The plus and minus operators now support lists for join and difference respectively

operator-sum.go

@@ -6,6 +6,7 @@ package expr
 
 import (
 	"fmt"
+	"slices"
 )
 
 //-------- plus term
@@ -39,6 +40,23 @@ func evalPlus(ctx exprContext, self *term) (v any, err error) {
 			rightInt, _ := rightValue.(int64)
 			v = leftInt + rightInt
 		}
+	} else if isList(leftValue) || isList(rightValue) {
+		var leftList, rightList []any
+		var ok bool
+		if leftList, ok = leftValue.([]any); !ok {
+			leftList = []any{leftValue}
+		}
+		if rightList, ok = rightValue.([]any); !ok {
+			rightList = []any{rightValue}
+		}
+		sumList := make([]any, 0, len(leftList)+len(rightList))
+		for _, item := range leftList {
+			sumList = append(sumList, item)
+		}
+		for _, item := range rightList {
+			sumList = append(sumList, item)
+		}
+		v = sumList
 	} else {
 		err = self.errIncompatibleTypes(leftValue, rightValue)
 	}
@@ -74,6 +92,16 @@ func evalMinus(ctx exprContext, self *term) (v any, err error) {
 			rightInt, _ := rightValue.(int64)
 			v = leftInt - rightInt
 		}
+	} else if isList(leftValue) && isList(rightValue) {
+		leftList, _ := leftValue.([]any)
+		rightList, _ := rightValue.([]any)
+		diffList := make([]any, 0, len(leftList)-len(rightList))
+		for _, item := range leftList {
+			if slices.Index(rightList, item) < 0 {
+				diffList = append(diffList, item)
+			}
+		}
+		v = diffList
 	} else {
 		err = self.errIncompatibleTypes(leftValue, rightValue)
 	}