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Marpa::R2::Rewrite - How Marpa rewrites grammars


Marpa rewrites grammars, adding internal symbols and rules in the process. These rewritings do not affect the semantics, but they do show up when you look at Marpa's grammars.

Marpa's internal symbols have tags at the end, enclosed in square brackets. This means that all Marpa internal symbols end in a right square bracket.

Marpa adds a new start rule

Many parsers add their own start rule and their own start symbol to grammars. Marpa is no exception. The new internal start symbol is the original start symbol with "[']" suffixed. An example of a Marpa internal start symbol is "Expression[']". If the grammar allows a null parse, there will also be a Marpa internal nulling start symbol, with "['][]" suffixed. An example of a Marpa internal nulling start symbol would be "Script['][]".

Marpa eliminates proper nullable symbols

A symbol is nulled if it produces the empty sentence. Nulling symbols are those which are always nulled. Nullable symbols are those which are sometimes nulled. Non-nullable symbols are those which are never nulled.

All nulling symbols are also nullable symbols. A proper nullable is any nullable symbol which is not a nulling symbol. In other words, a proper nullable is a symbol that might or might not be nulled.

Nullable symbols were a problem in previous versions of Earley parsers. Aycock and Horspool 2002 outlined a new approach for dealing with them. I use their ideas with modifications of my own.

Marpa rewrites its grammar to eliminate proper nullables. It does this by turning every proper nullable into two symbols: a non-nullable variant and a nulling variant. The non-nullable variant keeps the original symbol's name, but is no longer allowed to appear in places where it might be nulled.

The name of the nulling variant is that of the original symbol with the nulling tag ("[]") suffixed. When the nulling variant is used, it must be nulled.

The newly introduced nulling symbols will not appear on any left hand sides, with one exception: grammars that allow a null parse will have a nulling start rule. Except for the nulling start symbol, Marpa marks nulling symbols internally and recognizes them directly, without the need for empty rules.

Rules with proper nullables on the RHS have to be replaced with new rules covering every possible combination of the non-nullable and nulling variants. That rewrite is called the CHAF rewrite.

Marpa divides some rules up into smaller ones

Here's an example of a CHAF rewrite:

    {   lhs => 'statement',
        rhs => [
            qw/optional_whitespace expression
                optional_whitespace optional_modifier

This rule contains four instances of proper nullables, illustrating my fear that grammars of practical interest will have lots of proper nullables on right hand sides. optional_whitespace and optional_modifier are both proper nullables and optional_whitespace appears three times.

Here's is the output from show_rules, showing what Marpa does with this rule:

    0: statement -> optional_whitespace expression optional_whitespace optional_modifier optional_whitespace
    /* !used */
    15: statement -> optional_whitespace expression statement[R0:2] /* vrhs real=2 */
    16: statement -> optional_whitespace expression optional_whitespace[] optional_modifier[] optional_whitespace[]
    17: statement -> optional_whitespace[] expression statement[R0:2] /* vrhs real=2 */
    18: statement -> optional_whitespace[] expression optional_whitespace[] optional_modifier[] optional_whitespace[]
    19: statement[R0:2] -> optional_whitespace statement[R0:3] /* vlhs vrhs real=1 */
    20: statement[R0:2] -> optional_whitespace optional_modifier[] optional_whitespace[] /* vlhs real=3 */
    21: statement[R0:2] -> optional_whitespace[] statement[R0:3] /* vlhs vrhs real=1 */
    22: statement[R0:3] -> optional_modifier optional_whitespace /* vlhs real=2 */
    23: statement[R0:3] -> optional_modifier optional_whitespace[] /* vlhs real=2 */
    24: statement[R0:3] -> optional_modifier[] optional_whitespace /* vlhs real=2 */

Rule 0 is the original rule. Because Marpa has rewritten it, the rule is marked "!used", telling later stages in the precomputation to ignore it. Marpa breaks Rule 0 up into three pieces, each with no more than two proper nullables. Marpa then eliminates the proper nullables in each piece by factoring.

To factor a piece, Marpa first rewrites it into multiple rules, one for each possible combination of nulled and non-nulled symbols. Marpa then replaces each proper nullable with its nulling or non-nullable variant, as appropriate. There are two symbol variants for each proper nullable. With a maximum of two proper nullables for each piece, each with two variants, there are at most four combinations. A rule for one of these combinations is called a factored rule, or a factor.

In the example in the above display, the original rule (Rule 0) was broken into three pieces. Rule 0 had 5 RHS symbols, and the three pieces contain, respectively, the first two RHS symbols; the third RHS symbol; and the last two (that is, 4th and 5th) RHS symbols.

The first piece of Rule 0 is factored into four rules: Rules 15 to 18. The second piece is factored into three rules: Rules 19 to 21. The third and last piece of Rule 0 is also factored into three rules: Rules 22 to 24.

When a rule is broken into pieces, the left hand side of the first piece is the left hand side of the original rule. New symbols are introduced to be the left hand sides of the other pieces. The names of the new LHS symbols are formed by suffixing a tag to the name of the original left hand side. The suffixed tag begins with a capital "R" and identifies the location of the piece in the original rule. For example, the tag "[R0:3]" indicates that this symbol is the left hand side for the piece of Rule 0 which begins at right hand symbol 3 (the first symbol is symbol 0).

When a new LHS symbol is created for a piece, the worst case is that the new LHS is also a proper nullable. This worst case occurs when all the original symbols in the piece for the new LHS and all symbols in all subsequent pieces are proper nullables.

A newly created LHS symbol will always appear on the RHS of the previous piece. If the newly created symbol is a proper nullable, then it counts against the limit of two proper nullables for that previous piece, and it must be factored, just the same as if it was one of the proper nullables appearing in the original rule.

Rule 0 is such a worst case. The last three symbols of the right hand side are all proper nullables. That means that all the symbols in the last two pieces of the original rule are proper nullables. Therefore both of the newly created LHS symbols are proper nullables.

The original Rule 0 has 4 proper nullables. After the CHAF rewrite, there are 6 proper nullables: the original 4 plus the 2 symbols newly created to serve as left hand sides. This is why, in order to have at most 2 proper nullables per piece, the original rule must to be divided into 3 pieces.

CHAF preserves user semantics. Marpa, when it splits the rule into pieces and factors the pieces, inserts logic to gather and preserve the values of child nodes. The user's semantic actions see the original child values as if the CHAF rewrite had never occurred.

Marpa converts sequence productions to BNF rules

Internally, Marpa converts productions specified as sequences into BNF productions. The conversion is done in a standard way. For example,

    lhs       => 'statements',
    rhs       => [qw/statement/],
    separator => 'comma',
    min       => 1


    2: statements -> statements[Subseq:8:5] /* vrhs real=0 */
    3: statements -> statements[Subseq:8:5] comma /* vrhs real=1 */
    4: statements[Subseq:8:5] -> statement /* vlhs real=1 */
    5: statements[Subseq:8:5] -> statements[Subseq:8:5] comma statement /* vlhs vrhs real=2 */

In the added symbol, the tag "[Subseq:8:5]" indicates a special symbol introduced to serve as the LHS of the sequence. The "8:5" indentifies the sequence using the symbol ID's of the two symbols involved. "statements", the LHS symbol, has symbol ID 8. "statement", the RHS symbol, has symbol ID 5.

Here's another example, this time of a sequence without a separator. The rule

    { lhs => 'block', rhs => [qw/statements/], min => 0 },

is rewritten by Marpa as follows:

    7: block -> /* empty !used */
    8: block -> block[Subseq:0:8] /* vrhs real=0 */
    9: block[Subseq:0:8] -> statements /* vlhs real=1 */
    10: block[Subseq:0:8] -> block[Subseq:0:8] statements /* vlhs vrhs real=1 */

Note that rule 7, the empty rule with the block symbol as its LHS, is marked "!used". block is a proper nullable, and rules from sequence conversion undergo the same rewriting of proper nullables as any other rules. In this case a nulling variant of block (block[]) was created. That made the empty rule created by the sequence conversion useless, and that is why it was marked "!used".


  Copyright 2012 Jeffrey Kegler
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