Proposal: Relax rules for 'pure'

[Robert Jacques]

On Wed, 22 Sep 2010 11:53:22 -0400, Don <nospam at nospam.com> wrote:

> Robert Jacques wrote:
>> 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'.
>
> Yes they can. They just need to declare the parameters to be immutable.

What about value types?