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

[Aaron Hill]

On 2018-05-18 14:24, Thomas Morley wrote:
> Hi all,
>
> (1)
> consider the following code (working as expected):
>
> \paper {
>   #(if #t (set-paper-size "a8" 'landscape) #(set-paper-size "a8"))
> }
>
> \score { { R1 } \layout { ragged-right = ##f } }
>
> Switching from #t to #f results in different paper-size, as desired.
>
> (2)
> But trying to put it in a procedure, it always returns the true-case:
>
> #(define (proc bool x y)
>   (if bool x y))
>
> \paper {
>   #(proc #f (set-paper-size "a8" 'landscape) #(set-paper-size "a8"))
> }
>
> \score { { R1 } \layout { ragged-right = ##f } }
>
> Paper-size is always a8-landscape.
>
> (3)
> Trying:
> #(define (proc bool x y)
>   (if bool x y))
>
> \paper {
>   #(apply set-paper-size (proc #t '("a8" 'landscape) '("a8")))
> }
>
> \score { { R1 } \layout { ragged-right = ##f } }
>
> Paper-size is always a8
>
> ---------
>
> What's happening here and why?
> And how to make a procedure accepting set-paper-size work, with
> different settings and an if-condition?

My Scheme is a little rusty over the years, but I will try to explain 
what is going on.

Firstly, `if` has a rule that it only evaluates either the true-value or 
false-value based on the Boolean.  That means exactly one of the two 
expressions will evaluate.

Your `proc` function does not have this behavior, as the arguments 
passed in will be evaluated before you get to the inner `if`.  Now, 
`set-paper-size` has a side-effect, so the evaluation of that function 
alone is enough to have an impact.  Technically, the function should be 
named `set-paper-size!`, as the convention is to suffix an exclamation 
to indicate such functions.

So, calling `proc` with the two functions results in both being 
evaluated, which is not what you want.

Your `apply` approach is closer to what you want, since you are using 
`proc` as a means of selecting the arguments you want and calling 
`set-paper-size` only once.  This should work, except you have an extra 
quote.

%%%%
  #(apply set-paper-size (proc #t '("a8" landscape) '("a8")))
%%%%

The outer quote for invoking the list shorthand already results in 
`landscape` being a symbol.  The extra quote would put another layer of 
indirection, which `set-paper-size` does not expect.

Hope this helps,

-- Aaron Hill