refactor: move json related atoms to the atom file

x/logic/predicate/atom.go

@@ -7,3 +7,35 @@ import (
 // AtomPair are terms with principal functor (-)/2.
 // For example, the term -(A, B) denotes the pair of elements A and B.
 var AtomPair = engine.NewAtom("-")
+
+// AtomJSON are terms with principal functor json/1.
+// It is used to represent json objects.
+var AtomJSON = engine.NewAtom("json")
+
+// AtomAt are terms with principal functor (@)/1.
+// It is used to represent special values in json objects.
+var AtomAt = engine.NewAtom("@")
+
+// AtomTrue is the term true.
+var AtomTrue = engine.NewAtom("true")
+
+// AtomFalse is the term false.
+var AtomFalse = engine.NewAtom("false")
+
+// AtomNull is the term null.
+var AtomNull = engine.NewAtom("null")
+
+// MakeNull returns the compound term @(null).
+// It is used to represent the null value in json objects.
+func MakeNull() engine.Term {
+	return AtomAt.Apply(AtomNull)
+}
+
+// MakeBool returns the compound term @(true) if b is true, otherwise @(false).
+func MakeBool(b bool) engine.Term {
+	if b {
+		return AtomAt.Apply(AtomTrue)
+	}
+
+	return AtomAt.Apply(AtomFalse)
+}

x/logic/predicate/json.go

@@ -17,9 +17,6 @@ import (
 	"github.com/okp4/okp4d/x/logic/util"
 )
 
-// AtomJSON is a term which represents a json as a compound term `json([Pair])`.
-var AtomJSON = engine.NewAtom("json")
-
 // JSONProlog is a predicate that will unify a JSON string into prolog terms and vice versa.
 //
 // json_prolog(?Json, ?Term) is det
@@ -125,11 +122,11 @@ func termsToJSON(term engine.Term, env *engine.Env) ([]byte, error) {
 		}
 
 		switch {
-		case AtomBool(true).Compare(t, env) == 0:
+		case MakeBool(true).Compare(t, env) == 0:
 			return json.Marshal(true)
-		case AtomBool(false).Compare(t, env) == 0:
+		case MakeBool(false).Compare(t, env) == 0:
 			return json.Marshal(false)
-		case AtomNull.Compare(t, env) == 0:
+		case MakeNull().Compare(t, env) == 0:
 			return json.Marshal(nil)
 		}
 
@@ -153,9 +150,9 @@ func jsonToTerms(value any) (engine.Term, error) {
 		}
 		return engine.Integer(r.Int64()), nil
 	case bool:
-		return AtomBool(v), nil
+		return MakeBool(v), nil
 	case nil:
-		return AtomNull, nil
+		return MakeNull(), nil
 	case map[string]any:
 		keys := lo.Keys(v)
 		sort.Strings(keys)

x/logic/predicate/util.go

@@ -78,18 +78,6 @@ func ListToBytes(terms engine.ListIterator, env *engine.Env) ([]byte, error) {
 	return bt, nil
 }
 
-func AtomBool(b bool) engine.Term {
-	var r engine.Atom
-	if b {
-		r = engine.NewAtom("true")
-	} else {
-		r = engine.NewAtom("false")
-	}
-	return engine.NewAtom("@").Apply(r)
-}
-
-var AtomNull = engine.NewAtom("@").Apply(engine.NewAtom("null"))
-
 // ExtractJSONTerm is an utility function that would extract all attribute of a JSON object
 // that is represented in prolog with the `json` atom.
 //