compilation error patterns, how to add new ones

compilation error patterns, how to add new ones

[Ian Zimmerman]

What is the proper way for an external package (that is, one not
shipping with emacs) to add new error patterns?  Adding patterns
directly to compilation-error-regexp-alist seems a no-no, not only
because it messes up the value to make it unreadable but also because
that variable is a customization item.  So I tried to just add my
pattern to compilation-error-regexp-alist-alist unconditionally and
leave it to the user if she adds the symbol to the customizable list.

But now there is a new problem: if the symbol is in fact added and a
compilation runs before my package is loaded (compiling files in a
different language, for example), emacs throws up because it tries to
use the result of looking up the pattern in
compilation-error-regexp-alist-alist, and that is nil.

It seems like the only way is to give users a snippet of code to run at
init time, beyond just autoloading, and I was trying to avoid that.

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