Modernize FilterLink

opencog/atoms/atom_types/atom_types.script

@@ -1110,8 +1110,12 @@ DIRECTLY_EVALUATABLE_LINK <- LINK
 // for more information
 IMPLICATION_LINK <- ORDERED_LINK, DIRECTLY_EVALUATABLE_LINK
 
-// ImplicationScopeLink should be moved to PLN at earliest
-// convenience. XXX FIXME.
+// ImplicationScopeLink is a synonym for RuleLink, as far as I can tell.
+// It is used by PLN to do more or less the same thing as RuleLink.
+// Except that BackwardChainerUTest hangs when it inherits from
+// RuleLink, because RuleLink::execute() does something that URE isn't
+// expecting. At any rate, it should be moved there. XXX FIXME.
+// IMPLICATION_SCOPE_LINK <- RULE_LINK
 IMPLICATION_SCOPE_LINK <- SCOPE_LINK
 
 INHERITANCE_LINK <- ORDERED_LINK, DIRECTLY_EVALUATABLE_LINK

opencog/atoms/core/CMakeLists.txt

@@ -9,7 +9,6 @@ ADD_LIBRARY (atomcore
 	Context.cc
 	DefineLink.cc
 	DeleteLink.cc
-	FilterLink.cc
 	FindUtils.cc
 	FreeLink.cc
 	FreeVariables.cc
@@ -58,7 +57,6 @@ INSTALL (FILES
 	Context.h
 	DefineLink.h
 	DeleteLink.h
-	FilterLink.h
 	FindUtils.h
 	FreeLink.h
 	FreeVariables.h

opencog/atoms/core/FreeVariables.cc

@@ -412,7 +412,7 @@ void FreeVariables::canonical_sort(const HandleSeq& hs)
 	// Get free variables
 	HandleSet fv = get_free_variables(hs);
 
-	// Ignore free variables in body not in the FreeVariables object
+	// Ignore free variables in the body that are not in this object.
 	HandleSet ignored_vars = set_symmetric_difference(fv, varset);
 	Context ctx(Quotation(), ignored_vars, false);
 

opencog/atoms/core/FreeVariables.h

@@ -105,16 +105,14 @@ struct FreeVariables : Replacement
 	/// ordered link.
 	void find_variables(const HandleSeq& oset, bool ordered_link=true);
 
-	/// Sort the variables in a canonical order determined by their
-	/// positions in the given outgoing set, which is assumed ordered,
-	/// as outgoing sets of scopes are always ordered so far.  In
-	/// ordered link, the ordered is determined by the outgoing set
-	/// order (from left to right).  In unordered links, the ordered is
-	/// determined by some arbitrary, though semantically consistent
-	/// fix order.  The order only depends on variable names as last
-	/// resort, when no semantic property can be used to break the
-	/// symmetry.
-	void canonical_sort(const HandleSeq& outgoings);
+	/// Sort the variables into a canonical order, so that they appear
+	/// in the same order as in the provided HandleSeq.  That is, the
+	/// HandleSeq is presumed to be a list of trees, with variables
+	/// embeded in those trees. Each tree is walked, left-to-right,
+	/// depth-first. The order in which the variables are encountered
+	/// is the "canonical sort order". Variables that do NOT appear
+	/// in the HandleSeq are trimmed (removed) from this object.
+	void canonical_sort(const HandleSeq&);
 
 	/// Convert a variable->argument mapping into a sequence of
 	/// "arguments" that are in the same order as the free variables

opencog/atoms/core/ScopeLink.cc

@@ -23,8 +23,8 @@
 
 #include <string>
 
-// #include <opencog/util/Logger.h>
 #include <opencog/util/random.h>
+
 #include <opencog/atoms/atom_types/NameServer.h>
 #include <opencog/atoms/base/hash.h>
 #include <opencog/atoms/core/FindUtils.h>
@@ -181,6 +181,13 @@ void ScopeLink::init_scoped_variables(const Handle& vardecl)
 
 /* ================================================================= */
 
+void ScopeLink::trim(const HandleSeq& terms)
+{
+	_variables.trim(terms);
+}
+
+/* ================================================================= */
+
 inline Handle append_rand_str(const Handle& var)
 {
 	std::string new_var_name = randstr(var->get_name() + "-");

opencog/atoms/core/ScopeLink.h

@@ -81,14 +81,20 @@ class ScopeLink : public Link
 	const Handle& get_vardecl(void) const { return _vardecl; }
 	const Handle& get_body(void) const { return _body; }
 
+	// Remove any variables that do NOT appear anywhere in the provided
+	// HandleSeq. This can be used to clean up vardecls, when variables
+	// are declared but then never used (thus making them impossible to
+	// ground, or having other nasty side-effects.)
+	void trim(const HandleSeq&);
+
 	// Return an alpha-converted copy of this atom. Optionally, new
 	// variable names can be provided. If none are provided, then new
 	// randomly generated names are created.
 	//
-	// Warning: the atomspace treats all alpha-convertible atoms as
+	// Warning: the AtomSpace treats all alpha-convertible atoms as
 	// identical; if the new copy is inserted into the atomspace, the
 	// original version will be returned.  Alpha-converted atoms can
-	// only be used outside of the atomspace, for temporary operations.
+	// only be used outside of the AtomSpace, for temporary operations.
 	Handle alpha_convert() const;
 	Handle alpha_convert(const HandleSeq& vars) const;
 

opencog/atoms/core/TypedVariableLink.cc

@@ -43,7 +43,8 @@ void TypedVariableLink::init()
 	if (VARIABLE_NODE != stype and
 	    GLOB_NODE != stype)
 		throw SyntaxException(TRACE_INFO,
-			"Sorry, we expect type names to be variables!");
+			"Sorry, we expect type names to be variables! Got=%s",
+			to_short_string().c_str());
 
 	// Allow VARIABLE_NODE, although this is a bug in the URE,
 	// which should be using a SignatureLink for this case. XXX FIXME.

opencog/atoms/core/Variables.cc

@@ -571,6 +571,24 @@ void Variables::trim(const Handle& term)
 		erase(hu);
 }
 
+// Identical to above, except multiple terms are scanned.
+void Variables::trim(const HandleSeq& terms)
+{
+	// Find the vars in all of the terms.
+	FreeVariables fv;
+	fv.find_variables(terms);
+
+	// Find all vars not in any of the terms.
+	HandleSeq unused;
+	for (const Handle& hv: varseq)
+		if (not fv.varset_contains(hv))
+			unused.push_back(hv);
+
+	// Get rid of all vars not in any of the terms.
+	for (const Handle& hu: unused)
+		erase(hu);
+}
+
 /* ================================================================= */
 
 /// Return true if the other Variables struct is equal to this one,

opencog/atoms/core/Variables.h

@@ -158,6 +158,9 @@ struct Variables : public FreeVariables
 	/// Remove *all* variables that do not appear in the term.
 	void trim(const Handle&);
 
+	/// Remove *all* variables that do not appear in any of the terms.
+	void trim(const HandleSeq&);
+
 	/// Return the TypedVariableLink for the indicated variable.
 	/// Return just the Variable itself, if its not typed.
 	Handle get_type_decl(const Handle&, const Handle&) const;

opencog/atoms/flow/CMakeLists.txt

@@ -3,6 +3,7 @@
 INCLUDE_DIRECTORIES( ${CMAKE_CURRENT_BINARY_DIR})
 
 ADD_LIBRARY (atomflow
+	FilterLink.cc
 	FormulaPredicateLink.cc
 	NumberOfLink.cc
 	PromiseLink.cc
@@ -29,6 +30,7 @@ INSTALL (TARGETS atomflow EXPORT AtomSpaceTargets
 )
 
 INSTALL (FILES
+	FilterLink.h
 	FormulaPredicateLink.h
 	NumberOfLink.h
 	PromiseLink.h