by the Haskel program.