Re: Procedure for set-paper-size in \paper ?

[Aaron Hill]

On 2018-05-18 16:03, Thomas Morley wrote:
> 2018-05-19 0:36 GMT+02:00 Aaron Hill <address@hidden>:
> > On 2018-05-18 14:24, Thomas Morley wrote:
>
> > > #(define (proc bool x y)
> > >   (if bool x y))
>
> > Your `proc` function does not have this behavior, as the arguments 
> > passed in
> > will be evaluated before you get to the inner `if`.
>
> Hm, then I should reword my request.
> Is there a way to circumvent this behaviour?

Yes, you need to use some form of lazy evaluation.  Perhaps the most 
explicit way is to pass in lambdas:

%%%%
  \version "2.19.81"
  #(define (proc bool x y) ((if bool x y)))
  \paper {
    #(proc #t
      (lambda () (set-paper-size "a8" 'landscape))
      (lambda () (set-paper-size "a8")))
  }
  \markup { "Are we landscape or not?" }
%%%%

NOTE: There is an extra set of parentheses in the `proc` body in order 
to evaluate the selected parameter.

Another less verbose option is to simply quote the arguments and `eval` 
them as needed:

%%%%
  #(define (proc bool x y)
    (eval (if bool x y) (interaction-environment)))
  \paper {
    #(proc #t '(set-paper-size "a8" 'landscape) '(set-paper-size "a8"))
  }
%%%%

NOTE: Unlike before, we *need* the extra quote for `landscape` in this 
case, so there is a potential gotcha.

-- Aaron Hill