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    <p>I think of it like this: Programs have to do something with the outside world to be useful. What's happening (or should be happening) when you write code (in any language) is that you strive to write as much pure, side-effect-free code as possible and corral the IO into specific places.</p> <p>What we have in Haskell is that you're pushed more in this direction of writing to tightly control effects. In the core and in many libraries there is an enormous amount of pure code as well. Haskell is really all about this. Monads in Haskell are useful for a lot of things. And one thing they've been used for is containment around code that deals with impurity.</p> <p>This way of designing together with a language that greatly facilitates it has an overall effect of helping us to produce more reliable work, requiring less unit testing to be clear on how it behaves, and allowing more re-use through composition.</p> <p>If I understand what you're saying correctly, I don't see this as a something fake or only in our minds, like a "get out of jail free card." The benefits here are very real.</p>
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    1. POIs Haskell truly pure (is any language that deals with input and output outside the system)?
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    1. COWhen I say "get out of jail free card", I refer to the fact that nothing can be truly pure when your dealing with physical machines that operate in an empirical manner based on logic gates. I think that saying something is pure is a misnomer, as the same technique can be used in any language that supports functions and function passing as first class features. Don't get me wrong I love Haskell but I think that the statement "pure" is just misleading.
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    2. COPart of the problem here is that you're treating Haskell as a way of telling physical machines what to do. It isn't, although it of course can do that. Haskell expressions have a denotational semantics -- a meaning that exists independently of whether they're executed on a machine, or by hand. Because the definition of Haskell isn't tied to machine-concepts like memory addresses, this is straightforward to understand, while a denotational semantics for, say, C, is a tricky business.
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    3. CO@sclv, exactly, I think you have hit the mark spot on there, in the minds eye and in a mind experiment where we deal with the perfect nature of maths and logic then yes Haskell is very much a great meta language. But and here is my problem, Monads are the interface between the real world and the perfect world of Haskell. The Monad is defined in the language of Haskell and errors can occur at this point, the same as any other computer based programming language, where most errors do and will occur because of the impure nature of reality.
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