Proposal: Relax rules for 'pure'

[Lutger]

Don 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:
> 
> 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.

I understand now, this is a great proposal! What confused me was the label of 
'pure functions' for functions that actually aren't pure in the usual way that 
purity is defined.