[mercury-users] Field updates and state variables (more)

[Nicholas Nethercote]

On Wed, 19 Jul 2006, Peter Schachte wrote:

>> Peter Hawkins recently proposed this syntax for updating fields in state
>> variables:
>>
>>   map.insert(Key, Value, !S ^ my_map)
>>
>> to mean this:
>>
>>   map.insert(Key, Value, !.S ^ my_map, Temp),
>>   !:S = !.S ^ my_map := Temp,
>
> How about:
>
>    !S ^ my_map ^ elem(Key) := Value
>
> Should work right now, and it's less verbose than the syntax you
> suggest.

I'm getting this error:

   symbols.m:581: Error: expected `Field =^ field1 ^ ... ^ fieldN'
   symbols.m:581:   or `^ field1 ^ ... ^ fieldN := Field'.
   symbols.m:581:   in DCG field access goal.
   symbols.m:546: Error: no clauses for predicate `add_symbol/5'.

Also, this relies on have the ^elem setter, which is fine when the record 
element is a map, but not so good for anything else.  Of course you could 
write your own setter, but that's more typing, etc.

Nick
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