Enumeration variables

DyALog provides Enumeration Variables, i.e. variables that may take their values from some defined enumeration. For instance, the term X::tense[present,past] denotes a variable X with value in the sub-enumeration [present,past] of some user-defined enumeration tense. Note the use of enclosing [] instead of enclosing ().

To associate an enumeration to a symbole, use the directive finite_set/2.

     :-finite_set(tense,[present,past,futur]).
     

It is also possible to define sub-enumeration using the directive subset/2

     :-finite_set(letter,[a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,
                          q,r,s,t,u,v,w,x,y,z ]).
     :-subset(voyel,letter[a,e,i,o,u,y])
     

Enumeration are restricted to at most 30 elements. These elements should be ground objects. Enumeration variable may be unified with variables, enumeration variables based on the same enumeration and with elements of their enumeration.