[m-rev.] Add `:- mutable' declaration to the language.

[Ian MacLarty]

On 1 Sep 2005, at 15:11, Ralph Becket wrote:

> Ian MacLarty, Thursday,  1 September 2005:
>> On Thu, 1 Sep 2005, Ralph Becket wrote:
>>>
>>> :- semipure pred get_x(int::out(ground)) is det.
>>> :- impure   pred set_x(int::in(ground)) is det.
>>> :-          pred initialise_mutable_x(io::di, io::uo) is det.
>>
>> Can we have pure versions of get_x and set_x which take the I/O state?
>
> Sure, just write
>
> :- pred get_x_io(int::out, io::di, io::uo) is det.
> get_x_io(X, !IO) :- promise_pure(semipure get_x(X)).
>
> :- pred set_x_io(int::in, io::di, io::uo) is det.
> set_x_io(X, !IO) :- promise_pure(impure set_x(X)).
>

I still think these should be supplied.

If the pure versions are supplied then they can be used without having 
any knowledge of the impurity system (which can take a while to learn 
to use properly, especially if you are new to Mercury).
With the pure versions there is no burden on the user to promise that 
they are pure and therefore no need to understand the impurity system.
This would make the :-mutable declarations more generally accessible.

Ian.

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