[mercury-users] Field updates and state variables

[Peter Hawkins]

Hi...

I find myself writing code like this a lot:

:- type my_state ---> my_state(my_map :: map(int, int)).

:- pred foo(my_state::in, my_state::out) is det.
foo(!S) :-
...
!:S = !.S ^ my_map := map.insert(!.S ^ my_map, Key, Value)
...

Wouldn't it be nice if I could write the field update as:
map.insert(Key, Value, !S ^ my_map)
and have the compiler expand it to the code above? I'd like that a lot.

Cheers,
Peter

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