[Haskell-cafe] On the purity of Haskell

[Colin Adams]

On 30 December 2011 16:59, Gregg Reynolds <dev at mobileink.com> 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.
>>
>
>
>
> 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.)
>
> This seems such an obviously incorrect conclusion.

f42 is a funtion for returning a program for returning an int, not a
function for returning an int.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/haskell-cafe/attachments/20111230/bd728c41/attachment.htm>