package lambda

func Rename(expr Expression, target string, newName string) Expression {
	switch e := expr.(type) {
	case *Variable:
		if e.value == target {
			return NewVariable(newName)
		}
		return e

	case *Abstraction:
		newParam := e.parameter
		if e.parameter == target {
			newParam = newName
		}

		newBody := Rename(e.body, target, newName)

		if newParam == e.parameter && newBody == e.body {
			return e
		}

		return NewAbstraction(newParam, newBody)

	case *Application:
		newAbs := Rename(e.abstraction, target, newName)
		newArg := Rename(e.argument, target, newName)

		if newAbs == e.abstraction && newArg == e.argument {
			return e
		}

		return NewApplication(newAbs, newArg)

	default:
		return expr
	}
}