Re: Scheme help request: How can else of an if-statement be made to do nothing?

[Aaron Hill]

On 2020-12-11 10:43 am, Matthew Fong wrote:
> Hello everyone,
>
> Resurrecting an old thread. I've been trying my hand in Scheme 
> programming,
> via small examples on the project I'm working on.
>
> I wanted to extend the variable list to this function Harm wrote, to 
> take
> an extra boolean variable, which is pretty trivial. The boolean is 
> meant to
> change the color of the text depending if it's true or false. But I am 
> not
> clear on how to define the color using the boolean. The trivial way to 
> do
> it is adding a conditional under the first if, and duplicate all the 
> code
> except the color. Is there a proper Scheme-ish way to do this?
> (Admittingly, I have not caught on to Scheme just yet -- my brain tends 
> to
> work with Python)
>
> print-if-defined =
> #(define-void-function (sym text) (symbol? markup?)
>   (if (defined? sym)
>       (add-text #{ \markup \with-color #'(0.8 0.2 0.2) #text #})))
>
> symA = "Something"
>
> \print-if-defined #'symB "Text"
> \print-if-defined #'symA "Text"

Unless I am missing something from your specific use case, you should 
only need to nest another (if ...) expression in place of the actual 
color:

%%%%
print-if-defined =
#(define-void-function (foo sym text) ((boolean? #f) symbol? markup?)
  (if (defined? sym)
      (add-text #{ \markup \with-color
        #(if foo '(0.8 0.2 0.2) '(0.2 0.8 0.2))
        #text #})))

symA = "Something"

\print-if-defined #'symB "Text"     % hidden
\print-if-defined #'symA "Text"     % shown, green
\print-if-defined ##t #'symA "Text" % shown, red
%%%%


-- Aaron Hill