New operators 'but' and assignment ('=')

operator-assign.go

@@ -0,0 +1,43 @@
+// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// All rights reserved.
+
+// operator-assign.go
+package expr
+
+import "fmt"
+
+//-------- assign term
+
+func newAssignTerm(tk *Token) (inst *term) {
+	return &term{
+		tk:       *tk,
+		class:    classOperator,
+		kind:     kindUnknown,
+		children: make([]*term, 0, 2),
+		position: posInfix,
+		priority: priAssign,
+		evalFunc: evalAssign,
+	}
+}
+
+func evalAssign(ctx exprContext, self *term) (v any, err error) {
+	if err = self.checkOperands(); err != nil {
+		return
+	}
+
+	leftTerm := self.children[0]
+	if leftTerm.tk.Sym != SymIdentifier {
+		err = fmt.Errorf("left operand of %q must be a variable", self.tk.source)
+		return
+	}
+
+	if v, err = self.children[1].compute(ctx); err == nil {
+		ctx.SetValue(leftTerm.tk.source, v)
+	}
+	return
+}
+
+// init
+func init() {
+	registerTermConstructor(SymEqual, newAssignTerm)
+}