new syntax to get items from collection: collection[index]. Supported collections are string, list and dict

operator-index.go

@@ -0,0 +1,91 @@
+// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// All rights reserved.
+
+// operator-index.go
+package expr
+
+// -------- index term
+func newIndexTerm(tk *Token) (inst *term) {
+	return &term{
+		tk:       *tk,
+		children: make([]*term, 0, 2),
+		position: posInfix,
+		priority: priDot,
+		evalFunc: evalIndex,
+	}
+}
+
+func verifyKey(indexTerm *term, indexList *ListType) (index any, err error) {
+	if len(*indexList) != 1 {
+		err = indexTerm.Errorf("one index only is allowed")
+	} else {
+		index = (*indexList)[0]
+	}
+	return
+}
+
+func verifyIndex(indexTerm *term, indexList *ListType, maxValue int) (index int, err error) {
+	var v int
+
+	if len(*indexList) != 1 {
+		err = indexTerm.Errorf("one index only is allowed")
+	} else if v, err = toInt((*indexList)[0], "index expression"); err == nil {
+		if v < 0 && v >= -maxValue {
+			v = maxValue + v
+		}
+		if v >= 0 && v < maxValue {
+			index = v
+		} else {
+			err = indexTerm.Errorf("index %d out of bounds", v)
+		}
+	}
+	return
+}
+
+func evalIndex(ctx ExprContext, self *term) (v any, err error) {
+	var leftValue, rightValue any
+	var indexList *ListType
+	var ok bool
+
+	if err = self.checkOperands(); err != nil {
+		return
+	}
+	if leftValue, rightValue, err = self.evalInfix(ctx); err != nil {
+		return
+	}
+
+	if indexList, ok = rightValue.(*ListType); !ok {
+		err = self.Errorf("invalid index expression")
+		return
+	}
+	indexTerm := self.children[1]
+
+	switch unboxedValue := leftValue.(type) {
+	case *ListType:
+		var index int
+		if index, err = verifyIndex(indexTerm, indexList, len(*unboxedValue)); err == nil {
+			v = (*unboxedValue)[index]
+		}
+	case string:
+		var index int
+		if index, err = verifyIndex(indexTerm, indexList, len(unboxedValue)); err == nil {
+			v = string(unboxedValue[index])
+		}
+	case *DictType:
+		var ok bool
+		var indexValue any
+		if indexValue, err = verifyKey(indexTerm, indexList); err == nil {
+			if v, ok = (*unboxedValue)[indexValue]; !ok {
+				err = indexTerm.Errorf("key %v does not belong to the dictionary", rightValue)
+			}
+		}
+	default:
+		err = self.errIncompatibleTypes(leftValue, rightValue)
+	}
+	return
+}
+
+// init
+func init() {
+	registerTermConstructor(SymIndex, newIndexTerm)
+}