Detecting that something is empty

Suppose you need to know that the argument of your command is empty: that is, to distinguish between \cmd{} and \cmd{blah}. This is pretty simple:

The case where you want to ignore an argument that consists of nothing but spaces, rather than something completely empty, is more tricky. It’s solved in the code fragment ifmtarg, which defines commands \@ifmtarg and \@ifnotmtarg, which examine their first argument, and select (in opposite directions) their second or third argument. The package’s code also appears in the LaTeX memoir class.

Ifmtarg makes challenging reading; there’s also a discussion of the issue in number two of the “around the bend” articles by the late lamented Mike Downes.

Around the bend series