Re: [Haskell-cafe] On the purity of Haskell

On Fri, Dec 30, 2011 at 12:49 AM, Heinrich Apfelmus < apfelmus@quantentunnel.de> wrote:

The function

f :: Int -> IO Int f x = getAnIntFromTheUser >>= \i -> return (i+x)

is pure according to the common definition of "pure" in the context of purely functional programming. That's because

f 42 = f (43-1) = etc.

Put differently, the function always returns the same IO action, i.e. the same value (of type IO Int) when given the same parameter.

Two questions trouble me: How can we know whether this claim is true or not? What does the claim even mean, i.e., what does "the same IO action" mean, considering that we lack a denotational model of IO? - Conal