Adding first operator

pkg/yqlib/expression_parser.go

@@ -54,6 +54,12 @@ func (p *expressionParserImpl) createExpressionTree(postFixPath []*Operation) (*
 			switch numArgs {
 			case 1:
 				if len(stack) < 1 {
+					// Allow certain unary ops to accept zero args by interpreting missing RHS as nil
+					// TODO - make this more general on OperationType
+					if Operation.OperationType == firstOpType {
+						// no RHS provided; proceed without popping
+						break
+					}
 					return nil, fmt.Errorf("'%v' expects 1 arg but received none", strings.TrimSpace(Operation.StringValue))
 				}
 				remaining, rhs := stack[:len(stack)-1], stack[len(stack)-1]