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In this tutorial we are going to make an HTTP request without using the existing web HTTP modules in Guile Scheme using Emacs and geiser mode. This is something I like to do when picking up a new language as it forces me to use modules, learn concepts around strings and sequences, and generally start getting familiar with a language.

To follow along you will need to install guile. I recommend using Emacs with geiser-guile installed (although you can use a worse editor if you wish).

Make a new directory to contain your tutorial project. Add a main.scm file. Copy and paste the code blocks below as we go. Or just do it directly in the REPL. I personally prefer to have a text buffer open with a REPL connected.

Start geiser mode. Test that is is connected by running geiser-eval-region on the form below

(display "Hey there")

Next we need someone to make a request to. We can use getaddrinfo (again, in Guile's core) to get host and service info. Since it can return multiple results, we take the first with car:

(define server-addr (car (getaddrinfo "www.gnu.org" "http")))

We can inspect some attributes of our address like so:

(addrinfo:protocol server-addr)
(addrinfo:addr server-addr)

See more attributes here in the Guile docs.

Now let's connect our socket to the address:

(connect my-sock (addrinfo:addr server-addr))

This should return #t if successful. Let's send a basic TCP message so see it's working.

(send my-sock "test/n")

It fails because the socket is expecting a bytevector. Let's use a module to help us convert strings to bytevectors:

(use-modules (rnrs bytevectors))

(define test-tcp-msg (string->utf8 "test/n"))

(send my-sock test-tcp-msg)

This responds with the number of characters send (+ the null terminator).

Now let's try to receive some data back. In UNIX, pretty much everything is just a file, including sockets. So to read from a socket we will use the file associated with it. We open a file of my-sock and then read a line from it.

Aside:

(use-modules (ice-9 ports))

(define response-port
  (fdopen (fileno my-sock) "r"))

(use-modules (ice-9 rdelim))
;; ~ice-9~ is Guile's standard library for functionality
;; beyond the Scheme specification. Here we will use ~ports~,
;; which are not a network ports (eg: port 80)
;; but a Guile abstraction for I/O.

(read-line response-port)

We should find that the connection is reset by peer, this is because the server is expecting an HTTP message but we just send 'test'.

Let's try with an HTTP request, which can have a basic format as follows:

{http_method} {path protocol}\r\nHOST:{hostname} {instructions after response}\r\n

…where \r\n are carrage returns as required by the HTTP protocol.

(define my-first-http-request
  (string->utf8 "GET / HTTP/1.1\r\nHOST: gnu.org\r\nConnection: close\r\n\r\n")


(send my-sock my-first-http-request)

If this succeeds then we should be able to read a line from the file:

(read-line response-port)

Instead we would like to take from the response until we hit the End Of File. We will use unfold to walk through the lines until we get to an EOF.

(use-modules (srfi srfi-1))
;; `srfi` is Scheme Requests for Implementation.
;; They are numbered (which seem rather un-ergonomic).
;; `srfi-1` is for list operations.

(define (read-all-lines port)
  (unfold eof-object? ;; end condition
          values      ;; mapper (in this case do nothing)
          (lambda (_) (read-line port)) ;; successor
          (read-line port))) ;; init value

We start by reading a line, check if it's an eof, if not then apply the mapper [so here (values x) = x], and pass it to the successor lambda, which ignores its argument and just reads the next line. Then keeps looping until eof.

(read-all-lines response-port)

Thanks for following along, I hope it's helpful for you to get up and running en Guile Scheme. ✌️