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Nested quasiquoting and Scheme embedded in #{ #}

From: Jean Abou Samra
Subject: Nested quasiquoting and Scheme embedded in #{ #}
Date: Wed, 10 Aug 2022 13:32:41 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.12.0


A little request for feedback from Scheme power users:

Consider the following snippet from David K.

\version "2.23.11"

#(define-macro (pattern args result)
   `(define-music-function (parser location ,@args) ,(make-list (length args) 'ly:music?)
      #{ $@(list ,@result) #}))

$(pattern (A B C D) (A B D A C D)) { a' a' a' a' } { b' b' b' b' } { c'' c'' c'' c'' } { d'' d'' d'' d'' }

I was astounded that this works, but it does work. When you
quasiquote a S-expression containing embedded LilyPond #{ ... #},
and include a Scheme expression #... inside this, and use an unquote
inside that nested Scheme expression, it really gets substituted.

In the discussion
we (developers) are considering a change in the implementation of
#{ ... #} that would have benefits, but it would not work with this sort
of case. One would have to use plain Scheme here, replacing

#{ $@(list ,@result) #}


(make-sequential-music (list ,@result))

The benefits of the change would be making the parsing more robust for
situations like a # in a LilyPond % comment within #{ ... #}, which should
not be interpreted as a Scheme expression.  It would also simplify the

Note that you can't really do something useful with a quoted #{ ... #}
apart from evaluating it, so this use case is specific to macro contexts.

I believe it's OK calling this a very special case and no longer
supporting it, but I'd like to be sure. Would you have thought of
doing something like this? Would you have expected it to work?


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