# <</2

## Description

    Object<<Goal
    {Proxy}<<Goal

Debugging control construct. Calls a goal within the context of the specified object. The goal is called with the execution context (sender, this, and self) set to the object. The goal may need to be written between parenthesis to avoid parsing errors due to operator conflicts. This control construct should only be used for debugging or for writing unit tests. This control construct can only be used for objects compiled with the context_switching_calls compiler flag set to `allow`. Set this compiler flag to `deny` to disable this control construct and thus preventing using it to break encapsulation.

The `{Proxy}<<Goal` syntax allows simplified access to parametric object proxies. Its operational semantics is equivalent to the goal conjunction `(call(Proxy), Proxy<<Goal)`. I.e. `Proxy` is proved within the context of the pseudo-object user and, if successful, the goal term is used as a parametric object identifier. Exceptions thrown when proving `Proxy` are handled by the `<</2` control construct. This syntax construct supports backtracking over the `{Proxy}` goal.

Caveat: although the goal argument is fully compiled before calling, some necessary information for the second compiler pass may not be available at runtime.

## Modes and number of proofs

    +object_identifier<<+callable - zero_or_more
    {+object_identifier}<<+callable - zero_or_more

## Errors

Either Object or Goal is a variable:
`instantiation_error`
Object is neither a variable nor a valid object identifier:
`type_error(object_identifier, Object)`
Goal is neither a variable nor a callable term:
`type_error(callable, Goal)`
Object does not contain a local definition for the Goal predicate:
`existence_error(procedure, Goal)`
Object does not exist:
`existence_error(object, Object)`
Object was created/compiled with support for context switching calls turned off:
`permission_error(access, database, Goal)`
Proxy is a variable:
`instantiation_error`
Proxy is neither a variable nor an object identifier:
`type_error(object_identifier, Proxy)`
The predicate Proxy does not exist in the user pseudo-object:
`existence_error(procedure, ProxyFunctor/ProxyArity)`

## Examples

    % call the member/2 predicate in the
    % context of the "list" object:
    test(member) :-
        list << member(1, [1]).