metapredicate spec

[jjtolton]

Is this information correct?

Supported (please correct any inaccuracies)

argspec	meaning	scryer supported?
+	Argument will always be instantiated	yes
?	Argument may or may not be instantiated	yes
-	Always uninstantiated	yes
:	Module prefixed to argument	yes
I (integer)	"curry level" required for partial, predicate completion, (see below)	yes

Not supported

argspec	meaning	scryer supported?
^	"annotated" goal? Not sure what this is	No
//	DCG body	No

Curried/Partial Predicates

A predicate is often denoted P/N, where P is the name of the
predicate and N is the arrity. A partial predicate is a predicate
P/M where N #= I + M. In this case, as in the example below, I is
the number in the metapredicate directive and M is the number of
partial predicates.

So given

:- meta_predicate(foo(1, ?, ?)).

foo(Result, And, Out) :-
        call(Result, R),
        Out #= And + R.

bar(X,Y,Result) :-
        Result #= X + Y.


?- foo(bar(1,3), 5, Out).
%@    Out = 9.

?- foo(bar(1,B), 5, 7).
%@    B = 1.

In this case, we have bar/N where N=3. It is a partial goal to
foo/3's first argument as bar(1,3) wwhich is bar/M, where M=2.
Since foo/3's meta predicate directive specifies the first argument
as 1, as in meta_predicate(I, ?, ?) (note: not valid syntax,
for explanatory purposes only) where I=1, and
N#=M+I => 3#=2+I => I #= 1, we see that one additional argument is to
be supplied.

Questions, mostly answered

Questions: ==========

• ^... is this a thing? The word "annotation" doesn't even show
  up in the ISO-standard. (but I don't see metapredicates there
  either...). Saw this on the SWIPL docs:
  https://www.swi-prolog.org/pldoc/man?section=metapred
  ( Edlt: No, see metapredicate spec #2617 (comment) )

• The numerical value doesn't seem to do anything, it also doesn't
  matter if you use and of the other values either? (Edit: see metapredicate spec #2617 (comment))

• It seems like the most important thing metapredicates do from a
  technical standpoint is attach module information when required? (Edit: see metapredicate spec #2617 (comment))

---

#2617 (comment)

[triska]

Even without a module system, we need meta_predicate/1 declarations.

The reason is that it lets the compiler and development tools deduce statically which predicates are called when a partial goal is specified as an argument of a meta-predicate. You need this information for example when implementing a cross-referencer, or to detect which predicates are certainly called etc.

[UWN]

^ is an argument like the goal of setof/3. SWI specific (last time I heard of it)