Proposal: Relax rules for 'pure'

[Robert Jacques]

On Wed, 22 Sep 2010 04:13:34 -0400, Don <nospam at nospam.com> wrote:

> Don wrote:
>> The docs currently state that:
>
>> PROPOSAL:
>> Drop the first requirement. Only one requirement is necessary:
>>  A pure function does not read or write any global mutable state.
>>
>
> Wow. It seems that not one person who has responded so far has  
> understood this proposal! I'll try again. Under this proposal:

Funny, your re-iteration appears to coincided to my previous  
understanding. So either I've mis-understood twice, or I didn't  
sufficiently demonstrate my understanding when I made my critique. :) That  
said, I do think this version is much clearer and understandable.

> If you see a function which has mutable parameters, but is marked as  
> 'pure', you can only conclude that it doesn't use global variables.  
> That's not much use on it's own. Let's call this a 'weakly-pure'  
> function.
>
> However, if you see a function maked as 'pure', which also has only  
> immutable parameters, you have the same guarantee which 'pure' gives us  
> as the moment. Let's call this a 'strongly-pure' function.
>
> The benefit of the relaxed rule is that a strongly-pure function can  
> call a weakly-pure functions, while remaining strongly-pure.
> This allows very many more functions to become strongly pure.
>
> The point of the proposal is *not* to provide the weak guarantee. It is  
> to provide the strong guarantee in more situations.

The problem from my point of view is that the programmer can not declare  
that a function should be 'strongly-pure' or 'weakly-pure'. Essentially,  
the point of the proposal is *to* provide the weak guarantee and leave the  
strong guarantee up to a sufficiently smart compiler. And I really don't  
want a function to suddenly run 8x slower, just because Joe-coder changed  
a type somewhere and made my 'strongly-pure' inner-loop 'weakly-pure'.  
That said, if we can only have one type of purity in the language, I think  
'weakly-pure' is more powerful a concept than 'strongly-pure'.