Input/Output json file in lisp - json

Question

Good morning everyone,
To finish this project i needs your help, again.
So now i'm trying to create two functions to reading/writing files in lisp.
this is the description how the functions must work
(json-load filename) -> JSON
(json-write JSON filename) -> filename
The json-load function opens the file filename returns a JSON object (or generates an error). If
filename does not exist the function generates an error. The suggestion is to read the whole file in one
string and then to call json-parse.
The json-write function writes the JSON object to the filename file in JSON syntax. If
filename does not exist, it is created and if it exists it is overwritten. Of course it is expected that
CL-PROMPT> (json-load (json-write '(json-obj # | stuff | #) "foo.json"))
(json-obj # | stuff | #)
this is my json-load function
(defun json-load (filename)
(with-open-file (file-stream filename)
(let ((file-contents (make-string (file-length file-stream))))
(read-sequence file-contents file-stream)
file-contents)) (json-parse (file-contents)))
but it not working
i need some help to write function too.
thanks guys
edit 1:
(defun json-load (filename)
(with-open-file (in filename
:direction :input
:if-does-not-exist :error)
(file-get-contents filename))
(json-parse filename))
(defun file-get-contents (filename)
(with-open-file (stream filename)
(let ((contents (make-string (file-length stream))))
(read-sequence contents stream)
contents)))
so the function should be not far away to be correct but the problem, i think, is the file-get-contents function.
I think that because if i run this function the output is
"\"{\\\"nome\\\" : \\\"Arthur\\\",\\\"cognome\\\" : \\\"Dent\\\"}\""
and so the json-parse does not recognize json-object anymore.
Any ideas?
edit 2:
i try both functions but with the same result. if i call json-parse with the same json-object in the file it's all right but if i call json-load lisp respond me with my own error message "undefined JSON object (json-parse)".
Why?
Edit 3:
This is json-write function but, for now, it doesn't work.
(defun json-write (json filename)
(with-open-file (out filename
:direction :output
:if-exists :overwrite
:if-does-not-exist :create)
(pprint (json out))))
so the description at the beginning of the post says that the json-write function writes JSON object to the filename file in JSON syntax.
Now, 2 questions
1) it's my function partially correct?
2) how can i write a Json object in Json syntax?
Thanks

Answer 1

I'm working on the same project, hopefully the professors don't mind us sharing info ;)
This is the approach I took:
(defun json-load (filename)
(with-open-file (in filename
:direction :input
:if-does-not-exist :error)
(multiple-value-bind (s) (make-string (file-length in))
(read-sequence s in)
(json-parse s))))
Remember that read-sequence overwrites the given sequence, in this case s. I'm using multiple-value-bind simply so that I don't have to use neither variable declarations nor a lambda function (Although it is just a less idiomatic version of (let ((v form)) ...)), as #tfb pointed out).