On Mon, May 4, 2020 at 9:04 AM Jean Abou Samra <j...@abou-samra.fr> wrote: > > Date: Mon, 4 May 2020 12:21:30 +0200 > From: Jean Abou Samra <j...@abou-samra.fr> > To: lilypond-devel@gnu.org > Subject: Question (define and set!) > Message-ID: <8f79371e-400b-574b-3126-146d008cf...@abou-samra.fr> > Content-Type: text/plain; charset=utf-8; format=flowed > > Hi, > > Why do you often find constructs similar to this one in LilyPond's source? > > midi.scm line 26: > > (define-session-public absolute-volume-alist '()) > (set! absolute-volume-alist (append absolute-volume-alist etc.)) > > line 72: > > (define instrument-names-alist '()) > (set! instrument-names-alist ... > > Is there any difference with (define-session-public > absolute-volume-alist '(...)) ? I suspect there is one since this is > frequent but I can't figure out what the purpose is here.
Part of it is likely due to a subtle issue in Scheme. set! already assumes that the variable exists; define not only gives it a value, but tells Scheme that the variable actually represents something. Thus (define x (1+ x)) creates an error, because it assumes that x is undefined, but is trying to use the value of the (nonexistent) variable x. The proper command is set!. More information is available at: https://stackoverflow.com/questions/5406064/difference-between-define-let-and-set --- Christopher Heckman
