2.4.7 Revelations

A revelation introduces information about an opaque type into a scope. Unlike other declarations, revelations introduce no new names.

There are two kinds of revelations, partial and complete. A program can contain any number of partial revelations for an opaque type; it must contain exactly one complete revelation.

A partial revelation has the form:

    REVEAL T <: V

where V is a type expression (possibly just a name) and T is an identifier (possibly qualified) declared as an opaque type. It reveals that V is a supertype of T.

In any scope, the revealed supertypes of an opaque type must be linearly ordered by the subtype relation. That is, if it is revealed that T <: U1 and T <: U2, it must also be revealed either that U1 <: U2 or that U2 <: U1.

A complete revelation has the form:

    REVEAL T = V

where V is a type expression (not just a name) whose outermost type constructor is a branded reference or object type and T is an identifier (possibly qualified) that has been declared as an opaque type. The revelation specifies that V is the concrete type for T. It is a static error if any type revealed in any scope as a supertype of T is not a supertype of V. Generally this error is detected at link time.

Distinct opaque types have distinct concrete types, since V includes a brand and all brands in a program are distinct.

A revelation is allowed only in an interface or in the outermost scope of a module. A revelation in an interface can be imported into any scope where it is required, as illustrated by the stack example.

For example, consider:

    INTERFACE I; TYPE T <: ROOT; PROCEDURE P(x:T): T; END I.

    INTERFACE IClass; IMPORT I; REVEAL I.T <: MUTEX; END IClass.

    INTERFACE IRep; IMPORT I;
      REVEAL I.T = MUTEX BRANDED OBJECT count: INTEGER END;
    END IRep.

An importer of I sees I.T as an opaque subtype of ROOT, and is limited to allocating objects of type I.T, passing them to I.P, or declaring subtypes of I.T. An importer of IClass sees that every I.T is a MUTEX, and can therefore lock objects of type I.T. Finally, an importer of IRep sees the concrete type, and can access the count field.