[m-dev.] type classes and the color blue

[Ralph Becket]

> Does the following code make any sense at all:
> 
> :- typeclass thing(T) where
> 	[
> 		pred is_blue,
> 		mode is_blue is semidet
> 	].
> 
> :- instance thing(sky) where
> 	[
> 		pred(is_blue/1) is semidet_succeed
> 	].
> 
> :- instance thing(sun) where
> 	[
> 		pred(is_blue/1) is semidet_fail
> 	].

I can't see anything wrong, except that I don't
think semidet_succeed and semidet_fail take any
arguments (I haven't got the reference manual
handy at the mo.)  But wrapping them up should
work:

:- instance thing(sky) where
	[
		pred(is_blue/1) is sky_is_blue
	].

sky_is_blue(_) :- semidet_succeed.

> :- pred both_blue(A, B) <= (thing(A), thing(B)).
> :- mode both_blue(in, in) is semidet.
> 
> both_blue(A, B) :- ... what can go here? ...

both_blue(A, B) :- is_blue(A), is_blue(B).

should do the trick.

> Is there a way to qualify methods by variable name?

Method calls are decided on a per-instance basis, so
what happens at runtime depends upon what type of thing/1
arguments are passed to both_blue/2.  Each argument A
and B is accompanied by a hidden extra argument which
is used to work out which procedure to call for which
method.

Cheers,

Ralph
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