Defer in Scheme

In Scheme, we don’t have a direct equivalent of the defer keyword. However, we can achieve similar functionality using dynamic-wind or call-with-* procedures. For this example, we’ll use dynamic-wind to ensure that our file is closed after we’re done with it.

(use-modules (ice-9 format))

;; Suppose we wanted to create a file, write to it,
;; and then close when we're done. Here's how we could
;; do that with dynamic-wind.

(define (main)
  ;; dynamic-wind takes three thunks (functions with no arguments):
  ;; 1. A setup function
  ;; 2. The main function to be protected
  ;; 3. A cleanup function
  (dynamic-wind
    (lambda () (format #t "creating~%"))
    (lambda ()
      (let ((port (open-output-file "/tmp/defer.txt")))
        (format #t "writing~%")
        (format port "data~%")
        port))
    (lambda ()
      (format #t "closing~%")
      (close-port port))))

(define (create-file filename)
  (format #t "creating~%")
  (open-output-file filename))

(define (write-file port)
  (format #t "writing~%")
  (format port "data~%"))

(define (close-file port)
  (format #t "closing~%")
  (close-port port))

;; Run the main function
(main)

In this Scheme version:

1. We use dynamic-wind to ensure that our file is closed after we’re done with it, even if an error occurs.

2. The create-file, write-file, and close-file functions are defined separately, similar to the Go example.

3. Instead of using defer, we pass the cleanup function (closing the file) as the third argument to dynamic-wind.

4. Error handling in Scheme is typically done using the condition system, which is more complex than shown here. For simplicity, we’ve omitted explicit error handling.

5. We use format for printing, which is similar to fmt.Println in Go.

Running the program would produce output similar to the Go version:

$ guile defer.scm
creating
writing
closing

This example demonstrates how to use dynamic-wind in Scheme to ensure that cleanup operations (like closing a file) are performed, even if an error occurs during the main operation. While it’s not identical to Go’s defer, it serves a similar purpose in ensuring proper resource management.