Http Client in Standard ML

Here’s an idiomatic Standard ML example demonstrating the concept of an HTTP client:

(* HTTP Client Example *)

(* Import necessary modules *)
use "Basis.sml";
use "Socket.sml";
use "TextIO.sml";

(* Function to send an HTTP GET request and print the response *)
fun httpGet url =
    let
        (* Parse the URL *)
        val {host, port, path} = Url.parse url

        (* Create a socket and connect to the server *)
        val sock = INetSock.TCP.socket()
        val _ = Socket.connect(sock, INetSock.toAddr(NetHostDB.getByName host, port))

        (* Create input and output streams *)
        val out = Socket.outputStream sock
        val in = Socket.inputStream sock

        (* Send the HTTP GET request *)
        val _ = TextIO.output(out, "GET " ^ path ^ " HTTP/1.1\r\nHost: " ^ host ^ "\r\n\r\n")
        val _ = TextIO.flushOut out

        (* Read and print the response *)
        fun readResponse() =
            case TextIO.inputLine in of
                SOME line => (print line; readResponse())
              | NONE => ()
    in
        readResponse();
        Socket.close sock
    end
    handle e => (print ("Error: " ^ exnMessage e ^ "\n"); Socket.close sock)

(* Main function *)
fun main() =
    httpGet "http://example.com"

(* Run the program *)
val _ = main()

This Standard ML example demonstrates how to create a simple HTTP client that sends a GET request and prints the response. Here’s a breakdown of the code:

1. We import necessary modules: Basis.sml, Socket.sml, and TextIO.sml. These provide basic functionality, socket operations, and text I/O operations, respectively.

2. The httpGet function takes a URL as a string and performs the following steps:

   • Parses the URL to extract the host, port, and path.
   • Creates a TCP socket and connects to the server.
   • Sends an HTTP GET request to the server.
   • Reads and prints the response.

3. The main function calls httpGet with a sample URL (“http://example.com”).

4. Error handling is implemented using a handle clause, which catches any exceptions and prints an error message.

To run this program:

1. Save the code in a file, e.g., http_client.sml.

2. Make sure you have a Standard ML implementation installed (like MLton or SML/NJ).

3. Compile and run the program:

   For MLton:

   $ mlton http_client.sml
   $ ./http_client

   For SML/NJ:

   $ sml http_client.sml

This example demonstrates basic network programming in Standard ML, including socket operations, text I/O, and error handling. It’s important to note that Standard ML doesn’t have built-in HTTP client libraries like some modern languages, so we’ve implemented a basic HTTP client from scratch using sockets.

Keep in mind that this is a simplified example and doesn’t handle all aspects of HTTP (like redirects or complex headers). For production use, you might want to use a more robust HTTP client library if available for your Standard ML implementation.