Fix single element "in" clauses

evaluationStage.go

@@ -404,6 +404,16 @@ func makeAccessorStage(pair []string) evaluationOperator {
 	}
 }
 
+func ensureSliceStage(op evaluationOperator) evaluationOperator {
+	return func(left interface{}, right interface{}, parameters Parameters) (interface{}, error) {
+		orig, err := op(left, right, parameters)
+		if err != nil {
+			return orig, err
+		}
+		return []interface{}{orig}, nil
+	}
+}
+
 func separatorStage(left interface{}, right interface{}, parameters Parameters) (interface{}, error) {
 
 	var ret []interface{}

evaluation_test.go

@@ -505,6 +505,12 @@ func TestNoParameterEvaluation(test *testing.T) {
 			Input:    "!(1 in (1, 2, 3))",
 			Expected: false,
 		},
+		EvaluationTest{
+
+			Name:     "Single Element Array membership literal",
+			Input:    "1 in (1)",
+			Expected: true,
+		},
 		EvaluationTest{
 
 			Name:     "Logical operator reordering (#30)",

stagePlanner.go

@@ -385,12 +385,22 @@ func planValue(stream *tokenStream) (*evaluationStage, error) {
 	switch token.Kind {
 
 	case CLAUSE:
+		var prev ExpressionToken
+		if stream.index > 1 {
+			prev = stream.tokens[stream.index-2]
+		}
 
 		ret, err = planTokens(stream)
 		if err != nil {
 			return nil, err
 		}
 
+		// clauses with single elements don't trigger SEPARATE stage planner
+		// this ensures that when used as part of an "in" comparison, the array requirement passes
+		if prev.Kind == COMPARATOR && prev.Value == "in" && ret.symbol == LITERAL {
+			ret.operator = ensureSliceStage(ret.operator)
+		}
+
 		// advance past the CLAUSE_CLOSE token. We know that it's a CLAUSE_CLOSE, because at parse-time we check for unbalanced parens.
 		stream.next()