# Copyright 2011 Jeffrey Kegler
# This file is part of Marpa::PP.  Marpa::PP is free software: you can
# redistribute it and/or modify it under the terms of the GNU Lesser
# General Public License as published by the Free Software Foundation,
# either version 3 of the License, or (at your option) any later version.
#
# Marpa::PP is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
# Lesser General Public License for more details.
#
# You should have received a copy of the GNU Lesser
# General Public License along with Marpa::PP.  If not, see
# http://www.gnu.org/licenses/.

=head1 NAME

Marpa::PP::Semantics::Infinite - How Marpa Deals with Infinite Cycles

=head1 INFINITELY AMBIGUOUS GRAMMARS

Marpa will parse using an infinitely ambiguous grammar.
(In the technical literature, an infinite ambiguity is more usually
called a B<cycle> or a B<loop>.)

An example of an infinitely ambiguous grammar is the following:

=for Marpa::PP::Display
ignore: 1

    S ::= A
    A ::= B
    B ::= A
    B :: 'x'

=for Marpa::PP::Display::End

Given the input 'x', this grammar will produce
these parses 
 
=for Marpa::PP::Display
ignore: 1

    S -> A -> B -> x
    S -> A -> B -> A -> B -> x
    S -> A -> B -> A -> B -> A -> B -> x
    .
    .
    .

=for Marpa::PP::Display::End

Because of the two rules C<A ::= B> and C<B ::= A>,
this list of parses could go on forever.
The two rules C<A ::= B> and C<B ::= A> form what is called a B<cycle>.

Typically, if a user has written an grammar with an infinite cycle,
it was a mistake and
he wants to rewrite it before proceeding.
By default, an infinitely ambiguous grammar is a fatal error.
This is the behavior most users will want.

To proceed to producing parse results from an infinitely ambiguous grammar,
the user must set
the grammar's
L<C<infinite_action>|Marpa::PP::Grammar/"infinite_action">
named argument
to a value other than "C<fatal>".
The other choices are "C<warn>"
and "C<quiet>".

=head1 CYCLE LENGTH

Obviously,
Marpa cannot list all of an infinite number of parse results.
Marpa deals with potentially infinite parses by limiting the
cycle length.
B<Cycle length> is the number of times a parse derivation goes
around a potentially infinite cycle.

Marpa limits all cycles to a length of 1.
There will always be a finite number of these parse results.

Above I showed
a set of parses from an example of an
infinitely ambiguous grammar.
Here are those parses again, this time
labeled with their cycle length.

=for Marpa::PP::Display
ignore: 1

    Cycle length 1: S -> A -> B -> x
    Cycle length 2: S -> A -> B -> A -> B -> x
    Cycle length 3: S -> A -> B -> A -> B -> A -> B -> x

=for Marpa::PP::Display::End

Of the parse results in the above list, Marpa would return a value
only for the first,
the one whose cycle length is 1.

=head1 LIMITATIONS

The precise behavior of
Marpa's cycle detection is,
at this point,
not strictly defined and
applications should avoid
relying on the details of its semantics.
The exact point at which
a cycle is broken varies among
implementations.

In future releases,
Marpa's cycle detection may be
more carefully defined.
But cycles at this point are universally considered
useless,
and there seems to be literally nobody who
cares about the details of their semantics.
The quality of the present implementation
seems to be completely adequate in terms
of the present interest.

At this point it seems that a cycle should be
broken when it is about to loop back to the "same
rule".
But in current Marpa implementations,
the "same rule"
means the same rule
B<after Marpa's rewriting of the grammar>,
not the same rule as in the original grammar.
If a more careful semantics is created, it probably
should be defined in terms of the rules as
the user sees them,
rather than in terms of the rules as rewritten
by Marpa's internals.

=head1 COPYRIGHT AND LICENSE

=for Marpa::PP::Display
ignore: 1

  Copyright 2011 Jeffrey Kegler
  This file is part of Marpa::PP.  Marpa::PP is free software: you can
  redistribute it and/or modify it under the terms of the GNU Lesser
  General Public License as published by the Free Software Foundation,
  either version 3 of the License, or (at your option) any later version.
  
  Marpa::PP is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  Lesser General Public License for more details.
  
  You should have received a copy of the GNU Lesser
  General Public License along with Marpa::PP.  If not, see
  http://www.gnu.org/licenses/.

=for Marpa::PP::Display::End

=cut

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