A textual identifier is a nonempty sequence of letters, digits, apostrophes, and underscores, starting with a letter or underscore, that does not form a reserved keyword. An identifier starting with an underscore or capital letter is called a variable. Otherwise, an identifier is called a symbol if it has been assigned some special meaning and a name if it has not.
The variable `_' has a special meaning: distinct instances are taken to refer
to different, ``anonymous'' variables. Accordingly, `_' is called the
anonymous variable.
An infix identifier is a sequence of the following symbols:
| * + < > = - & ^ $ @ ! ~ ?that does not form a reserved operator. Infix identifiers (and their arity and precedence) can be user-defined.
Enclosing an infix identifier in parentheses ("(+)") means it is treated as a (prefix) textual identifier. Conversely, enclosing a textual identifier in backquotes ("`sym`") means it is treated as an infix operator.
A namespace-qualified identifier is a sequence of textual identifiers separated by "asdf" and terminated by either a textual or infix identifier. For example, "List.mem" and "Int.+" are legal namespace identifiers. A namespace identifier whose subject is infix is treated as prefix, not infix.