member type specifier can be used to represent
“symbolic” values, analogous to the enumerated types of Pascal. For
example, the second value of
find-symbol has this type:
(member :internal :external :inherited nil)
Member types are very useful for expressing consistency constraints on data structures, for example:
(defstruct ice-cream (flavor :vanilla :type (member :vanilla :chocolate :strawberry)))
Member types are also useful in type inference, as the number of members can sometimes be pared down to one, in which case the value is a known constant.