Duplicate parses are eliminated

When evaluating the parse trees in a parse series, Marpa never evaluates the same parse tree twice. What this means probably matches the programmer intuition of what it should mean. Marpa considers two parse trees to be the same if they are semantic equivalents.

Two parse trees are semantic equivalents if and only if a recursive, top-down evaluation of each applies the same rules in the same order at the same earleme locations. This definition implies that, given any deterministic semantics, two parse trees which are semantic equivalents will always produce the same parse result -- hence the term. When the Marpa documentation refers to duplicate parses, it will mean that the two are semantic equivalents, unless otherwise stated.

Default parse order

In this document, the term arbitrary parse order is used to mean an arbitrary choice among the strict total orders of the equivalence classes that contain the semantically equivalent parse trees. This set of equivalence classes is finite.

Traversal of the parse trees in arbitrary parse order will be always be well-behaved in the sense that no two parse trees will be semantic duplicates, and no unique (semantic non-duplicate) parse tree will be omitted in it, No other property of arbitrary parse order is guaranteed. For example, the order may change each time the parse series is traversed.

By calling the recognizer's "value" method repeatedly, Marpa can produce all the parse results in the current parse series. The default is for the parse results to be returned in an arbitrary parse order. This corresponds to the ""none"" value of the recognizer's "ranking_method" named argument.

Ranking methods

Marpa recognizer objects have a "ranking_method" named argument, whose value can be the name of a ranking method, or ""none"", indicating that the default ranking method is to be used.

The "rule" ranking method

The rule method ranks alternative parses according to their rules. Every rule has a rule rank. A rule's rank can be specified using the the "rank" named argument for that rule. Rule ranks must be integers. If no rule rank is specified, the rule rank is 0.

The "high_rule_only" ranking method

The "high_rule_only" ranking method is similar to the "rule" ranking method, except that, at every choice point, it discards all of the choices which have a rank lower than that of the highest ranked alternative.

Since the "high_rule_only" ranking method eliminates some parse trees, it can reduce or eliminate the ambiguity of a parse, but it does not necessarily do either. This is because, at each choice point among the parse trees, it is possible that several of the choices, or all of them, will have the same rank as the highest ranked alternative.

Rule ranking

A parse series is kept in a structure called a parse bocage. The parse bocage is a tree-like structure, whose root node is the common root of all the parse trees of the parse series. In an unambiguous parse, there will be only one parse tree, and the parse bocage will be equivalent to that parse tree. In an ambiguous parse, there will be choice points in the parse bocage. At the choice points, there will be two or more alternatives -- choices which result in different parse trees.

When ranking, the logic traverses the parse bocage, looking for choice points. From the point of view of the individual parse trees, this traversal will be top-down and left-to-right. At the choice points, the alternatives are ranked (in the "rule" ranking method) or selected (in the "high_rule_only" ranking method), by comparing them as follows:

• Different ranks: If the two alternatives have different rule ranks, they must also have different rules. The alternative with the higher rule rank will rank high.
• Same Rule: If the two alternatives have the same rule, they rank as described under "Null variant ranking".
• Same rank, different rules: Two different rules can have the same rank. If the two alternatives are for different rules, but the two rules have the same rank, the relative order of the two alternatives is arbitrary.

Null variant ranking

Some rules have a RHS which contains proper nullables: symbols which may be nulled, but which are not nulling symbols. (Nulling symbols are symbols which are always nulled.)

When a rule contains proper nullables, each application of that rule to a section of input creates a nulling variant -- that rule with a specific pattern of null and non-null symbols. In many cases, this creates an ambiguity -- different nulling variants could apply to the same substring of input. In ambiguous parsings of this kind, some applications may want to rank nulling variants that start with non-null symbols higher. Other applications may want to do the opposite -- to rank nulling variants that start with null symbols higher.

The "null_ranking" named argument for rules specifies which nulling variants are ranked high or low. Ranking of nulling variants is done left-to-right, with the null preference as indicated by the "null_ranking" named argument. Specifically, if the "null_ranking" is ""low"", then the closer a nulling variant places its visible (non-null) symbols to the start of the rule, the higher it ranks. "low" null ranking is the default. If the "null_ranking" is ""high"", then the closer a nulling variant places its null symbols to the start of the rule, the higher it ranks.

A general approach to sorting parses

The most general way to sort Marpa parses is for the application to take control. The application can set up the Marpa semantic actions so that the parse result of every parse tree is a "<rank, true_value>" duple. The duples can then be sorted by "rank". Once the resuls are sorted, the "rank" element of the duple can be discarded. (Those familiar with the Schwartzian transform may note a resemblance. In Perl, duples can be implemented as references to arrays of 2 elements.)

The user needs to be careful. In theory, ambiguity can cause an exponential explosion in the number of results. In practice, ambiguity tends to get out of hand very easily. Producing and sorting all the parses can take a very long time.