feat: better recursive descent

pkg/saccharine/ast/visit.go

@@ -0,0 +1,22 @@
+package ast
+
+import "fmt"
+
+type Visitor[T any] interface {
+	VisitAtom(*Atom) T
+	VisitAbstraction(*Abstraction) T
+	VisitApplication(*Application) T
+}
+
+func Visit[T any](visitor Visitor[T], node Expression) T {
+	switch node := node.(type) {
+	case *Atom:
+		return visitor.VisitAtom(node)
+	case *Abstraction:
+		return visitor.VisitAbstraction(node)
+	case *Application:
+		return visitor.VisitApplication(node)
+	default:
+		panic(fmt.Sprintf("unknown node %t", node))
+	}
+}