[m-dev.] Is this a bug?

[Ralph Becket]

Ralph Becket, Monday, 18 June 2007:
> I'm trying to use field update syntax to update a map of maps, but it
> isn't working:
> 
> ::: map_map_update_test.m :::
> 
> :- module map_map_update_test.
> :- interface.
> :- import_module io.
> 
> :- pred main(io::di, io::uo) is det.
> 
> :- implementation.
> :- import_module map.
> 
> main(!IO) :-
>     A0 = map.init,
>     A  = A0 ^ elem(42) ^ elem(99) := 123,
>     io.write_int(A ^ det_elem(42) ^ det_elem(99), !IO),
>     io.nl(!IO).
> 
> ::: end of map_map_update_test.m :::

Ah, I've understood the problem:

	A  = A0 ^ elem(42) ^ elem(99) := 123

expands according to the following rule in the reference manual

	transform(Term ^ Field(Arg1, ...) := FieldValue) =
			'Field :='(Arg1, ..., Term, FieldValue)).

	transform(Term0 ^ Field(Arg1, ...) ^ Rest := FieldValue) = Term :-
		OldFieldValue = Field(Arg1, ..., Term0),
		NewFieldValue = transform(OldFieldValue ^ Rest := FieldValue),
		Term = 'Field :='(Arg1, ..., Term0, NewFieldValue).

The OldFieldValue uses the `elem' field access function which is
semidet.

This is somewhat counterintuitive.

-- Ralph
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