我正在研究一个
Marpa::R2语法,它将文本中的项目分组.每个组只能包含某种类型的项目,但不会明确分隔.这会导致问题,因为x … x(其中.表示可以成为组的一部分的项目)可以分组为x(…)x,x(..)(.)x,x(.) (..)x,x(.)(.)(.)x.换句话说,语法非常模糊.
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”(就像它在Perl正则表达式中那样),我怎么能消除这种歧义呢？
在下面的语法中,我尝试将秩副词添加到序列规则中,以便优先处理Group over Sequence,但这似乎不起作用.
下面是一个练习此行为的测试用例.
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.
解决方法
序列规则是针对非棘手案例而设计的.当事情变得棘手时,序列规则总是可以重写为BNF规则,这就是我在这里建议的.以下内容使您的测试工作：
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,'expected number of parses';
done_testing;