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)))
On Sun, Mar 1, 2020 at 12:50 AM liendolucas84 <[email protected]>
wrote:
> 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!
>
> On Sunday, March 1, 2020 at 7:43:20 PM UTC+11, Sorawee Porncharoenwase
> wrote:
>>
>> 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")))
>>
>>
>> On Sun, Mar 1, 2020 at 12:31 AM liendolucas84 <[email protected]>
>> wrote:
>>
>>> 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.
>>>
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>>>
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