Proposal: Relax rules for 'pure'

[Don]

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.