[Haskell-cafe] On the purity of Haskell

[Heinrich Apfelmus]

Steve Horne wrote:
> Heinrich Apfelmus wrote:
>>
>> Purity has nothing to do with the question of whether you can express 
>> IO in Haskell or not.
>>
> ....
> 
>> The beauty of the IO monad is that it doesn't change anything about 
>> purity. Applying the function
>>
>>    bar :: Int -> IO Int
>>
>> to the value 2 will always give the same result:
>>
> Yes - AT COMPILE TIME by the principle of referential transparency it 
> always returns the same action. However, the whole point of that action 
> is that it might potentially be executed (with potentially 
> side-effecting results) at run-time. Pure at compile-time, impure at 
> run-time. What is only modeled at compile-time is realized at run-time, 
> side-effects included.

Well, it's a matter of terminology: "impure" /= "has side effects". The 
ability of a language to describe side effects is not tied to its 
(im)purity.

Again, purity refers to the semantics of functions (at run-time): given 
the same argument, will a function always return the same result? The 
answer to this question solely decides whether the language is pure or 
impure. Note that this depends on the meaning of "function" within that 
language. In C, side-effects are part of the semantics of functions, so 
it's an impure language. In Haskell, on the other hand, functions will 
always return the same result, so the language is pure. You could say 
that side effects have been moved from functions to some other type 
(namely IO) in Haskell.


Best regards,
Heinrich Apfelmus

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