Enumerated Types
Enumerated type declarations take the form
def LHS ::= RHS
where
LHS is either an atom, the name of the type, or a compound term with distinct type variables as arguments and
RHS is an enumeration or a range of integers.
So, for example,
def man ::= roger | tom | bill | alan | graham | keith |
sam | fred
def woman ::= june | mary | rose | penny | sue | helen |
veronica
def noun ::= "boy" | "fox" | "girl" | "ball" | "man" |
"woman" | "lady"
declares
man,
woman and
noun to be the names of types. The right hand sides enumerate the possible values of these types.
It's also possible to have enumerations of numbers as above but also another common enumeration for integers is to use a range as in
def age ::= 0 .. 110
def digit ::= 0 .. 9
In all the above enumerations, all the values are atomic. It it also possible to have compound terms (constructors) in enumerations as in
def tree(T) ::= empty() | tr(tree(T),T,tree(T))
def noun_phrase_tree::= np(article,noun_exp_tree)
The first declaration above is a recursive polymorphic type declaration. All the arguments in the constructors on the right hand side must be types (or type variables listed as arguments on the left hand side).
