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.\"
.IX Title "Marpa::R2::Semantics::Null 3pm"
.TH Marpa::R2::Semantics::Null 3pm "2021-01-22" "perl v5.32.0" "User Contributed Perl Documentation"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
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.nh
.SH "NAME"
Marpa::R2::Semantics::Null \- How the SLIF evaluates null rules and symbols
.SH "Overview"
.IX Header "Overview"
In Marpa parses, rules and  symbols can be nulled \*(--
in other words they can derive the zero-length, or null, string.
Which symbols can be, or are, nulled, depends on the grammar
and the input.
When a symbol or rule is not nulled,
the symbol is said to be \fBvisible\fR.
.PP
Even the start symbol can be nulled,
in which case the entire parse derives the null string.
A parse in which the start symbol is nulled is
called a \fBnull parse\fR.
.PP
When evaluating a parse, nulled rules and symbols are
assigned values as described
in the semantics document.
This document provides additional detail on the assignment
of values to nulled symbols.
.SH "Description"
.IX Header "Description"
.SS "Null values come from rules"
.IX Subsection "Null values come from rules"
Nulled subtrees are pruned back to their topmost symbol.
Lexemes are never nulled, so a nulled symbol is always the \s-1LHS\s0 of a rule instance,
and the action is determined from the rule alternative.
.PP
A complication arises if the symbol appears on the \s-1LHS\s0 of more than one
nullable rule alternative.  Because the symbol is nulled, the input is no help in determining
which rule alternative to use.  The rule alternative whose semantics are used for a nulled symbol
is determined as follows:
.IP "\(bu" 4
If all nullable rule alternatives have the same semantics, that semantics is used.
.IP "\(bu" 4
If one of the nullable rule alternatives
is empty (that is, has a zero-length \s-1RHS\s0),
then the empty alternative's semantics are used.
.IP "\(bu" 4
In the remaining case,
two or more of the rule alternatives have different action names,
but none of the alternatives has a zero-length \s-1RHS.\s0
When this happens, Marpa throws an exception.
One easy way
to fix the issue,
is to add an empty rule with the intended semantics.
.PP
In determining whether the semantics of two nullable rule alternatives
is \*(L"the same\*(R",
the blessing is taken into account.
Two rule alternatives are considered to have different semantics if
they are blessed differently.
.PP
The \*(L"lost\*(R" semantics of the non-topmost symbols and rules
of null subtrees are usually not missed.
Nulled subtrees cannot contain input,
and therefore do not contain token symbols.
So no token values are lost when
nulled subtrees are pruned.
As bushy as a null subtree might be,
all of its symbols and rules are nulled.
.PP
Since nulled symbols and rules correspond to zero-length strings,
so we are literally dealing here with
the \*(L"semantics of nothing\*(R".
In theory the semantics of nothing can be arbitrarily complex.
In practice it should be possible to keep them simple.
.SH "Example"
.IX Header "Example"
As already stated,
Marpa prunes every null subtree back to its topmost
null symbol.
Here is an example:
.PP
.Vb 4
\&    sub do_L {
\&        shift;
\&        return \*(AqL(\*(Aq . ( join q{;}, map { $_ // \*(Aq[ERROR!]\*(Aq } @_ ) . \*(Aq)\*(Aq;
\&    }
\&
\&    sub do_R {
\&        return \*(AqR(): I will never be called\*(Aq;
\&    }
\&
\&    sub do_S {
\&        shift;
\&        return \*(AqS(\*(Aq . ( join q{;}, map { $_ // \*(Aq[ERROR!]\*(Aq } @_ ) . \*(Aq)\*(Aq;
\&    }
\&
\&    sub do_X { return \*(AqX(\*(Aq . $_[1] . \*(Aq)\*(Aq; }
\&    sub do_Y { return \*(AqY(\*(Aq . $_[1] . \*(Aq)\*(Aq; }
\&
\&    ## no critic (Variables::ProhibitPackageVars)
\&    our $null_A = \*(Aqnull A\*(Aq;
\&    our $null_B = \*(Aqnull B\*(Aq;
\&    our $null_L = \*(Aqnull L\*(Aq;
\&    our $null_R = \*(Aqnull R\*(Aq;
\&    our $null_X = \*(Aqnull X\*(Aq;
\&    our $null_Y = \*(Aqnull Y\*(Aq;
\&    ## use critic
\&
\&    my $slg = Marpa::R2::Scanless::G\->new(
\&        {   source => \e<<\*(AqEND_OF_DSL\*(Aq,
\&    :start ::= S
\&    S ::= L R     action => do_S
\&    L ::= A B X   action => do_L
\&    L ::=         action => null_L
\&    R ::= A B Y   action => do_R
\&    R ::=         action => null_R
\&    A ::=         action => null_A
\&    B ::=         action => null_B
\&    X ::=         action => null_X
\&    X ::= \*(Aqx\*(Aq     action => do_X
\&    Y ::=         action => null_Y
\&    Y ::= \*(Aqy\*(Aq     action => do_Y
\&    END_OF_DSL
\&        }
\&    );
\&
\&    my $slr = Marpa::R2::Scanless::R\->new(
\&        {   grammar           => $slg,
\&            semantics_package => \*(Aqmain\*(Aq,
\&        }
\&    );
\&
\&    $slr\->read( \e\*(Aqx\*(Aq );
.Ve
.PP
If we write the unpruned parse tree
in pre-order, depth-first, indenting children
below their parents, we get something like this:
.PP
.Vb 9
\&        0: Visible Rule: S := L R
\&             1: Visible Rule L := A B X
\&                 1.1: Nulled Symbol A
\&                 1.2: Nulled Symbol B
\&                 1.3: Token, Value is \*(Aqx\*(Aq
\&             2: Nulled Rule, Rule R := A B Y
\&                 2.1: Nulled Symbol A
\&                 2.2: Nulled Symbol B
\&                 2.3: Nulled Symbol Y
.Ve
.PP
In this example, five symbols and a rule are nulled.
The rule and three of the symbols are in a single subtree: 2, 2.1, 2.2 and 2.3.
Marpa prunes every null subtree back to its topmost symbol,
which in this case is the \s-1LHS\s0 of the rule numbered 2.
.PP
The pruned tree looks like this
.PP
.Vb 6
\&        0: Visible Rule: S := L R
\&             1: Visible Rule L := A B X
\&                 1.1: Nulled Symbol A
\&                 1.2: Nulled Symbol B
\&                 1.3: Token, Value is \*(Aqx\*(Aq
\&             2: LHS of Nulled Rule, Symbol R
.Ve
.PP
Nulled nodes 1.1, 1.2 and 2 were all kept, because they are topmost in their
nulled subtree.
All the other nulled nodes were discarded.
.PP
Here is the output:
.PP
.Vb 1
\&    S(L(null A;null B;X(x));null R)
.Ve
.PP
In the output we see
.IP "\(bu" 4
The null value for symbol 1.1: "\f(CW\*(C`null A\*(C'\fR".
This comes from the empty rule for \f(CW\*(C`A\*(C'\fR.
.IP "\(bu" 4
The null value for symbol 1.2: "\f(CW\*(C`null B\*(C'\fR".
This comes from the empty rule for \f(CW\*(C`B\*(C'\fR.
.IP "\(bu" 4
The token value for symbol 1.3: "\f(CW\*(C`x\*(C'\fR".
.IP "\(bu" 4
An application of the rule evaluation closure for the rule
\&\f(CW\*(C`L := A B X\*(C'\fR.
.IP "\(bu" 4
The null value for rule 2: "\f(CW\*(C`null R\*(C'\fR".
This comes from the empty rule for \f(CW\*(C`R\*(C'\fR.
.IP "\(bu" 4
An application of the rule evaluation closure for the rule
\&\f(CW\*(C`S := L R\*(C'\fR
.PP
We \fBdo not\fR see any output
for symbols
2.1 (\f(CW\*(C`A\*(C'\fR),
2.2 (\f(CW\*(C`B\*(C'\fR),
or 2.3 (\f(CW\*(C`Y\*(C'\fR)
because they were not topmost
in the pruned subtree.
We \fBdo not\fR see an application of the rule evaluation closure for rule \f(CW\*(C`R := A B Y\*(C'\fR,
because there is an empty rule for \f(CW\*(C`R\*(C'\fR, and that takes priority.
.SH "Copyright and License"
.IX Header "Copyright and License"
.Vb 5
\&  Copyright 2014 Jeffrey Kegler
\&  This file is part of Marpa::R2.  Marpa::R2 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::R2 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::R2.  If not, see
\&  http://www.gnu.org/licenses/.
.Ve