How do you access get parameters in racket server application

Question

I have a project in my Organization of Programming Languages course and am building a web API using Racket. I have managed to get a server up and running that has multiple landing pages such as

localhost:8080/api/add 
localhost:8080/api/subtract

I am new to Racket and have just used PHP in the past to handle GET parameter passing via the URL.

such as...

localhost:8080/api/add/?num1=1&num2=3

num1 would be accessed by PHP with

$_GET[ 'num1' ] ;

How should this be done in Racket? I have not found anything in the Racket documentation that shows an equivalent method. My final intent is to pass JSON strings as the GET parameter JSON

localhost:8080/api/add/?json={ some json }

Answer 1

Ok so I found the solution in

POST/GET bindings in Racket

essentially to get

localhost:8080/api/add/?json={ some json }

you would use

(bytes->string/utf-8 
    (binding:form-value 
        (bindings-assq (string->bytes/utf-8 "[field_name_here]") 
                       (request-bindings/raw req))))

Now this answers my question but lets me run into exceptions when the binding searched for is not present, so I decided to take it an extra step and perform an existence check. In PHP I want to perform something like...

<?php 

    function get_param_as_string( $param ) {
        if ( isset( $_GET[ $param ] ) )
            return $_GET[ $param ] ;
        return "" ;
    }

?>

So I created the racket procedure

(define (get-param->string req param)
    (if (eq? #f (bindings-assq (string->bytes/utf-8 param)
                               (request-bindings/raw req)))
        ""
        (bytes->string/utf-8 
           (binding:form-value 
               (bindings-assq (string->bytes/utf-8 param)
                              (request-bindings/raw req))))))

Now as in the case specified by the question of the url

localhost:8080/api/add/?json={ some json }

The following would retrieve the json GET variable if present or would return "" if missing. so...

(get-param->string req "json")

would result in...

{ some json }

and

localhost:8080/api/add/?tmp={ some json }

or

localhost:8080/api/add/

would result in...

""