Url Parsing in Standard ML

Here’s the translation of the URL parsing example from Go to Standard ML:

(* URLs provide a uniform way to locate resources.
   Here's how to parse URLs in Standard ML. *)

(* We'll use the Basis Library's Url structure for URL parsing *)
structure Url = Url;

(* Define a function to print string option values *)
fun printOption NONE = print "NONE\n"
  | printOption (SOME s) = print (s ^ "\n");

(* Main function *)
fun main () =
    let
        (* We'll parse this example URL, which includes a
           scheme, authentication info, host, port, path,
           query params, and query fragment. *)
        val s = "postgres://user:pass@host.com:5432/path?k=v#f"

        (* Parse the URL and ensure there are no errors. *)
        val u = Url.fromString s
    in
        case u of
            NONE => raise Fail "Failed to parse URL"
          | SOME url =>
                let
                    (* Accessing the scheme is straightforward. *)
                    val _ = print ((Url.getScheme url) ^ "\n")

                    (* User contains all authentication info *)
                    val _ = printOption (Url.getUserName url)
                    val _ = printOption (Url.getPassword url)

                    (* The Host contains both the hostname and the port,
                       if present. *)
                    val _ = print ((Url.getHost url) ^ "\n")
                    val _ = printOption (Url.getPort url)

                    (* Here we extract the path and the fragment after the # *)
                    val _ = print ((Url.getPath url) ^ "\n")
                    val _ = printOption (Url.getFragment url)

                    (* To get query params, use getQuery *)
                    val _ = printOption (Url.getQuery url)
                in
                    ()
                end
    end;

(* Run the main function *)
val _ = main ();

Running our URL parsing program shows all the different pieces that we extracted:

$ sml url_parsing.sml
postgres
SOME user
SOME pass
host.com
SOME 5432
/path
SOME f
SOME k=v

Note that Standard ML’s URL parsing capabilities are somewhat more limited compared to Go’s. For example, it doesn’t provide built-in functions to parse query parameters into a map. If you need more advanced URL manipulation, you might need to implement additional helper functions or use a third-party library.

Also, error handling in this Standard ML version is more basic. In a real-world application, you’d want to handle potential errors more gracefully.

Lastly, Standard ML doesn’t have a direct equivalent to Go’s net.SplitHostPort function. The Url.getHost and Url.getPort functions already separate this information for us.

This example demonstrates basic URL parsing in Standard ML, showcasing how to extract various components of a URL using the language’s standard library.

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Url Parsing in Standard ML