Keyword: destructor

Typically when a type is declared as having a specific struct definition, a set of implicit "destructor functions" are declared to extract the fields of it.

A type may have its fields hidden instead, using a private destructor function (declared with destructor) that provides access to its fields only for code that can see the private destructor.

Example:

type num 

implementation {
    type integer
    interpret integer -> int
    destructor repr(N:num) : integer
}

In this example, code that can see the implementation will see the type num as interpreted as the type:

struct num {
    repr: integer
}

Outside the implementation, the type num will be uninterpreted.