Http Client in Lisp

Here’s an idiomatic Lisp example that demonstrates the same concept as the provided HTTP client example:

(defpackage :http-client-example
  (:use :cl :dexador :babel))

(in-package :http-client-example)

(defun main ()
  ;; Make an HTTP GET request to a server
  (handler-case
      (multiple-value-bind (body status headers uri stream)
          (dex:get "https://example.com")
        ;; Print the HTTP response status
        (format t "Response status: ~A~%" status)
        
        ;; Print the first 5 lines of the response body
        (let ((lines (split-sequence:split-sequence #\Newline body)))
          (loop for line in lines
                for i from 0 below 5
                do (format t "~A~%" line))))
    (error (e)
      (format t "An error occurred: ~A~%" e))))

;; Run the main function
(main)

This Lisp example demonstrates how to make an HTTP GET request and process the response. Let’s break it down:

1. We define a package http-client-example and use the dexador library for HTTP requests and babel for text encoding.

2. The main function is where the HTTP client logic is implemented.

3. We use dex:get from the Dexador library to make an HTTP GET request to “https://example.com”. This is equivalent to the http.Get function in the Go example.

4. The handler-case macro is used for error handling, similar to the if err != nil checks in the Go code.

5. We use multiple-value-bind to destructure the response from dex:get, which returns the body, status, headers, URI, and stream.

6. The response status is printed using format, which is similar to fmt.Println in Go.

7. To print the first 5 lines of the response body, we split the body into lines using split-sequence:split-sequence and then use a loop to print the first 5 lines.

8. If an error occurs during the HTTP request or processing, it’s caught in the handler-case and printed.

To run this Lisp program:

1. Ensure you have a Common Lisp implementation installed (e.g., SBCL, CCL).
2. Install the required libraries (dexador, babel, split-sequence) using Quicklisp:

(ql:quickload '(:dexador :babel :split-sequence))

3. Save the code in a file, e.g., http-client.lisp.
4. Load and run the file in your Lisp REPL:

(load "http-client.lisp")

This example demonstrates how to make HTTP requests, handle responses, and process the response body in Lisp. It showcases Lisp’s powerful error handling with handler-case and its ability to return multiple values from a function, which is used here with dex:get.