How do I serialize an object into JSON including defined variables in Racket?

Question

I am trying to to write a json structure to a file using racket. The actual function for writing works fine but when creating the data structure which I write to the file, I simply cannot figure out how to include values from bound variables.

    (start_location . "h")
    (end_location . "t")
    (ETA . 30)
    (route . ("g"))
    (optional_parameters . #hasheq((accessibility . #f)))
))

(define write-json-wrapper (lambda (jsexpr filename)
    (call-with-output-file filename (lambda (x) (write-json jsexpr x)) #:exists 'replace)
))  

The above structure works when I write it to the file.

(define x "h")
(define b #hasheq(
    (start_location . x)
    (end_location . "t")
    (ETA . 30)
    (route . ("g"))
    (optional_parameters . #hasheq((accessibility . #f)))
))```

However when I try to use the value stored in x as the value for start_location it throws an error.

write-json: expected argument of type <legal JSON value>; given: 'x

Any help or explanation would be much appreciated.

Answer 1

#hasheq is literal for hash table and its content isn't evaluated:

#hasheq((a . 5) (b . 7)) reads equal to (make-...eq '((b . 7) (a . 5)))

If you want to use values of variables, you have to use make-hasheq like this:

#lang racket
(require json)

(define x 1)
(define y 2)
(define z 3)

(define jsexpr
  (make-hasheq (list (cons 'x x)
                     (cons 'y y)
                     (cons 'z z))))

(define write-json-wrapper (lambda (jsexpr filename)
    (call-with-output-file filename
      (lambda (x) (write-json jsexpr x)) #:exists 'replace))) 

Example:

(write-json-wrapper jsexpr "foo.txt")

If you're familiar with quasiquote, you can do also this:

(define jsexpr
  (make-hasheq `((x . ,x)
                 (y . ,y)
                 (z . ,z))))

So for your data:

(make-hasheq `((start_location . ,x)
                 (end_location . "t")
                 (ETA . 30)
                 (route . ("g"))
                 (optional_parameters . #hasheq((accessibility . #f)))))