[Haskell-cafe] On the purity of Haskell

[Gregg Reynolds]

On Dec 30, 2011, at 10:19 AM, Conal Elliott wrote:

> 
> 
> On Fri, Dec 30, 2011 at 12:49 AM, Heinrich Apfelmus <apfelmus at 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?


time t:  f 42   (computational process implementing func application begins…)
t+1:   <keystroke> = 1
t+2:  43   (… and ends)

time t+3:  f 42
t+4:  <keystroke> = 2
t+5:  44

Conclusion:  f 42 != f 42

(This seems so extraordinarily obvious that maybe Heinrich has something else in mind.)

-Gregg
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