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Author image Jeffrey Kegler
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NAME

Marpa::API::Implementation - Marpa Implementation

OVERVIEW

This document describes the Marpa data structures that are useful for tracing and debugging grammars. It assumes that the reader knows the Marpa API, and that she has a general knowledge of parsing. All the examples in this document assume that none of the data has been stripped.

GRAMMAR REWRITING

Marpa rewrites grammars, adding internal symbols and rules in the process. This rewriting does not affect the semantics, but it does show up when you examine the internals.

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.

Adding a 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['][]".

Elminating 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.

Pedantically, 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 described in the next section.

CHAF Rewriting

To deal with the splitting of rhs proper nullables into two symbols, one non-nullable and one nulling, Aycock and Horspool created new rules covering all possible rhs combinations of non-nullable and nulling symbols. This factoring was exponential in the theoretical worst case, and I believed there would be efficiency issues in practice as well.

A result due to Chomsky shows that any grammar can be rewritten as a grammar with at most two symbols on the right hand side. Relaxing Chomsky's rewrite to allow right hand sides with any number of symbols, but at most two proper nullables, produces a rewrite I call CHAF (Chomsky-Horspool-Aycock Form).

CHAF changes the worst case to linear, and in practical cases lowers the multiplier. Here's an example of a CHAF rewrite. First, the rule:

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

This rule contains four proper nullables, illustrating my fear that grammars of practical interest might lots of proper nullables on the right hand side. 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 did with this rule:

    0: statement -> optional_whitespace expression optional_whitespace optional_modifier optional_whitespace
    /* !used */
    13: statement -> optional_whitespace expression statement[R0:2][xe] /* vrhs real=2 */
    14: statement -> optional_whitespace expression optional_whitespace[] optional_modifier[] optional_whitespace[]
    15: statement -> optional_whitespace[] expression statement[R0:2][xe] /* vrhs real=2 */
    16: statement -> optional_whitespace[] expression optional_whitespace[] optional_modifier[] optional_whitespace[]
    17: statement[R0:2][xe] -> optional_whitespace statement[R0:3][xf] /* vlhs vrhs real=1 */
    18: statement[R0:2][xe] -> optional_whitespace optional_modifier[] optional_whitespace[] /* vlhs real=3 */
    19: statement[R0:2][xe] -> optional_whitespace[] statement[R0:3][xf] /* vlhs vrhs real=1 */
    20: statement[R0:3][xf] -> optional_modifier optional_whitespace /* vlhs real=2 */
    21: statement[R0:3][xf] -> optional_modifier optional_whitespace[] /* vlhs real=2 */
    22: statement[R0:3][xf] -> 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. Rules 13 to 16 are the first piece, with the first two symbols from Rule 0. Rules 17 to 19 are the second piece, with the 3rd symbol. Rules 20 to 22 are the third piece, with the 4th and 5th symbols from Rule 0.

Each piece is factored, so that every combination of nulling and non-nullable symbols is included. New symbols are introduced to be the left hand sides of the pieces. The names of the new symbols are the name of the original left hand side with two suffixed tags. The first 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). The second suffixed tag begins with a lowercase "x" and is an arbitrary hex value to quarantee uniqueness.

Rule 0 is a worst case for CHAF, because the last three symbols of the right hand side are all proper nullables. That means that the last two pieces of the original rule can be either empty or non-empty, and therefore that both of the newly created lhs symbols are also proper nullables.

With the CHAF rewrite, there are now a total of 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 only 2 proper nullables per piece, the original rule needed to be divided into 3 pieces. The newly created lhs symbols, because they are proper nullables, need to be split into nulling and non-nullable variants and factored, just as the proper nullables found in the original rule had to be split and factored.

Even though this is CHAF's worst case, CHAF does this factoring with 10 rules, while the original Aycock-Horspool factoring (NNF) required 16 rules. After more than 4 proper nullables, the advantage of CHAF becomes overwhelming. With 5 proper nullables, there would be 13 rules for CHAF versus 32 for NNF. With 6 proper nullables, 16 versus 64.

The user's semantics are preserved, because Marpa, when it splits the rule into pieces and factors the pieces, inserts logic to gather and preserve the values of child nodes. These values are presented to the user's semantic actions as if no CHAF rewrite had occurred.

Converting Sequence Productions to BNF

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
    }

becomes

    1: statements -> statement[Seq:1-*][Sep:comma][x9] /* vrhs discard_sep real=0 */
    2: statements -> statement[Seq:1-*][Sep:comma][x9] comma /* vrhs discard_sep real=1 */
    3: statement[Seq:1-*][Sep:comma][x9] -> statement /* vlhs real=1 */
    4: statement[Seq:1-*][Sep:comma][x9] -> statement[Seq:1-*][Sep:comma][x9] comma statement /* vlhs vrhs real=2 */

In the added symbol, the tag "[Seq:1-*]" indicates this is a symbol for a sequence of from 1 to an infinite number of symbols and the tag "[Sep:comma]" that it is comma separated.

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:

    5: block -> /* empty !used nullable */
    6: block -> statements[Seq:1-*][xa] /* vrhs real=0 */
    7: statements[Seq:1-*][xa] -> statements /* vlhs real=1 */
    8: statements[Seq:1-*][xa] -> statements[Seq:1-*][xa] statements /* vlhs vrhs real=1 */

Note that rule 5, 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".

EARLEY SETS

Speaking pedantically, creating the Earley sets is is not parsing in the strict sense. Creating the Earley sets is recognition -- determining whether or not a grammar recognizes an input. During recognition, Marpa creates Earley items and puts them into Earley sets. The input is recognized if an Earley item of the correct form is in the Earley set located at the end of parsing. In Marpa, parsing in the strict sense takes place after the recognition phase is done, using the Earley sets built by the recognizer.

Every Earley item has a current earleme. An Earley item's current earleme is also called its dot earleme, its current location, or its dot location. An Earley set is the set of all the Earley items with the same dot location.

In the default, token-stream, input model, the earleme location is exactly the same as the "token stream location". Marpa allows other input models, and in those models the term earleme can take on other meanings. User interested in the alternative input models should look at Marpa::API::Models.

In addition to its current earleme, each Earley item has a QDFA state, and an origin. The origin of an Earley item can also be called its start location. Here's a representation of an Earley item, as you might see it in debugging output from Marpa:

    S4@2-3

This Earley item is for QDFA state 4 (that is the "S4" part), The "@2-3" part says that this Earley item originates at earleme 2, and is current at earleme 3. The number of an Earley item's current earleme (also called its dot earleme) is always the same as number of the Earley set that contains the item.

(Note to experts: Those familiar with Earley parsing will note that S4@2-3 does not look like a traditional Earley item. QDFA states are not traditional -- they come from Aycock and Horspool. The use of the at-sign as punctuation is from Grune and Jacobs. And I have added the dot earleme to the notation.)

QDFA STATES

To understand what is going on in the Earley items, it will be necessary to understand QDFA's (Quasi-Deterministic Finite Automata). Let's look at an Earley item:

    S5@0-3

This states that this item is for QDFA state 5; that it originates at earleme 0; and that it is currently at (or has its dot position at) earleme 3. We can get a description of the QDFA states with the show_QDFA method. (For those who like the big picture, the entire show_QDFA output, along with the other trace and debugging outputs for this example, is in the appendices.)

Here's what show_QDFA says about QDFA state 5:

    S5: 2,8
    Expression -> Term .
    Term -> Term . Add Term
     <Add> => S8; S9

The first line of show_QDFA's description of QDFA state 5 begins with its name: S5. The two numbers on the first line after the QDFA state name are the numbers of the NFA (Non-deterministic Finite Automata) states from which this QDFA state was formed. The NFA state numbers may be ignored. They will not be mentioned again in this document.

Marpa uses QDFA states to track the parse. Every QDFA state has one or more LR(0) items. In this example, the second and third lines are LR(0) items. LR(0) items are rules with a dot added to indicate how far recognition has proceeded into the rule.

A dot between two symbols indicates that all symbols before the dot have been recognized, but that none of the symbols after the dot have been recognized. A dot at the beginning of an LR(0) item indicates that none of its symbols have yet been recognized. A dot at the end indicates that the rule is complete -- that all of the rule's symbols have been recognized.

The location of the dot is called the dot position. The symbol before the dot position in an LR(0) item is called the pre-dot symbol. The symbol after the dot position in an LR(0) item is called the post-dot symbol.

The last line in the above display shows transitions. It says that, when Marpa sees an Add token, it will transition both to QDFA state S8 and to QDFA state S9. We will use both transitions in our examples.

It's important not to confuse Earley items with LR(0) items. Earley items are built from one or more LR(0) items. In traditional Earley parsing, each Earley item contained one and only one LR(0) item. This made the internals simpler, but was not as efficient. Marpa combines LR(0) items into QDFA states based on the ideas in Aycock and Horspool 2002.

Marpa places an important restriction on LR(0) items. The post-dot symbol is never a nullable symbol. In a completed rule, the dot position will be at the very end of a rule, after all symbols. When an LR(0) item is not a completed rule, the post-dot symbol will always be non-nullable. Intuitively, it may be helpful to think of Marpa as recognizing nulling symbols instantly whenever they are encountered, so that the dot position never stops in front of a nulling symbol.

Because nulling symbols are never post-dot symbols, the dot position is tracked in non-nulling symbols. Nulling symbols are skipped over as if they didn't exist. This document speaks of the non-null symbol position in a rule, or simply of the non-null position. Recognition of a rule starts with the dot position at the first non-null position. An LR(0) item with the dot position at the first non-null position is called a rule initialization. Beginning from the rule initialization, as each post-dot symbol is recognized, the dot position advances to the next non-null position. Once all of the zero or more symbols are recognized, the LR(0) item becomes a completed rule.

Every QDFA state contains one or more LR(0) items. This means that every QDFA state is a set of statements about rule recognition. An Earley item is present in the Earley sets if and only if every rule in every LR(0) item in the QDFA state of the Earley item has been recognized as far as its dot position.

Each Marpa Earley item records the location in the earleme stream where a set of LR(0) items begin and end. The origin of each LR(0) item corresponds to the origin of the Earley item, and the dot position of each LR(0) item corresponds to the dot location of the Earley item.

HOW EARLEY SETS ARE BUILT

New items come into the Earley sets in four ways: scanning, prediction, completion, and initialization. Scanning is the addition of Earley items to represent the recognition of symbols directly in the input. Rule completion (or simply completion) occurs when one of the rules of the grammar is fully recognized, or "completed". Rule completions add Earley items which represent the recognition of symbols that are not found directly in the input, but that are found indirectly by applying the rules of the grammar.

Prediction puts Earley items into the Earley sets which represent rules that might start at a given location. Initialization adds one or two Earley items to Earley set 0 to represent the start rules.

Scanning

Scanning adds Earley items to indicate which tokens have been recognized in the input, and where. Look again at Earley item S5@0-3. The current earleme for S5@0-3 is earleme 3. Here, again, is the information for QDFA state S5, the QDFA state in Earley item S5@0-3.

    S5: 2,8
    Expression -> Term .
    Term -> Term . Add Term
     <Add> => S8; S9

Marpa knows that it can instruct the lexer to look for a Add token at earleme 3, because in QDFA state S5, there is a transition on an Add token. In this example, an Add token corresponds to a plus sign ("+").

Marpa's state machine is not fully deterministic, it is quasi-deterministic. That is, Marpa's state machine is not a DFA (Deterministic Finite Automata), it is a QDFA. A DFA state, for any given token, has at most one transition. A QDFA state, for any given token, can have up to two transitions. In the case of QDFA state S5, on an Add token there are two transitions: one to QDFA state S8 and one to QDFA state S9. We will use both both transitions in our examples.

The transition to QDFA state S9 is an example of adding a predicted Earley item to an Earley set. The transition to QDFA state S8 is an example of adding a scanned Earley item to an Earley set. Let's look at scanning, and the transition to QDFA state 8, first.

    S8: 9
    Term -> Term Add . Term
     <Term> => S12

Which earley set does the new earley item for QDFA state S8 go into? In this example, the Add token has a length (in earlemes) of 1. The current earleme for S5@0-3 is earleme 3. Since 3+1 equals 4, the current earleme for the new Earley item will be earleme 4.

The Earley item containing the transition and from-state used to create a scanned Earley item is called the predecessor of the scanned Earley item. In this example the predecessor of the scanned Earley item is the Earley item S5@0-3. The origin of a scanned Earley item is always the same as the origin of its predecessor.

Collecting what we've determined so far, the new, scanned, Earley item will have its dot location at earleme 4, will have QDFA state S8, and will have its origin at earleme 0. This means that the new, scanned, Earley item will be S8@0-4.

Any item which is added to an Earley set based on a QDFA state transition from a predecessor, is called that predecessor's successor. In this example, S8@0-4 is the successor of S5@0-3.

    S8@0-4 [p=S5@0-3; s=Add; t=\undef]

In the current example, the tokens are "42", "*", "7", "+ and "1". The current earleme for Earley item S8@0-4 is earleme 4, indicating that the 4th token has just been seen. The fourth token is a plus sign ("+"), which the lexer reads as an Add token.

After the name of the Earley item is an Earley item source choice, shown in brackets. When the context makes the meaning clear, an Earley item source choice may be called a source or a source choice. In this example, the source choice is [p=S5@0-3; s=Add; t=\undef]. This parse is not ambiguous, so all Earley items have at most one source choice entry.

In this case the source choice entry is for a token choice. Token choices are shown as [p=predecessor; s=token_name; t=token_value] 3-tuples. The token choice [p=S5@0-3; s=Add; t=\undef] indicates that the scanned token was an Add symbol, that the scanned token value is "undef", and that the predecessor of S8@0-4 is S5@0-3.

Token Lengths

Above, we calculated the location of the new Earley item assuming that the Add token had a length, in earlemes, of 1. Where did this token length come from?

A length of 1 is the default length for a token. In fact, in the default, token-stream, model of input, a token length of 1 is the only token length possible. When other, non-standard, input models are used, tokens can be variable length. For details on alternative input models, see Marpa::API::Models.

Prediction

In the previous section, we looked at one of the transitions from QDFA state S5 at earleme 3 on an Add token with a token length of 1. There were two transitions and we looked at one of them. The transition we looked at was to QDFA state S8. Now we will look at the other transition, to QDFA state S9. Here is QDFA state S9:

    S9: predict; 3,5,7,11
    Term -> . Factor
    Factor -> . Number
    Term -> . Term Add Term
    Factor -> . Factor Multiply Factor
     <Factor> => S3
     <Number> => S4
     <Term> => S13

In the first line of the show_QDFA output for QDFA state S9, immediately after the name of the state, comes the string "predict". This means that S9 is a prediction state, and that the new Earley item we add as a result of the transition to S9 will be a predicted Earley item.

The current location of a predicted Earley item is determined in the same way as the current location of a scanned item. We take the current location of the Earley item containing the transition and add the length of the token causing the transition. In this case the transition is in S5@0-3, whose current location is 3. The token is the same as it was in the previous section: an Add token with a token length of 1. As before, 3+1 equals 4, so that the new current location will be 4.

Predicted Earley items determine their start location, or origin, in a special way. The origin of a predicted Earley item is always the same as its current location. So the origin of this new, predicted, Earley item will be 4.

Now that we know its QDFA state, current location and origin, we are in a position to add the predicted Earley item. It goes into Earley set 4, and looks like this:

    S9@4-4

Note that no source choice is shown. Source choices are not recorded for predicted items.

More about Prediction States

Prediction states exist when the presence of LR(0) items at a certain location gives rise to the expectation of other LR(0) items at that same location. When an LR(0) item with a post-dot symbol is present at a given location, we call that symbol an expected symbol. For every expected symbol at a given location, we can also expect the rule initializations for all the rules with that expected symbol on their left-hand side. Any rule initialization we introduce will have a post-dot symbol. If that post-dot symbol is not already an expected symbol, it may cause further rule initializations to be expected at the same location. In other words, LR(0) item expectations recurse.

The QDFA states invented by Aycock and Horspool handle these recursive LR(0) item expectations efficiently. Above we looked at QDFA states S8 and S9. We'll examine the recursive sequence of LR(0) item expectations that originates in S8 and is collected in S9.

The QDFA's invented by Aycock and Horspool pair every transition to a prediction state with the transition to a kernel (or non-prediction) state. In this case, S8 and S9 are a pair, where S8 is the kernel state. S8 contains only one LR(0) item:

    Term -> Term Add . Term

In this LR(0) item the post-dot symbol is Term. The paired prediction QDFA state for S8 will contain all the LR(0) item expectations caused by the expectation of the Term symbol.

Expectation of the Term symbols gives rise to the expectation of rule initializations for all rules with Term on their left hand side. There are two such rules and their rule initializations are

    Term -> . Term Add Term
    Term -> . Factor

Marpa LR(0) expectations recurse. The rule initializations above have two post-dot symbols: Term and Factor. Term is already an expected symbol.

But Factor is a new expected symbol. That means that all rules with Factor on their left hand side will have rule initializations at this earleme location. There are two such rules. The rule initialization for the first is

    Factor -> . Factor Multiply Factor
    Factor -> . Number

The post-dot symbols in these rule initializations are Factor and Number. Factor is already an expected symbol, but Number is new as an expected symbol.

We need to add all rule initializations for any rules that the Number on the left hand side. But there is no rule in our grammar with Number on its left hand side. So there are no new LR(0) item expectations to add and our recursion is finished.

Starting with the expected symbols in QDFA state S8, we found four new LR(0) item expectations. The prediction state paired with S8 is S9. Let's look at it again:

    S9: predict; 3,5,7,11
    Term -> . Factor
    Factor -> . Number
    Term -> . Term Add Term
    Factor -> . Factor Multiply Factor
     <Factor> => S3
     <Number> => S4
     <Term> => S13

We see that it does contain all four of the LR(0) item expectations that we found in this section.

Completion

Above, we saw a scanned Earley item, added as the result of recognizing a symbol directly in the input. New symbols are also recognized indirectly, when rules are completed. When a rule is recognized all the way to the end, so that its dot location is after the rightmost right hand side symbol, then the left hand side symbol of that rule is also recognized.

As a reminder, an LR(0) item with the dot at the end of the rule is called a completed rule. Whenever an Earley item contains a completed rule, that indicates that the left hand side symbol of the completed rule was recognized. The left hand side symbol of a completed rule is a completed symbol.

Earley items with completed symbols can cause other Earley items to be added to the Earley sets. Let's look at one example. S4@2-3, in Earley set 3, contains a completed symbol. Here is what QDFA state 4 looks like:

    S4: 6
    Factor -> Number .

Every completed symbol has an origin and a current earleme. The origin and the current earleme of a completed symbol are exactly the same as the origin and the current earleme of Earley item containing the completed symbol. In this example, the Earley item containing the completed symbol Factor is S4@2-3 so that Factor is a completed symbol with origin at 2 and current earleme at 3.

An Earley item containing a completed rule is called a completion cause Earley item, or simply a cause. As a result of recognizing a completion cause Earley item, Marpa may add one or more completion effect Earley items to the Earley sets. (A completion effect Earley item is often called simply a completion effect, or just an effect.)

For a completion effect Earley item to be added to the Earley sets, a completed symbol is necessary but not sufficient. There must also be a matching predecessor Earley item. A matching predecessor Earley item is an Earley item whose origin is the same as the origin of the completed symbol, and whose the post-dot symbol is the completed symbol.

In our example, Factor is a completed symbol with origin 2. We will add an Earley item as a completion effect if we find a matching predecessor: an Earley item in Earley set 2 which has Factor as one of the post-dot symbols.

In this example we find two matching predecessors: S6@0-2 and S7@2-2. Completion effects are added for both of these. We'll look only at the completion effect added using S6@0-2 as its matching predecessor. Here is QDFA state 6.

    S6: 13
    Factor -> Factor Multiply . Factor
     <Factor> => S10

The transition for Factor in QDFA state S6 is to S10, so the new completion effect will have QDFA state S6. A completion effect always has the same current earleme as its completion cause. The origin of a completion effect is always the same as the origin of its matching predecessor. The means our new new completion effect will have QDFA state S10, origin 0 and current earleme 3. Here it is:

    S10@0-3 [p=S6@0-2; c=S4@2-3]

Note the source choice in brackets after the name: [p=S6@0-2; c=S4@2-3]. Previously we saw a token source choice. This is another kind of source choice, a completion source choice. This source choice says the reason for S10@0-3 to be in the earley sets is the completion cause S4@2-3 and its matching predecessor, S6@0-2.

When a parse is ambiguous, Earley items can have multiple source choices. In this example, the parse in unambiguous, and all scanned Earley items and all completion effect Earley items will have one and only one source choice.

Completion effects recurse. A completion effect becomes the cause of another Earley item if the completion effect also contains a completed symbol and if there is a matching predecessor.

The completion effect that we just added (S10@0-3) is also a completion cause and leads to a recursion step. Earley item S10@0-3 has QDFA state S10. QDFA state S10 contains a completed rule:

    S10: 14
    Factor -> Factor Multiply Factor .

The completed symbol is Factor with origin 0 and current earleme 3. Looking for matching predessors at earleme 0, we find one: S1@0-0. Here is what QDFA state 1 looks like:

    S1: predict; 1,3,5,7,11
    Expression -> . Term
    Term -> . Factor
    Factor -> . Number
    Term -> . Term Add Term
    Factor -> . Factor Multiply Factor
     <Factor> => S3
     <Number> => S4
     <Term> => S5

The completed symbol is Factor and its transition in S1 is to S3, so we add S3@0-3 to earley set 3.

    S3@0-3 [p=S1@0-0; c=S10@0-3]

In our previous example of scanning, we referred to S5@0-3. One more completion cause-effect pair and we will see S5@0-3 again. The origin of S3@0-3 is at earleme 0. Term is only the completed symbol in QDFA state 3. S1@0-0 contains an LR(0) item which has Term as the post-dot symbol. In QDFA state S1, the transition on Term is to QDFA state S5. This means S5@0-3 will be added to earley set 3 as a completion effect, with S3@0-3 as the cause and S1@0-0 as the matching predecessor:

    S5@0-3 [p=S1@0-0; c=S3@0-3]

With completed items, if one Earley item is the matching predecessor of a second Earley item, then the second Earley item is the successor of the first one. For example, S5@0-3 is the successor of S1@0-0. The matching predecessor of an Earley item is often simply called its predecessor. For example, S1@0-0 is the predecessor of S5@0-3.

Initialization

One or two Earley items are put into Earley set 0 to start things off. In our example, the initial Earley items are

    S0@0-0
    S1@0-0

S0@0-0 will always be present in Earley set 0. QDFA state 0 contains the start rules, with the dot pointer at the beginning. Only Earley set 0 will contain an Earley item for QDFA state 0.

S1@0-0 contains the rules predicted by S0@0-0. For some very simple grammars S1@0-0 is not necessary.

EVALUATION

Choosing LR(0) Items

As we have seen, Marpa uses QDFA states in its Earley items, and these can contain multiple LR(0) items. To produce an actual parse result, Marpa must, for every Earley item, choose exactly one of the LR(0) items. To do this, it looks for an applicable LR(0) item.

If the parse is not ambiguous, there will be exactly one applicable LR(0) item for every Earley item. If the parse is ambiguous, some Earley items may have more than one applicable LR(0) item.

An LR(0) item in a cause is applicable if it matches the LR(0) item in the completion effect, and vice versa. An LR(0) item in a cause matches the LR(0) item in a completion effect, if the left hand side of the cause's LR(0) item is the same as the first non-null symbol before the dot position in the LR(0) item in the completion effect.

An LR(0) item in a predecessor is applicable if it matches the LR(0) item in the successor, and vice versa. An LR(0) item in a predecessor matches the LR(0) item in its successor if the two LR(0) items are for the same rule, and if the dot position is exactly one non-null symbol earlier in the predecessor than it is in the successor.

The matching of LR(0) items is symmetric: An LR(0) item in a completion effect matches the LR(0) item in its cause if and only if the LR(0) item in the cause matches the LR(0) item in the completion effect. An LR(0) item in a predecessor matches the LR(0) item in its successor if and only if the LR(0) item in the successor matches the LR(0) item in the predecessor.

Ambiguous Parsing

The parse in this example is not ambiguous. Marpa allows ambiguous parses. When a parse is ambigious, the procedure for adding Earley items is the same as above, except when Marpa attempts to add the same Earley item twice.

Two Earley items are considered "the same" if they have the same QDFA state, the same origin and the same current earleme. Marpa will never add the same Earley item twice. In cases where Marpa might add the same Earley item twice, Marpa instead adds another source choice to the already existing Earley item. This means that some Earley items might have multiple source choices. It is possible for an Earley item to end up with

  • Both a token source choice and a completion source choice;

  • Multiple token source choices;

  • Multiple completion source choices;

  • Multiple token source choices and multiple completion source choices;

A parse is ambigious if and only if it has one or more points of ambiguity. A point of ambiguity is an Earley item with two or more source choices, or an Earley item with two or more applicable LR(0) items.

Marpa's Single Parse Evaluator will produce one result from an ambiguous parse, but only one result. When the Single Parse Evaluator encounters a point of ambiguity, it picks just one of the alternatives, doing so arbitrarily.

Marpa's Multi-parse Evaluator will evaluate all the results of an ambiguous parse. The Multi-parse Evaluator also allows the user to control the order in which the parse results are produced.

Calculating the Parse Result

Marpa calculates parse results using fairly traditional methods. Marpa first derives a pre-order tree from the Earley sets using either the Single Parse Evaluator or the Multi-parse Evaluator. These two evaluators create the tree differently. In the case of an unambiguous parse the two evaluators will produce exactly the same tree.

Marpa evaluates the pre-order tree using an evaluation stack. The evaluation stack is initialized to empty. Every time Marpa evaluates a tree node, it pushes the result onto the evaluation stack. If a tree node needs values as input, perhaps because a Perl closure requires arguments, the values are popped from the evaluation stack. When Marpa reaches the root tree-node, computes its value and pushes it onto the stack, that value will be the only item on the stack. That one remaining stack item becomes the value of the parse.

The trace_values output in the appendix traces the calculation of the parse result in our example.

Optimizations

In some very important cases, the naive implementation of the traditional method of evaluating a parse tree is extremely wasteful. Grammars of great practical interest often contain very long sequences. A C language "translation unit" is defined as sequence of definitions and declarations, and can be extremely long. A Perl lexical unit is also a sequence.

In the case of sequences written as BNF rules, the pieces of the sequence often have no real semantics of their own. This means that popping and pushing the symbols for every piece of the sequence is pointless. It is most efficient to leave the items in the sequence on the stack until the sequence is complete. Often the semantics of individual pieces of the sequence are a no-op, and the stack can be left untouched.

Sequences are not the only case where symbols are best left on the stack. To optimize these cases, Marpa marks some of its internal symbols as "virtual". A symbol is virtual if it has no "real" semantics. When a symbol is virtual, Marpa will not bother to do the usual pop and/or push of symbols on the evaluation stack.

Marpa uses these three special rule properties to track virtual symbols: vlhs, vrhs and real. vlhs means that the left hand side is a virtual symbol. vrhs means that the right hand side contains a virtual symbol. real is an integer, indicating the number of real (that is, non-virtual) symbols on the right hand side.

DETERMINING WHETHER A PARSE IS SUCCESSFUL

As mentioned, what's usually called Earley's algorithm is just a recognizer, an algorithm to build the Earley sets. The input is recognized successfully if, at the earleme location designated as the end of parsing, there is an Earley item for a completed start rule state. A completed start rule state is a QDFA state containing a completed start rule. A completed start rule is an LR(0) item that is for the start rule and that has a dot position after the last symbol of the rule.

At most two of the QDFA states can be completed start rule states. One is a special case. In grammars which allow a null parse, the start state, QDFA state 0, is also a completed start rule state.

The other completed start rule state contains the LR(0) item which has Marpa's internal start rule with the dot pointer at the end of the rule. The lhs of the internal start rule is the internal start symbol. The rhs of the internal start rule is the grammar's original start symbol. QDFA state S2 is the completed start rule state for our example parse:

    S2: 16
    Expression['] -> Expression .

A parse starting at earleme S is successful at earleme N if it contains an Earley item for a completed start rule state with a origin earleme of S and a dot earleme of N. As of this writing, Marpa parses always start at earleme 0. The parse in our example is successful, because the following Earley item appears in Earley set 5:

    S2@0-5 [p=S0@0-0; c=S5@0-5]

APPENDIX: THE EXAMPLE

Below are the code and the trace outputs for the example used in this document.

Code for the example

    my $grammar = Marpa::Grammar->new(
        {   start          => 'Expression',
            actions        => 'My_Actions',
            default_action => 'first_arg',
            strip          => 0,
            rules          => [
                { lhs => 'Expression', rhs => [qw/Term/] },
                { lhs => 'Term',       rhs => [qw/Factor/] },
                { lhs => 'Factor',     rhs => [qw/Number/] },
                { lhs => 'Term', rhs => [qw/Term Add Term/], action => 'do_add' },
                {   lhs    => 'Factor',
                    rhs    => [qw/Factor Multiply Factor/],
                    action => 'do_multiply'
                },
            ],
        }
    );

    $grammar->precompute();

    my $recce = Marpa::Recognizer->new( { grammar => $grammar } );

    my @tokens = (
        [ 'Number', 42 ],
        [ 'Multiply', ],
        [ 'Number', 1 ],
        [ 'Add', ],
        [ 'Number', 7 ],
    );

    $recce->tokens( \@tokens );

    sub My_Actions::do_add {
        my ( undef, $t1, undef, $t2 ) = @_;
        return $t1 + $t2;
    }

    sub My_Actions::do_multiply {
        my ( undef, $t1, undef, $t2 ) = @_;
        return $t1 * $t2;
    }

    sub My_Actions::first_arg { shift; return shift; }

    my $value_ref = $recce->value();
    my $value = $value_ref ? ${$value_ref} : 'No Parse';

show_symbols Output

    0: Expression, lhs=[0] rhs=[5] terminal
    1: Term, lhs=[1 3] rhs=[0 3] terminal
    2: Factor, lhs=[2 4] rhs=[1 4] terminal
    3: Number, lhs=[] rhs=[2] terminal
    4: Add, lhs=[] rhs=[3] terminal
    5: Multiply, lhs=[] rhs=[4] terminal
    6: Expression['], lhs=[5] rhs=[]

show_rules Output

    0: Expression -> Term
    1: Term -> Factor
    2: Factor -> Number
    3: Term -> Term Add Term
    4: Factor -> Factor Multiply Factor
    5: Expression['] -> Expression /* vlhs real=1 */

show_QDFA Output

    Start States: S0; S1
    S0: 15
    Expression['] -> . Expression
     <Expression> => S2
    S1: predict; 1,3,5,7,11
    Expression -> . Term
    Term -> . Factor
    Factor -> . Number
    Term -> . Term Add Term
    Factor -> . Factor Multiply Factor
     <Factor> => S3
     <Number> => S4
     <Term> => S5
    S2: 16
    Expression['] -> Expression .
    S3: 4,12
    Term -> Factor .
    Factor -> Factor . Multiply Factor
     <Multiply> => S6; S7
    S4: 6
    Factor -> Number .
    S5: 2,8
    Expression -> Term .
    Term -> Term . Add Term
     <Add> => S8; S9
    S6: 13
    Factor -> Factor Multiply . Factor
     <Factor> => S10
    S7: predict; 5,11
    Factor -> . Number
    Factor -> . Factor Multiply Factor
     <Factor> => S11
     <Number> => S4
    S8: 9
    Term -> Term Add . Term
     <Term> => S12
    S9: predict; 3,5,7,11
    Term -> . Factor
    Factor -> . Number
    Term -> . Term Add Term
    Factor -> . Factor Multiply Factor
     <Factor> => S3
     <Number> => S4
     <Term> => S13
    S10: 14
    Factor -> Factor Multiply Factor .
    S11: 12
    Factor -> Factor . Multiply Factor
     <Multiply> => S6; S7
    S12: 10
    Term -> Term Add Term .
    S13: 8
    Term -> Term . Add Term
     <Add> => S8; S9

show_earley_sets Output

    Last Completed: 5; Furthest: 5
    Earley Set 0
    S0@0-0
    S1@0-0
    Earley Set 1
    S4@0-1 [p=S1@0-0; s=Number; t=\42]
    S3@0-1 [p=S1@0-0; c=S4@0-1]
    S5@0-1 [p=S1@0-0; c=S3@0-1]
    S2@0-1 [p=S0@0-0; c=S5@0-1]
    Earley Set 2
    S6@0-2 [p=S3@0-1; s=Multiply; t=\undef]
    S7@2-2
    Earley Set 3
    S4@2-3 [p=S7@2-2; s=Number; t=\1]
    S10@0-3 [p=S6@0-2; c=S4@2-3]
    S11@2-3 [p=S7@2-2; c=S4@2-3]
    S3@0-3 [p=S1@0-0; c=S10@0-3]
    S5@0-3 [p=S1@0-0; c=S3@0-3]
    S2@0-3 [p=S0@0-0; c=S5@0-3]
    Earley Set 4
    S8@0-4 [p=S5@0-3; s=Add; t=\undef]
    S9@4-4
    Earley Set 5
    S4@4-5 [p=S9@4-4; s=Number; t=\7]
    S3@4-5 [p=S9@4-4; c=S4@4-5]
    S12@0-5 [p=S8@0-4; c=S3@4-5]
    S13@4-5 [p=S9@4-4; c=S3@4-5]
    S5@0-5 [p=S1@0-0; c=S12@0-5]
    S2@0-5 [p=S0@0-0; c=S5@0-5]

trace_values Output

    Pushed value from S4@0-1L2o12a12: Number = \42
    Popping 1 values to evaluate S4@0-1L2o12a12, rule: 2: Factor -> Number
    Calculated and pushed value: 42
    Pushed value from S6@0-2R4:2o10a10: Multiply = \undef
    Pushed value from S4@2-3L2o9a9: Number = \1
    Popping 1 values to evaluate S4@2-3L2o9a9, rule: 2: Factor -> Number
    Calculated and pushed value: 1
    Popping 3 values to evaluate S10@0-3L2o8a8, rule: 4: Factor -> Factor Multiply Factor
    Calculated and pushed value: 42
    Popping 1 values to evaluate S3@0-3L1o7a7, rule: 1: Term -> Factor
    Calculated and pushed value: 42
    Pushed value from S8@0-4R3:2o5a5: Add = \undef
    Pushed value from S4@4-5L2o4a4: Number = \7
    Popping 1 values to evaluate S4@4-5L2o4a4, rule: 2: Factor -> Number
    Calculated and pushed value: 7
    Popping 1 values to evaluate S3@4-5L1o3a3, rule: 1: Term -> Factor
    Calculated and pushed value: 7
    Popping 3 values to evaluate S12@0-5L1o2a2, rule: 3: Term -> Term Add Term
    Calculated and pushed value: 49
    Popping 1 values to evaluate S5@0-5L0o1a1, rule: 0: Expression -> Term
    Calculated and pushed value: 49
    New Virtual Rule: S2@0-5L6o0a0, rule: 5: Expression['] -> Expression
    Symbol count is 1, now 1 rules

LICENSE AND COPYRIGHT

Copyright 2007-2010 Jeffrey Kegler, all rights reserved. Marpa is free software under the Perl license. For details see the LICENSE file in the Marpa distribution.