xexpr cond failing

[liendolucas84]

Hi everyone! Just joined the Racket User's group. I'm trying to conditionally render a paragraph element using a cond and an xexpr but I can't make the "xexpr?" function return a #t value and of course the response/xexpr function call fails as well. I've kind of fix this by always returning a paragraph element regardless is needed or not (of course this is not what I want). Here's the working code:

(define (login-template (login-error ""))
  `(html
     (head
       (link ((rel "icon") (href "data:,")))
       (title "Please login"))
     (body
       (form ((action "") (method "post"))
             (label ((for "username")) "Username:")
             (input ((type "text") (name "username") (maxlength "64")))
             (label ((for "password")) "Password:")
             (input ((type "password") (name "password") (maxlength "64")))
             (button ((type "submit") (value "Login")) "Login"))
       ,(cond
          ((not (eq? login-error "")) `(p, login-error))
          (else '(p "Enter your username/password."))))))

However I don't want to display at all the "else" clause as is not needed at all, here's the failing code:

(define (login-template (login-error ""))
  `(html
     (head
       (link ((rel "icon") (href "data:,")))
       (title "Please login"))
     (body
       (form ((action "") (method "post"))
             (label ((for "username")) "Username:")
             (input ((type "text") (name "username") (maxlength "64")))
             (label ((for "password")) "Password:")
             (input ((type "password") (name "password") (maxlength "64")))
             (button ((type "submit") (value "Login")) "Login"))
       ,(cond
          ((not (eq? login-error "")) `(p, login-error))))))

The last example fails if the login-error is indeed "" (an empty string). Am I missing something like a "null xexpr" usage for the "else" clause or something different that needs to be used in these situations? Of the all examples out there that I could find and read none of them shows how to do something like this. Is there a doc page where can I read about caveats using xexprs (if any) or situations like the one I've described?

Thanks for your time,

Lucas.

[Sorawee Porncharoenwase]

One possible way is to have the first case returns a list of one element, and the else case returns an empty list. You then splice this list into body via ,@.

`(body
  (form ((action "") (method "post"))
        (label ((for "username")) "Username:")
        (input ((type "text") (name "username") (maxlength "64")))
        (label ((for "password")) "Password:")
        (input ((type "password") (name "password") (maxlength "64")))
        (button ((type "submit") (value "Login")) "Login"))
  ,@(cond
      [(not (eq? login-error "")) `((p ,login-error))]
      [else '()]))

If the then branch is taken, the value will be:

`(body
  (form ((action "") (method "post"))
        (label ((for "username")) "Username:")
        (input ((type "text") (name "username") (maxlength "64")))
        (label ((for "password")) "Password:")
        (input ((type "password") (name "password") (maxlength "64")))
        (button ((type "submit") (value "Login")) "Login"))
  (p ,login-error))

On the other hand, if the else branch is taken, the value will be:

`(body
  (form ((action "") (method "post"))
        (label ((for "username")) "Username:")
        (input ((type "text") (name "username") (maxlength "64")))
        (label ((for "password")) "Password:")
        (input ((type "password") (name "password") (maxlength "64")))
        (button ((type "submit") (value "Login")) "Login")))

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[liendolucas84]

Thank you very much for your reply Sorawee. I'll go with your proposed solution for the time being. However it feels like I'm adding unnecessary code to make the xexpr valid which can be really annoying if having nested cases or other more complex situations. Thanks again for your fast reply!

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[Sorawee Porncharoenwase]

Well, you can always use macro!

(define-syntax-rule (when/splice test body ...)
  (if test (list (begin body ...) '())))

`(body
  (form ((action "") (method "post"))
        (label ((for "username")) "Username:")
        (input ((type "text") (name "username") (maxlength "64")))
        (label ((for "password")) "Password:")
        (input ((type "password") (name "password") (maxlength "64")))
        (button ((type "submit") (value "Login")) "Login"))
  ,@(when/splice (not (eq? login-error ""))
      `(p ,login-error)))

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[Sorawee Porncharoenwase]

Actually you might want to use (list (let () body ...)) rather than (list (begin body ...)) so that you can use define inside when/splice, just like how you can do that in when.