Proposal: Relax rules for 'pure'

[Don]

Robert Jacques wrote:

>>>>>> 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?
>>
>> Value types are implicitly convertable to immutable, so they can be 
>> strongly-pure.
> 
> No their not. Remember, arrays and other structs are value types in the 
> type system. Logically, they may be reference types, but as far as their 
> type signature goes, they are value types.
> 
>> The problem I think Robert is referring to is, a person cannot always 
>> tell just from looking at a signature that a type is strongly-pure 
>> qualified or not.
> 
> And that you can not manually specify one or the other.

Yes you can. Just add const to the parameter declaration. This works 
because const is transitive.

> 
>> For example, is this function weak or strong?
>>
>> pure int foo(T t);

pure int foo(const T t);

is always strong pure, regardless of what T is.