pl_search::Engine Class Reference

The Engine class manages the execution of predicates and backtracking. More...

#include <engine.hpp>

Public Member Functions
	Engine ()
bool	unify (TermPtr t1, TermPtr t2)
	Unifies two terms.
bool	execute (PredPtr p, bool unbind)
	Executes a predicate.

Private Member Functions
void	trail (PVarPtr v)
	Trails a variable.
void	backtrack ()
	Performs backtracking.
bool	call_predicate (PredPtr p)
	Call a predicate.
bool	retry_predicate (PredPtr p)
	Retries the call on a predicate.
bool	make_choice_and_continue (PredPtr p)
	Makes the current choice, checks it, and continues execution (using the predicates continuation) if the choice is valid.
void	push (PredPtr p)
	Pushes a predicate onto the environment stack.
void	pop_pred_call ()
	Pops the top predicate call from the environment stack.
void	cut_to_choice_point (int env_index)
	Cuts the environment stack to the choice point at the given index.
void	clear_stacks ()
	Clears the environment and trail stacks.

Private Attributes
std::stack< trail_entry * >	trail_stack
	Stack for trail entries.
std::stack< env_entry * >	env_stack
	Stack for environment entries.

Friends
class	Cut
class	DisjPred
class	IfThenElse
class	::EngineTest
void	Pred::wrap_with_once ()

Detailed Description

The Engine class manages the execution of predicates and backtracking.

Execution uses a continuation-passing style for predicates that represent a conjunction of goals. The engine implements backtracking and choice points.

Constructor & Destructor Documentation

Engine()

pl_search::Engine::Engine ( )

inline

Member Function Documentation

backtrack()

void pl_search::Engine::backtrack ( )

private

Performs backtracking.

call_predicate()

bool pl_search::Engine::call_predicate ( PredPtr  p )

private

Call a predicate.

If the call is non-deterministic, the predicate is pushed onto the environment stack before the call. Otherwise the is not pushed and the call is made directly.

Parameters

p	The predicate to call.

Returns

True if the call succeeds, false otherwise.

clear_stacks()

void pl_search::Engine::clear_stacks ( )

private

Clears the environment and trail stacks.

cut_to_choice_point()

void pl_search::Engine::cut_to_choice_point ( int  env_index )

private

Cuts the environment stack to the choice point at the given index.

Parameters

env_index

The index to cut back to.

execute()

bool pl_search::Engine::execute	(	PredPtr	p,
		bool	unbind
	)

Executes a predicate.

Parameters

p	The predicate to execute.
unbind	Whether to unbind variables after execution.

Returns

True if the execution succeeds, false otherwise.

make_choice_and_continue()

bool pl_search::Engine::make_choice_and_continue ( PredPtr  p )

inlineprivate

Makes the current choice, checks it, and continues execution (using the predicates continuation) if the choice is valid.

Parameters

p	The predicate to make a choice on.

Returns

True if the choice is valid and the predicates continuation succeeds, false otherwise.

pop_pred_call()

void pl_search::Engine::pop_pred_call ( )

private

Pops the top predicate call from the environment stack.

push()

void pl_search::Engine::push ( PredPtr  p )

private

Pushes a predicate onto the environment stack.

Parameters

p	The predicate to push.

retry_predicate()

bool pl_search::Engine::retry_predicate ( PredPtr  p )

private

Retries the call on a predicate.

If the predicate has no more choices, it is popped from the environment stack. Otherwise, the predicate is called. Note that only predicates that have been pushed onto the environment stack can be retried (as they are non-deterministic).

Parameters

p	The predicate to retry.

trail()

void pl_search::Engine::trail ( PVarPtr  v )

private

Trails a variable.

Parameters

v	The variable to trail.

unify()

bool pl_search::Engine::unify	(	TermPtr	t1,
		TermPtr	t2
	)

Unifies two terms.

Parameters

t1	The first term.
t2	The second term.

Returns

True if the terms unify, false otherwise.

Friends And Related Symbol Documentation

::EngineTest

friend class ::EngineTest

friend

Cut

friend class Cut

friend

The following predicates need access to private attributes.

DisjPred

friend class DisjPred

friend

IfThenElse

friend class IfThenElse

friend

Pred::wrap_with_once

void Pred::wrap_with_once ( )

friend

Member Data Documentation

env_stack

std::stack<env_entry *> pl_search::Engine::env_stack

private

Stack for environment entries.

trail_stack

std::stack<trail_entry *> pl_search::Engine::trail_stack

private

Stack for trail entries.

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The documentation for this class was generated from the following files:

• include/pl_search/engine.hpp
• src/engine.cpp