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For the benefit of everybody else, #+ does feature checking. The Common Lisp standard defines a standard "special variable" (kind of like a global) named * fea

For the benefit of everybody else, #+ does feature checking. The Common Lisp standard defines a standard "special variable" (kind of like a global) named * features * (shouldn't be spaces, but HN italicizes it otherwise), and #+ (and #-) take a "feature expression" and conditionally execute the expression that immediately follows it when the feature expression is true. If the feature check is false, the next expression is ignored. For example: #+freebsd(print "This is FreeBSD") #+(not (and 64-bit linux))(print "This is not 64-bit Linux.") #+quicklisp (ql:quickload :some-library) The CL standard lists a few standard features (:common-lisp, :ieee-floating-point, etc.) but for the most part implementations and libraries define their own. It's not perfect, but works well enough in practice.

TeMPOraL 6y ago
The common thing done with it is: #+nil(this expression (won't be read)) (Also done with #+and, and #+or.) This is an unofficial Common Lisp s-expression level comment. One of these incredibly nice things that's missing in most programming languages. It comments out a single s-expression, no matter how complex. It's like a #|block comment|# in that it can span multiple lines, and like ; line comment in that you only put something at the beginning. Have a complex expression within a function, that you need to temporarily shut down? Just shove #+nil in front of it. (The #+nil version is discouragd because there's a remote chance somebody will define a * feature * called :nil for some reason.) A less-common thing done with it is library interop. Many libraries tend to add a tag to * features * to indicate they've been loaded, so in your library, you can use #+library_tag to detect that another library is a part of the same project, and conditionally compile-in some extensions or interoperability code. (@dang, I think HN could use a way to escape the * * formatting marks. For a site and community that has a soft spot for Lisp, not being able to type in the "earmuffs" is something that always bewildered me.)
- rjsw 6y ago
  I would guess that there was a feature :nil defined in the NIL [1] system. [1] https://en.wikipedia.org/wiki/NIL_(programming_language) https://en.wikipedia.org/wiki/NIL_(programming_language)
  - TeMPOraL 6y ago
    That must be it, thanks!

TeMPOraL 6y ago

The common thing done with it is: #+nil(this expression (won't be read)) (Also done with #+and, and #+or.) This is an unofficial Common Lisp s-expression level comment. One of these incredibly nice things that's missing in most programming languages. It comments out a single s-expression, no matter how complex. It's like a #|block comment|# in that it can span multiple lines, and like ; line comment in that you only put something at the beginning. Have a complex expression within a function, that you need to temporarily shut down? Just shove #+nil in front of it. (The #+nil version is discouragd because there's a remote chance somebody will define a * feature * called :nil for some reason.) A less-common thing done with it is library interop. Many libraries tend to add a tag to * features * to indicate they've been loaded, so in your library, you can use #+library_tag to detect that another library is a part of the same project, and conditionally compile-in some extensions or interoperability code. (@dang, I think HN could use a way to escape the * * formatting marks. For a site and community that has a soft spot for Lisp, not being able to type in the "earmuffs" is something that always bewildered me.)

rjsw 6y ago
I would guess that there was a feature :nil defined in the NIL [1] system. [1] https://en.wikipedia.org/wiki/NIL_(programming_language) https://en.wikipedia.org/wiki/NIL_(programming_language)
- TeMPOraL 6y ago
  That must be it, thanks!