introduced mutators

mutator.go

@@ -0,0 +1,31 @@
+package main
+
+type NumMutator struct {
+	Op    NumOperator
+	Var   bool
+	Value int
+}
+
+type NumOperator string
+
+const (
+	NumOperatorAdd NumOperator = "+"
+	NumOperatorSub NumOperator = "-"
+	NumOperatorMul NumOperator = "*"
+	NumOperatorDiv NumOperator = "/"
+)
+
+func NewNumOperatorFromString(str string) NumOperator {
+	switch str {
+	case "+":
+		return NumOperatorAdd
+	case "-":
+		return NumOperatorSub
+	case "*":
+		return NumOperatorMul
+	case "/":
+		return NumOperatorDiv
+	default:
+		panic("invalid number operator: " + str)
+	}
+}