TypeTag

TypeTag is an enumeration that corresponds with the basic type structure of Avail's type lattice. Even though the type lattice contains an infinite collection of infinitely long chains of types, some of which have an infinite number of direct ancestors and direct descendants, we're still able to extract a pragmatic tree of types from the lattice.

Since this restricted set of types form a tree, they're defined in such an order that all of a type's descendants follow it. Since this is recursively true, the types are effectively listed in depth-last order. The ordinals are assigned in the order of definition, but each type keeps track of the maximum ordinal of all of its descendants (which occupy a contiguous span of ordinals just after the type's ordinal). We can test if type A is a subtype of B by checking if a.ordinal ≥ b.ordinal and a.highOrdinal ≤ b.highOrdinal. For a proper subtype test, we turn the first condition into an inequality.

Author

Mark van Gulik