[Haskell-cafe] On the purity of Haskell

[Gregg Reynolds]

On Dec 30, 2011, at 11:04 AM, Colin Adams wrote:

> 
> 
> 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.


My conclusion holds:  f 42 != f 42.  Obviously, so I won't burden you with an explanation. ;)

-Gregg
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