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"and then the runtime takes it from there" is of course the right answer, but no one upon first encountering Haskell understands the distinction between "the la

"and then the runtime takes it from there" is of course the right answer, but no one upon first encountering Haskell understands the distinction between "the language" and "the runtime", so all the insistence that "the language" is "purely functional" is utterly misleading. Furthermore, the claim that "getLine is really the same value all the time" is false. If that were the case, then any operation on getLine would return the same value, which it does not. Edit: And in any case, the question is not whether "getLine" is a function, but whether "notpurelyFunctional" is. It has the signature of a function, but its return value certainly doesn't depend only on its arguments, or we would be able to say if value1 == value2 was true or not, and we can't. Basically, any claim with the word "really" in it evaluates to false (in philosophy claims about what is "really" or "really really" true are sometimes enhanced by what is known as "a certain percussive emphasis.") I think monads are an incredibly clever way of working side-effects into a language while maintaining an interesting level of functional purity. But the way the concepts are presented are just not conducive to conveying them to newbs, who will rightly call bullshit based on what words like "the language" are normally construed to mean.

cousin_it 12y ago
> but its return value certainly doesn't depend only on its arguments Note that value1 and value2 are not, and cannot possibly be, return values of notPurelyFunctional. The type of notPurelyFunctional says its return value has type IO Double, but value1 and value2 have type Double. IO Double is not Double, doesn't contain a Double, and cannot be converted to Double by a function call. It sounds like you still think that the syntax "do x <- foo" means something like "let x be the result of some function call involving foo". Actually it can't be anything like that, it translates to something more compicated. > If that were the case, then any operation on getLine would return the same value Yes, and moreover, any operation on notPurelyFunctional 5.0 always returns the same value. You have my honest word that I have some experience with Haskell and believe these things to be 100% true, with no wishy-washy philosophical caveats. I know exactly how to write an interpreter in which the machine representation of something like "notPurelyFunctional 5.0" will be immutable bit-for-bit, without using any dirty tricks like pointers to mutable memory. And I know that it's immutable in actual Haskell implementations as well.

cousin_it 12y ago

> but its return value certainly doesn't depend only on its arguments Note that value1 and value2 are not, and cannot possibly be, return values of notPurelyFunctional. The type of notPurelyFunctional says its return value has type IO Double, but value1 and value2 have type Double. IO Double is not Double, doesn't contain a Double, and cannot be converted to Double by a function call. It sounds like you still think that the syntax "do x <- foo" means something like "let x be the result of some function call involving foo". Actually it can't be anything like that, it translates to something more compicated. > If that were the case, then any operation on getLine would return the same value Yes, and moreover, any operation on notPurelyFunctional 5.0 always returns the same value. You have my honest word that I have some experience with Haskell and believe these things to be 100% true, with no wishy-washy philosophical caveats. I know exactly how to write an interpreter in which the machine representation of something like "notPurelyFunctional 5.0" will be immutable bit-for-bit, without using any dirty tricks like pointers to mutable memory. And I know that it's immutable in actual Haskell implementations as well.