Question

What language(s) have comments with side effects? In essence, comments which are not comments….

Answer 1

I can think of several places where comments aren’t really comments.

• HTML and script tags (providing support for browsers that don’t allow or support scripts).

And then, considerably more obscurely:

• IBM Informix 4GL (I4GL) and 4J’s Genero (successor to Informix Dynamic 4GL, D4GL). The notation ‘--#‘ was used by D4GL to include material only applicable to D4GL; I4GL would see that as a comment. The inverse notation was ‘--@‘, which looked like a comment to D4GL but was treated as active material by I4GL.

And, even more obscurely:

• I wrote an I4GL file which was dual-languaged, exploiting I4GL’s multiple comment facilities. Material starting ‘#‘ (hash) marked the start of a comment outside of strings – up to the next newline, as does ‘--‘ (double-dash). Also, ‘{...}‘ (braces) enclose multiline comments.
• The top of the source file was actually a shell script, mostly enclosed in ‘{...}‘ which is, of course, perfectly legitimate in shell. The shell script was a data-driven code generator that copied itself to the top of the output, and then generated about 100 functions which were all depressingly similar but slightly different (in a language without templates or a pre-processor). The code had to validate what was in the database for a given ship against incoming data from an external source (Lloyds of London, in fact), to see what had changed since the last time the external data was received. Non-trivial comparison work, especially since it had to deal with database (SQL) nulls.
• The file was not really a Quine program, but it had some points in common with it. In particular, you could feed the script broken I4GL code and the regenerated file would be perfect again, basically because it ignored the existing I4GL code.