Marpa::R2::Semantics::Order - How the SLIF ranks ambiguous parses


Marpa allows ambiguous parses. While an unambiguous parse can produce at most one parse tree and one parse result, an ambiguous parse will produce a parse series. A parse series is a sequence of parse trees, each of which will have its own parse result.

This document describes ways of controlling the order in which the SLIF recognizer's value() method evaluates the parse trees of an ambiguous parse. It also describes ways to exclude selected parse trees from the parse series.

Default parse order

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.

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.


When ranking, the logic traverses each node of the parse bocage. In this context, the nodes are also called "choicepoints". From the point of view of the individual parse trees, the traversal will be top-down and left-to-right.

Each choicepoint has one or more "choices". Often a choicepoint has only a single choice, in which case the choicepoint is called "trivial". For two rule instances to be choices of the same choicepoint, they must end at the same location, and their rules must have the same LHS.

The terms "choicepoint" and "choice" are defined carefully in a separate document. That document also gives several examples of ranking, which are explained in detail.

Ranking methods

SLIF 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 rule alternatives. Every rule alternative has a numeric rank. A rule's rank can be specified using the the rank adverb argument for that RHS alternative. Rule ranks must be integers. They may be negative. If no numeric rank is specified, the numeric 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 choicepoint, it discards all of the choices which have a rank lower than that of the highest ranked choice.

The high_rule_only ranking method can reduce the ambiguity of a parse, but it does not necessarily do so. This is because, at each choicepoint 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 choice.

Rule ranking

At each choicepoint, the choices are ranked as follows:

  • Different numeric ranks:

    If the two parse choices have different numeric ranks, they must also have different rule alternatives. The parse choice whose rule alternative has the higher numeric rank will rank high.

  • Same rule alternative:

    If the two parse choices have the same rule alternative, they rank as described under "Null variant ranking".

  • Same numeric rank, different rule alternatives:

    Two different rule alternatives can have the same numeric rank. If the two parse choices are for rule alternatives that are different, but that have the same numeric rank, the relative order of the two parse choices is arbitrary.

Rule alternatives may be part of a single rule in the DSL -- for example, a prioritized rule. Lexical order within a DSL rule makes no difference when ranking rule alternatives. For example, it makes no difference if two rule alternatives come from the same prioritized rule; or from two different prioritized rules.

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 alternative contains proper nullables, each instance of that rule creates a nulling variant. A nulling variant is a specific pattern of null and non-null symbols in a rule instance's RHS. In many cases, this creates an ambiguity -- different nulling variants can match the same substring in the 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 adverb for RHS alternatives specifies which nulling variants are ranked high or low. If the null-ranking is "low", then the closer a nulling variant places its visible (non-null) symbols to the start of the rule instance, the higher it ranks. A null ranking of low 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 instance, the higher it ranks. In ranking nulling variants with more than one proper nullable, major-to-minor is left-to-right.

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 results 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.


This section contains additional explanations, not essential to understanding the rest of this document. Often they are formal or mathematical. While some people find these helpful, others find them distracting, which is why they are segregated here.

Duplicate parses

When evaluating the parse trees in a parse series, Marpa never evaluates the same parse tree twice. What this means probably matches the programmer's 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 G1 locations. If the semantics are deterministic, and if two parse trees are semantic equivalents according to this definition, the two parse trees will always produce the same parse result.

The two parse trees are called semantic equivalents, because from the point of view of a deterministic semantics they are indistinguishable. When the Marpa documentation refers to duplicate parses, unless otherwise stated, it means that the two are semantic equivalents.

Formally, semantic equivalence is defined as follows: Call the set of parse trees, T. Semantic equivalence is an equivalence relation on T. Call this relation ~. Call E, the quotient set of T by ~. In this document, the term arbitrary parse order is used to mean an arbitrary choice among the relations which are strict total orders of E.

Copyright and License

  Copyright 2018 Jeffrey Kegler
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