In Standard ML, there are syntactically two kinds of identifiers.

• Alphanumeric: starts with a letter or prime (') and is followed by letters, digits, primes and underbars (_).

  Examples: abc, ABC123, Abc_123, 'a.

• Symbolic: a sequence of the following

   ! % & $ # + - / : < = > ? @ | ~ ` ^ | *

  Examples: +=, <=, >>, $.

With the exception of =, reserved words can not be identifiers.

There are a number of different classes of identifiers, some of which have additional syntactic rules.

• Identifiers not starting with a prime.

• value identifier (includes variables and constructors)

• type constructor

• structure identifier

• signature identifier

• functor identifier

• Identifiers starting with a prime.

• type variable (must start with prime)

• Identifiers + numeric labels (1, 2, ...).

• record label