我是玛尔巴的新手。我尝试了几种方法来描述我的语法中包含 0 个或多个术语的列表,并且我想避免使用多个解析树。
我的语言将只有 1 个组件,后跟 0+ 个子组件:
package => component-rule [subcomponent-rule ...]
我首先尝试的是:
{ lhs => 'Package', rhs => [qw/component-rule subcomponents/] },
{ lhs => 'subcomponents', rhs => [qw/subcomponent-list/] },
{ lhs => 'subcomponent-list', rhs => [qw/subcomponent-rule/], action => 'do_subcomponent_list' },
{ lhs => 'subcomponent-list', rhs => [qw/subcomponent-list subcomponent-rule/], action => 'do_subcomponent_list' },
{ lhs => 'subcomponent-list', rhs => [qw//], action => 'do_subcomponent_empty_list' },
{ lhs => 'subcomponent-rule', rhs => [qw/subcomponent subcomponent-name/], action => 'do_subcomponent' },
(完整代码在帖子末尾。)
这是我的输入:
$recce->read( 'component', );
$recce->read( 'String', 'MO Factory');
$recce->read( 'subcomponent', );
$recce->read( 'String', 'Memory Wipe Station');
$recce->read( 'subcomponent', );
$recce->read( 'String', 'DMO Tour Robot');
我得到两个解析树,第一个带有不受欢迎的 undef,第二个我更喜欢。两者都将列表作为固有的树返回。
$VAR1 = [
{
'Component' => 'MO Factory'
},
[
[
{
'Subcomponent' => undef
},
{
'Subcomponent' => 'Memory Wipe Station'
}
],
{
'Subcomponent' => 'DMO Tour Robot'
}
]
];
$VAR2 = [
{
'Component' => 'MO Factory'
},
[
{
'Subcomponent' => 'Memory Wipe Station'
},
{
'Subcomponent' => 'DMO Tour Robot'
}
]
];
subcomponent-list 的可空规则是允许 0 个子组件的情况,但它在 1+ 个子组件列表的前面引入了 null 元素,这是一种替代解析。(玛尔巴只下轮一次,谢天谢地。)
我的另一个想法是使子组件列表不可为空,并引入一个中间规则,即 0 或 1 个子组件列表:
{ lhs => 'subcomponents', rhs => [qw//] },
{ lhs => 'subcomponents', rhs => [qw/subcomponent-list/] },
这至少消除了多重解析,但我仍然有一个循环,还有一个凌乱的嵌套树要压缩。
是否有更直接的方法来制作 0+ 长度列表或以其他方式使符号可选?
完整示例代码:
#!/usr/bin/perl
use Marpa::R2;
use Data::Dumper;
my $grammar = Marpa::R2::Grammar->new(
{ start => 'Package',
actions => 'My_Actions',
default_action => 'do_what_I_mean',
rules => [
{ lhs => 'Package', rhs => [qw/component-rule subcomponents/] },
{ lhs => 'component-name', rhs => [qw/String/] },
{ lhs => 'component-rule', rhs => [qw/component component-name/], action => 'do_component' },
{ lhs => 'subcomponent-name', rhs => [qw/String/] },
{ lhs => 'subcomponent-rule', rhs => [qw/subcomponent subcomponent-name/], action => 'do_subcomponent' },
{ lhs => 'subcomponents', rhs => [qw//] },
{ lhs => 'subcomponents', rhs => [qw/subcomponent-list/] },
{ lhs => 'subcomponent-list', rhs => [qw/subcomponent-rule/], action => 'do_subcomponent_list' },
{ lhs => 'subcomponent-list', rhs => [qw/subcomponent-list subcomponent-rule/], action => 'do_subcomponent_list' },
# { lhs => 'subcomponent-list', rhs => [qw//], action => 'do_subcomponent_empty_list' },
# { lhs => 'subcomponent-list', rhs => [qw//], },
],
}
);
$grammar->precompute();
my $recce = Marpa::R2::Recognizer->new( { grammar => $grammar } );
$recce->read( 'component', );
$recce->read( 'String', 'MO Factory');
if (1) {
$recce->read( 'subcomponent', );
$recce->read( 'String', 'Memory Wipe Station');
$recce->read( 'subcomponent', );
$recce->read( 'String', 'DMO Tour Robot');
$recce->read( 'subcomponent', );
$recce->read( 'String', 'SMO Break Room');
}
my @values = ();
while ( defined( my $value_ref = $recce->value() ) ) {
push @values, ${$value_ref};
}
print "result is ",Dumper(@values),"\n";
sub My_Actions::do_what_I_mean {
print STDERR "do_what_I_mean\n";
# The first argument is the per-parse variable.
# At this stage, just throw it away
shift;
# Throw away any undef's
my @children = grep { defined } @_;
# Return what's left
return scalar @children > 1 ? \@children : shift @children;
}
sub My_Actions::do_component {
my ( undef, $t1 ) = @_;
print STDERR "do_component $t1\n";
my $href = { 'Component' => $t1 };
return $href;
}
sub My_Actions::do_subcomponent{
my ( undef, $t1 ) = @_;
print STDERR "do_subcomponent $t1\n";
my $href = { 'Subcomponent' => $t1 };
return $href;
}
sub My_Actions::do_subcomponent_empty_list
{
print STDERR "do_subcomponent_empty_list\n";
my $href = { 'Subcomponent' => undef };
return $href;
}
sub My_Actions::do_subcomponent_list{
# The first argument is the per-parse variable.
# At this stage, just throw it away
shift;
# Throw away any undef's
my @children = grep { defined } @_;
print STDERR "do_subcomponent_list size ",scalar(@children),"\n";
# Do this to collapse recursive trees to a list:
# @children = map { ref $_ eq "ARRAY" ? @{$_} : $_; } @children;
return scalar @children > 1 ? \@children : shift @children;
}