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

[Ian MacLarty]

On 1 Sep 2005, at 16:24, Ralph Becket wrote:

> Ian MacLarty, Thursday,  1 September 2005:
>>
>> 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.
>
> We don't want `mutable' declarations to be generally accessible, for
> they are an abomination in the sight of God.  Albeit sometimes
> necessary.
>

They are only an abomination in the sight of God because God feels left 
out of the picture because he lives in the I/O state :-)

Seriously, it would be useful for a Mercury interface to a foreign API 
to be able to store and retrieve Mercury structures from the I/O state 
without using io.{get,set}_globals (since then the user of the library 
can't use io.{get,set}_globals) and without requiring the user to pass 
around additional state.  :- mutable declarations are therefore 
generally useful and we should therefore provide a pure method to 
access them so that people can use them without having to understand 
impurity.

Ian.

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