如何让玛尔帕的序列规则变得贪婪？

Question

我正在研究一种Marpa::R2语法，该语法将文本中的项目分组。每个组只能包含特定类型的项，但不能显式分隔。这会引起问题，因为x...x（其中.表示可以是组的一部分的项目）可以被分组为x(...)x、x(..)(.)x、x(.)(..)x、x(.)(.)(.)x。换句话说，语法是高度模糊的。

如果我只想要x(...)x解析，即如果我想强制+量词只表现为"贪婪"（就像在Perl正则表达式中一样），我如何消除这种歧义？

在下面的语法中，我尝试将rank副词添加到序列规则中，以便将Group优先于Sequence，但这似乎不起作用。

下面是一个练习这种行为的测试用例。

use strict;
use warnings;
use Marpa::R2;
use Test::More;
my $grammar_source = <<'END_GRAMMAR';
inaccessible is fatal by default
:discard ~ space
:start ::= Sequence
Sequence
    ::= SequenceItem+  action => ::array
SequenceItem
    ::= WORD    action => ::first
    |   Group   action => ::first
Group
    ::= GroupItem+  action => [name, values]
GroupItem
    ::= ('[') Sequence (']')  action => ::first
WORD    ~ [a-z]+
space   ~ [s]+
END_GRAMMAR
my $input = "foo [a] [b] bar";
diag "perl $^V";
diag "Marpa::R2 " . Marpa::R2->VERSION;
my $grammar = Marpa::R2::Scanless::G->new({ source => $grammar_source });
my $recce = Marpa::R2::Scanless::R->new({ grammar => $grammar });
$recce->read($input);
my $parse_count = 0;
while (my $value = $recce->value) {
    is_deeply $$value, ['foo', [Group => ['a'], ['b']], 'bar'], 'expected structure'
        or diag explain $$value;
    $parse_count++;
}
is $parse_count, 1, 'expected number of parses';
done_testing;

测试用例的输出（FAIL）：

# perl v5.18.2
# Marpa::R2 2.09
ok 1 - expected structure
not ok 2 - expected structure
#   Failed test 'expected structure'
#   at - line 38.
#     Structures begin differing at:
#          $got->[1][2] = Does not exist
#     $expected->[1][2] = ARRAY(0x981bd68)
# [
#   'foo',
#   [
#     'Group',
#     [
#       'a'
#     ]
#   ],
#   [
#     ${$VAR1->[1][0]},
#     [
#       'b'
#     ]
#   ],
#   'bar'
# ]
not ok 3 - expected number of parses
#   Failed test 'expected number of parses'
#   at - line 41.
#          got: '2'
#     expected: '1'
1..3
# Looks like you failed 2 tests of 3.

Answer 1

序列规则是为非棘手的情况设计的。当事情变得棘手时，序列规则总是可以重写为BNF规则，这就是我在这里的建议。以下使您的测试工作：

use strict;
use warnings;
use Marpa::R2;
use Test::More;
my $grammar_source = <<'END_GRAMMAR';
inaccessible is fatal by default
:discard ~ space
# Three cases
# 1.) Just one group.
# 2.) Group follows by alternating words and groups.
# 3.) Alternating words and groups, starting with words
Sequence ::= Group action => ::first
Sequence ::= Group Subsequence action => [values]
Sequence ::= Subsequence action => ::first
Subsequence ::= Words action => ::first
# "action => [values]" makes the test work unchanged.
# The action for the next rule probably should be
# action => [name, values] in order to handle the general case.
Subsequence ::= Subsequence Group Words action => [values]
Words ::= WORD+ action => ::first
Group
::= GroupItem+  action => [name, values]
GroupItem
::= ('[') Sequence (']')  action => [value]
WORD    ~ [a-z]+
space   ~ [s]+
END_GRAMMAR
my $input = "foo [a] [b] bar";
diag "perl $^V";
diag "Marpa::R2 " . Marpa::R2->VERSION;
my $grammar = Marpa::R2::Scanless::G->new( { source  => $grammar_source } );
my $recce   = Marpa::R2::Scanless::R->new( { grammar => $grammar } );
$recce->read( $input );
my $parse_count = 0;
while ( my $value = $recce->value ) {
is_deeply $$value, [ 'foo', [ Group => ['a'], ['b'] ], 'bar' ],
    'expected structure'
    or diag explain $$value;
$parse_count++;
} ## end while ( my $value = $recce->value )
is $parse_count, 1, 'expected number of parses';
done_testing;

Answer 2

一元语法：

Sequence           : WORD+ SequenceAfterWords
                   | Group SequenceAfterGroup
SequenceAfterWords : Group SequenceAfterGroup
                   |
SequenceAfterGroup : WORD+ SequenceAfterWords
                   |

---

Jeffrey Kegler说，在Marpa中，递归引导的处理效率更高。上面采取的相同方法可以前后颠倒来产生这种效果。

Sequence            : SequenceBeforeWords WORD+
                    | SequenceBeforeGroup Group
SequenceBeforeWords : SequenceBeforeGroup Group
                    |
SequenceBeforeGroup : SequenceBeforeWords WORD+
                    |

---

在这两种情况下，

Group     : GroupItem+
GroupItem : '[' Sequence ']'