Context in Scheme

Based on the provided instruction, here’s the translation of the Go code to Scheme, along with explanations in Markdown format suitable for Hugo:

In this example, we’ll look at how to set up a simple HTTP server in Scheme. We’ll demonstrate the usage of a concept similar to context.Context for controlling cancellation. While Scheme doesn’t have built-in HTTP server capabilities or a direct equivalent to Go’s context.Context, we’ll use similar concepts to illustrate the idea.

(import (rnrs) (rnrs eval) (srfi :48) (srfi :18))

;; Simulate an HTTP response writer
(define (response-writer message)
  (lambda ()
    (format #t "Response: ~a~%" message)))

;; Simulate an HTTP request
(define (make-request)
  (cons (make-condition-variable) (make-mutex)))

;; Simulate the context
(define (make-context)
  (let ((done #f))
    (cons
     (lambda () done)      ; is-done?
     (lambda () (set! done #t))))) ; set-done!

;; Handler function
(define (hello response-writer request)
  (let* ((ctx (make-context))
         (is-done? (car ctx))
         (set-done! (cdr ctx)))
    (format #t "server: hello handler started~%")
    (let ((t (make-thread
              (lambda ()
                (thread-sleep! 10)
                (if (not (is-done?))
                    ((response-writer "hello\n")))))))
      (thread-start! t)
      (thread-sleep! 2) ; Simulate work
      (if (not (is-done?))
          (begin
            (set-done!)
            (format #t "server: request cancelled~%")
            ((response-writer "Internal Server Error"))))
      (thread-join! t))
    (format #t "server: hello handler ended~%")))

;; Main function
(define (main)
  (format #t "Server starting on port 8090~%")
  (let ((request (make-request)))
    (hello (response-writer "Success") request)))

(main)

In this Scheme version:

1. We simulate an HTTP server environment using Scheme’s threading capabilities.

2. The make-context function creates a simple context with is-done? and set-done! functions to check and set the cancellation state.

3. The hello function simulates handling an HTTP request. It starts a thread that waits for 10 seconds before sending a response. Meanwhile, the main thread waits for 2 seconds and then simulates a cancellation.

4. If the cancellation occurs before the response is sent, an error message is returned instead.

5. We use thread-sleep! to simulate waiting and thread-join! to wait for the response thread to finish.

To run this program, save it to a file (e.g., context-example.scm) and execute it using a Scheme interpreter that supports the required libraries (like Chez Scheme):

$ scheme --script context-example.scm
Server starting on port 8090
server: hello handler started
server: request cancelled
server: hello handler ended

This example demonstrates how you might implement a cancellable context in Scheme, even though it doesn’t have built-in HTTP server capabilities. The concepts of concurrent execution and cancellation are represented using Scheme’s threading primitives.