On Dec 30, 2011, at 10:19 AM, Conal Elliott wrote:
>
>
> On Fri, Dec 30, 2011 at 12:49 AM, Heinrich Apfelmus
> <apfel...@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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