[mercury-users] Field updates and state variables

[Peter Schachte]

On Sun, Jul 02, 2006 at 12:28:06PM -0700, Peter Hawkins wrote:
> Hi...
> 
> I find myself writing code like this a lot:
> ...
> !: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)

That seems pretty magical; the former is not what I would naturally
expect the latter to expand to.

I think it would be nicer to use the elem getter/setter for maps,
writing

	!.S ^ my_map ^ elem(Key)

(as an expression) to lookup a key in a map, and

	!:S = !.S ^ my_map ^ elem(Key) := Value

(as a goal) to update a map.  In fact, it wouldn't be hard to write
your own elem/2 getter and 'elem :='/3 setter functions for your
my_map type that directly operate on the map, to get rid of the
annoying "^ my_map" part, so you would write

	!.S ^ elem(Key)

(as an expression) to lookup a key in a map, and

	!:S = !.S ^ elem(Key) := Value

(as a goal) to update a map.  See section 3.4.3 in the Mercury
reference manual.

What would be *really* useful would be to integrate state variable and
setter syntax, by translating

	!Var ^ Acessors := Value

to !:Var = !.Var ^ Accessors := Value.  Maybe mercury already does
that, but I couldn't find it in the reference manual.  Then you could
just write

	!S ^ elem(Key) := Value

That reads better to me than map.insert(Key, Value, !S ^ my_map).

-- 
Peter Schachte              Beware of bugs in the above code; I have only
schachte at cs.mu.OZ.AU        proved it correct, not tried it.
www.cs.mu.oz.au/~schachte/      -- Donald Knuth 
Phone: +61 3 8344 1338      
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