# Copyright 2012 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::Advanced::Models - Other Input Models
=head1 ABOUT THIS DOCUMENT
The alternative input models described in this document are an
advanced technique.
If you are starting out with Marpa, you
probably want to ignore this document.
If you are an experienced Marpa user,
it is still safe to ignore this document,
but you might find the possibilities it discusses
interesting.
=head1 DESCRIPTION
=head2 What is an Alternative Input Model?
An alternative input model
is anything that is not the default, token-stream model.
More helpfully, Marpa allows variable-length tokens and ambiguous tokens,
and an alternative input model is any input model which
=over
=item * Allows a token whose length is not exactly 1, or
=item * Allows locations which have more than one token.
=back
To do either of these things,
a user must use the recognizer's L<C<alternative>|/"alternative">
method.
In other words,
if you are not using the recognizer's C<alternative> method call,
you are not using an alternative input method.
Many concepts, such as parsing location,
parse exhaustion,
and the end of parsing,
are somewhat more complicated when alternative
input models are involved.
These concepts were explained in L<the main document for
the recognizer|Marpa::PP::Recognizer> on the assumption
that the default input model was in use.
This document revises those explanations as necessary
to take into
account the alternative input models.
=head2 Token Streams
Marpa's default input model is the traditional one --
a token stream.
Token streams are very standard in parsing applications --
so much so
that most texts do not take the trouble
of defining the term.
A B<token stream> is input structured as
a sequence of tokens,
where each token occupies one location
and every location has a token.
In the token stream model, all tokens are
of the same length.
Conventionally, all tokens are of length 1,
and the token stream starts at location 0.
Following this convention,
the I<N>th token would start at
location I<N-1> and end
at location I<N>.
For example,
the first token would start at location 0 and end at location 1.
The second token would start at location 1 and end at location 2.
=head2 Earlemes
For most parsers, position is location in a token stream.
To deal with variable-length and overlapping tokens,
Marpa needs a more flexible idea of location.
Marpa's tracks position in terms of B<earlemes>.
B<Earlemes> are named after Jay Earley,
the inventor of the first algorithm
in Marpa's lineage.
Every token has a start earleme and an end earleme.
The token stream model may also be called the token-per-earleme
model.
In the token stream model,
token location and earleme location
are exactly identical an one-to-one basis.
If a user's application uses the token stream model,
he can ignore earlemes, and think entirely in terms of
tokens and position in a token stream.
Because of this, the main Marpa documents speak
simply of the "location" in the parse.
=head2 The Furthest Earleme
The B<furthest earleme> is the last earleme at which a token ends.
In the default input model, the furthest earleme is not an important
concept.
In the default input model,
the furthest earleme and the current earleme
are always the same.
In alternative input models,
tokens may be longer than 1 earleme, and
the furthest earleme and the current earleme may be far apart.
This becomes an issue when
parsing is finished.
Alternative input models use
the recognizer's L<C<end_input>|/"end_input"> method to ensure
that processing of input catches up to the furthest earleme.
=head1 METHODS
=head2 alternative
=for Marpa::PP::Display
name: Recognizer alternative Synopsis
normalize-whitespace: 1
defined $recce->alternative( 'a', 42, 1 )
or return 'First alternative failed';
=for Marpa::PP::Display::End
The C<alternative> method is the most general method for reading
input, and is used in alternative input models.
It takes three arguments, only the first of which is required.
The first two arguments are similar to those of the recognizer's
C<read> method: the token type and the token value.
As with the C<read> method, the token value is optional and,
if omitted, defaults to a Perl C<undef>.
The third argument to the C<alternative> method is a token
length.
If omitted, token length defaults to 1,
which is the correct value for the token stream model.
Its value can be any integer B<greater> than zero.
Marpa does not allow zero length tokens in any input model.
Unlike the C<read> method, the C<alternative> method does
B<not> advance the current location
(current earleme) on each call.
This allows the application to read several tokens
at the same earleme.
This is how ambiguous input is created.
To advance the current earleme when
input is read using the C<alternative> method,
the C<complete_earleme> method must be called.
On success, C<alternative> returns the current earleme.
On failures, other than the rejection of a token in interactive mode,
C<alternative> throws an exception.
If the token is rejected, C<alternative> returns a Perl C<undef>.
=head2 current_earleme
=for Marpa::PP::Display
name: current_earleme Example
normalize-whitespace: 1
$current_earleme = $recce->current_earleme();
=for Marpa::PP::Display::End
Returns the current parse location,
also known as the current earleme.
Not often needed.
=head2 earleme_complete
=for Marpa::PP::Display
name: Recognizer earleme_complete Synopsis
normalize-whitespace: 1
$recce->earleme_complete();
=for Marpa::PP::Display::End
Processes all tokens at the current earleme and advances the current
earleme by 1.
Returns the number of terminals acceptable at the new current earleme.
Note that, in alternative input models,
a return value of 0 does B<not> necessarily indicate an
exhausted parser.
When reading input using the C<alternative> method,
C<earleme_complete> is used to move
forward in
the input stream.
All tokens read using the C<alternative> method are read at the same
location -- the current earleme.
A C<earleme_complete> call causes the tokens
read using the C<alternative> method to be processed,
and the current earleme to be advanced.
C<earleme_complete> may be called even if the C<alternative>
method has been not called since the last
call to C<earleme_complete>.
This will create an earleme with no tokens,
which is often useful.
=head2 exhausted
=for Marpa::PP::Display
name: Recognizer exhausted Synopsis
normalize-whitespace: 1
$recce->exhausted() and die 'Recognizer exhausted';
=for Marpa::PP::Display::End
The C<exhausted> method returns a Perl true if parsing
in a recognizer is exhausted, and a Perl false
otherwise.
Parsing is exhausted when the recognizer will not accept
any further input.
This method is seldom used.
For most applications, the return values of the other
methods provide sufficient information about the parse status,
and there is no need to specifically test for parse exhaustion.
In the default input model, a recognizer was exhausted if
the C<read> method returned 0, indicating that no tokens
would be accepted at the new current earleme.
In alternative input models,
C<earleme_complete> will always return 0 when the parser is exhausted,
but the converse is not always true:
C<earleme_complete> may return 0 even in some cases when the parser is not exhausted.
When C<earleme_complete> returns 0, no input will be
accepted at the current earleme.
But in input models which allow tokens longer than 1,
it is possible for input to be accepted at earlemes
after the current earleme,
even if no input is accepted
at the current earleme.
=head2 end_input
=for Marpa::PP::Display
name: Recognizer end_input Synopsis
normalize-whitespace: 1
$recce->end_input();
=for Marpa::PP::Display::End
Indicates that input is finished.
Calling C<end_input> is not necessary
or useful
in the default input model,
because in the default input model no token
has a length greater than 1.
The C<end_input> method takes no arguments.
The C<end_input> method returns a Perl true value on success.
On failure, it throws an exception.
In alternative input models, calling the C<earleme_complete> method
once input is finished does not
ensure that all input has been processed.
The C<earleme_complete> method completes the current earleme,
but in alternative models, tokens may extend well past the current earleme.
The C<end_input> method ensures that all input is processed.
Calling C<end_input> multiple times on the same recognizer object
is harmless,
but useless.
The second and subsequent calls will
return a Perl true,
but will have no effect.
=head1 ALTERNATIVE MODELS AND THE C<read> METHOD
A recognizer can mix
calls to
its L<C<read>|Marpa::PP::Recognizer/"read">
method with calls to its L<C<alternative>|/"alternative">
method.
The C<read> method has the same effect as a single call
to the C<alternative> method, followed immediately
by a call of the C<earleme_complete> method.
More precisely,
the C<read> method is equivalent to
=for Marpa::PP::Display
ignore: 1
sub Marpa::PP::Recognizer::read {
my $recce = shift;
return defined $recce->alternative(@_) ? $recce->earleme_complete() : undef;
}
=for Marpa::PP::Display::End
=head1 AMBIGUOUS LEXING
Marpa allows ambiguous tokens.
Several Marpa tokens can start at a single parsing location.
Ambiguous tokens can be of various lengths.
Tokens can also overlap.
B<Potentially
ambiguous lexing>
occurs when more than one token starts
at a single earleme.
When potentially ambiguous lexing occurs,
it becomes possible for there to be more
than one sequence of tokens.
An B<actual lexical ambiguity> only occurs if
more than one of the potential token sequences is consistent with
the grammar.
If there is no actual lexical ambiguity,
Marpa will use the only token choice that is
consistent with the grammar.
When lexing is B<actually ambiguous>, Marpa
will use all the alternatives
consistent with the grammar.
When the lexing in a parse is actually ambiguous,
the parse will be ambiguous.
From the point of view of Marpa's semantics,
ambiguities caused by lexing look the
same as ambiguities caused by an ambiguous grammar.
In the standard
terminology,
if a grammar produces more than one parse result,
then that grammar must be ambiguous.
In Marpa this is not strictly true.
In Marpa,
if the input is ambiguous,
even an unambiguous grammar can produce than one parse.
=head1 DUPLICATE TOKENS
A duplicate token is a token of the same type
and the same length as another
that was read at the same earleme.
Duplicate tokens are impossible in the default, token-stream,
model.
This is because in the token-stream model only one token can be
read at each earleme.
In alternative models, more than one token may be read at
an earleme, and duplicates B<are> possible.
Marpa detects duplicate tokens and treats them as
"hard errors" --
Marpa throws an exception
when it sees a duplicate token.
Marpa's assumption is that
duplicate tokens indicate
an error at the application level.
An application can retry input after
a duplicate token, if it
catches the exception.
In the future, if recovery from duplicate tokens is found
to be a useful technique, Marpa may provide an option to change
its behavior, so that a soft failure is returned
when there is a duplicate token.
=head1 EARLEMES: THE DETAILS
While scanning, Marpa keeps track of the B<current earleme>.
Earlemes in a parse start at earleme 0 and increase numerically.
The earleme immediately following earleme 0 is earleme 1,
the earleme immediately following earleme 1 is earleme 2,
and so on.
The earleme immediately following earleme I<N> is always earleme I<N+1>.
B<Distance> in the earleme stream is
what you would intuitively expect it to be.
The distance between earleme I<X> and earleme I<Y> is
the absolute value of the difference between I<X> and I<Y>,
I<|X-Y|>.
The distance from earleme 3 to earleme 6,
for example, is 3 earlemes.
Whenever a token is given to Marpa to be scanned,
it starts at the current earleme.
In addition to the type and value of the token,
Marpa must be told token's B<length> in earlemes.
The length of a Marpa token must be greater than zero.
This earleme length will become
the distance from the start of the
token to the end of the token.
If the length of the token is I<L>,
and the number of the current earleme is I<C>,
the end of the token will be at the earleme whose number is I<C+L>.
=head1 THE CHARACTER-PER-EARLEME MODEL
Many different models of the relationship between tokens and earlemes
are possible, but two are particularly important.
One is the one-token-per-earleme model,
which is the default,
and which has already been described.
The other is the one-character-per-earleme model.
In the one-character-per-earleme model,
every character will be treated as being exactly one
earleme in length.
If a token is more than one character in length,
that token will span earlemes.
When the lexing is ambiguous, tokens may overlap.
When a one-character-per-earleme model of input is used,
there may be many earlemes at which no tokens start.
For example,
in a straightforward character-per-earleme implementation
of a grammar for a language which allows
comments,
no tokens will start at
any earlemes which corresponds to character locations inside
a comment.
=head1 OTHER INPUT MODELS
So far only the token-per-earleme and
character-per-earleme models have seen any
real use in Marpa programs.
But other models are certainly possible.
Using earlemes,
you can structure your input in almost any way you like.
There are only three restrictions:
=over 4
=item 1
Scanning always starts at earleme 0.
=item 2
All tokens starting at
earleme I<N> must be scanned before
any tokens starting at earleme I<N+1>.
In other words, the tokens must be scanned in non-decreasing order
by start earleme.
=item 3
Every token must have a length, in earlemes,
which is greater than zero.
In other words,
token length can never
be zero or negative.
=back
=head1 COPYRIGHT AND LICENSE
=for Marpa::PP::Display
ignore: 1
Copyright 2012 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