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Re: Expansion of #$ in byte-compiled files

From: Stefan Monnier
Subject: Re: Expansion of #$ in byte-compiled files
Date: Thu, 03 Sep 2015 22:12:04 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/25.0.50 (gnu/linux)

> Please correct me if I am not right here, but it appears that in
> byte-compiled files (don't always?) expand #$ to the current elisp
> file name.  I wrote a module containing the following code:

>   (message "x: %S" #$)

The issue here is that #$ is a "reader macro", so it is replaced by the
content of load-file-name when this expression is *read*.  When the .el
file is loaded, this expression is read when load-file-name has the
intended value, but when it is compiled, this expression is read during
byte-compilation (i.e. not during any kind of "load"ing) when
load-file-name is nil.  So #$ will be read as if nil appeared there in
the file.  Hence the byte-compiler will generate code which behaves just like

   (message "x: %S" nil)

-- Stefan

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