Re: simple scheme in lilypond

[David Kastrup]

Gianmaria Lari <address@hidden> writes:

> Probably these are very trivial things. Sorry to ask them but would like to
> understand.
>
> If I write (1):
>
>
> \version "2.19.82"
> $(object->string (+ 3 2))
>
>
> Lilypond is happy and compile to a pdf file showing "5".
>
> Even this simpler code works... (2)
>
> \version "2.19.82"
> $"Hello"
>
>
> and generate a pdf file showing "Hello".
>
> It is clear to me that $ introduce a scheme expression that is evaluated.
> And in the previous two examples the scheme code is evaluated to strings.
> But I don't understand why lilypond outputs them. If I write (3):
>
> \version "2.19.82"
> "Test"

blabla = "Test"
##f
\blabla

will compile again (sorry for the ##f : it is necessary to defeat
lookahead).  It is indistinguishable from

blabla = \markup "Test"
##f
\blabla

in output and in its effect on the variable.  Basically, $x and \x are
the same (apart from the characters allowed in and after x).  The
logical thing would be to permit "Test" on its own but it appears rather
error-prone.  This is similar to

blabla = c4
##f
\blabla

compiling and producing a result while

c4

on its own doesn't.

> I tried to rewrite (1) and (2) with the hash mark instead of dollar (6):
>
> \version "2.19.82"
> #(object->string (+ 3 2))
>
>
> and (7):
>
> \version "2.19.82"
> #"Hello"
>
>
> This time, there is no output in the pdf file. Why this difference?

At the top level of the file, assignments and assignment-like statements
may be executed.  The return value of an assignment in Scheme is
unspecified.  There are things like

#(set! (hashq-ref x y) z)

that somewhat inscrutably return an actual value other than
*unspecified*.  For that reason, # expressions in contexts where
assignments are allowed are evaluated and ignored.  Use $ if you want
them to be heeded.

-- 
David Kastrup