#grammar #generation #symbol #string #rules


Provides tools for generating strings and sequences using context-free grammars

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0.2.1 May 14, 2021
0.2.0 Dec 31, 2019
0.1.1 Dec 29, 2019
0.1.0 Dec 29, 2019

#1172 in Algorithms


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This crate provides basic tools for generation of text strings and other sequences using context-free grammars.

Text generation example

The following example demonstrates random generation of a short sentence based on a sequence of input symbols and a set of rules, which may be applied to the symbol sequence until it is fully expanded.

use branchy::{

let input = vec![Symbol::Nonterminal("person"), Symbol::Terminal("comes from a"), Symbol::Nonterminal("location")];

let rules = vec![
    Rule::new("person", vec![Symbol::Nonterminal("name")]),
        vec![Symbol::Nonterminal("name"), Symbol::Terminal("the"), Symbol::Nonterminal("occupation")]
    Rule::new("name", vec![Symbol::Terminal("Alice")]),
    Rule::new("name", vec![Symbol::Terminal("Bob")]),
    Rule::new("occupation", vec![Symbol::Terminal("blacksmith")]),
    Rule::new("occupation", vec![Symbol::Terminal("baker")]),
    Rule::new("location", vec![Symbol::Nonterminal("size"), Symbol::Nonterminal("settlement_type")]),
    Rule::new("size", vec![Symbol::Terminal("small")]),
    Rule::new("size", vec![Symbol::Terminal("big")]),
    Rule::new("settlement_type", vec![Symbol::Terminal("village")]),
    Rule::new("settlement_type", vec![Symbol::Terminal("town")])

let mut expander = ExpanderBuilder::from(rules).build();

let expansion_result = expander.expand(input);

let expanded_string = expansion_result.unwrap().join(" ");
println!("{}", expanded_string);

When run, this example prints out a sentence, similar to the ones below:

Alice the blacksmith comes from a big village

Bob comes from a small town

Bob the baker comes from a big town

As you can see, both the input sequence and the rules of the grammar are described in terms of Nonterminal (ones that can be further expanded) and Terminal symbols. All of the rules have a non-terminal symbol value on their left-hand side and a sequence which may contain both Nonterminal and Terminal symbols on their right-hand side.

The "magic" happens in Expander's expand() method, which repeatedly selects and applies matching rules until the sequence is fully expanded (i.e contains only terminal symbols).

By default, UniformRandomRuleSelector is used to select rules while expanding, therefore the result is randomized. As we'll see below, this can be changed, if needed, via ExpanderBuilder.

Using a custom rule selector

When constructing an Expander, you can provide your own rule selector via ExpanderBuilder's with_rule_selector() method.

The following example defines a custom rule selector, which always chooses the first matching rule, and then uses it in generation of a short phrase. As you can see, rule selectors need to implement at least the select_matching_rule() method from the RuleSelector trait.

use branchy::{

struct AlwaysFirstRuleSelector;

impl<Nt, T> RuleSelector<Nt, T> for AlwaysFirstRuleSelector {
    fn select_matching_rule<'a>(&self, matching_rules: &[&'a Rule<Nt, T>]) -> Option<&'a Rule<Nt, T>> {
        if matching_rules.is_empty() {
        } else {

let input = vec![Symbol::Terminal("Have a"), Symbol::Nonterminal("adjective"), Symbol::Nonterminal("time_of_day")];

let mut expander = ExpanderBuilder::new()
    .with_new_rule("adjective", vec![Symbol::Terminal("wonderful")])
    .with_new_rule("adjective", vec![Symbol::Terminal("great")])
    .with_new_rule("time_of_day", vec![Symbol::Terminal("afternoon")])
    .with_new_rule("time_of_day", vec![Symbol::Terminal("evening")])

let expanded_string = expander.expand(input).unwrap().join(" ");

assert_eq!(expanded_string, "Have a wonderful afternoon");

This example also sets the rules of the grammar directly on ExpanderBuilder via the with_new_rule() method. See the documentation of ExpanderBuilder for more helper methods.


To help you debug your grammars, branchy provides the ExpansionLogger trait, which you can implement in order to be notified of the progress of the expansion and the steps it takes.

For example, in order to log the rules selected at each step of the expansion, you can implement the on_nonterm_expanded() method. The following example writes a message via println!() on every step.

use branchy::{

struct StdOutLogger;

impl<Nt, T> ExpansionLogger<Nt, T> for StdOutLogger
    where Nt: NonterminalValue + std::fmt::Debug,
          T:  TerminalValue + std::fmt::Debug
    fn on_nonterm_expanded(&mut self, expanded_nonterm_value: &Nt, rule: &Rule<Nt, T>) {
        println!("expanded {:?} to {:?}", expanded_nonterm_value, rule.replacement);

let input = vec![
    Symbol::Terminal("There is a"),
    Symbol::Terminal("to the"),
    Symbol::Terminal("of the town.")

let rules = vec![
    Rule::new("site_description", vec![Symbol::Nonterminal("adjective"), Symbol::Nonterminal("site")]),
    Rule::new("adjective", vec![Symbol::Terminal("huge")]),
    Rule::new("adjective", vec![Symbol::Terminal("dark")]),
    Rule::new("site", vec![Symbol::Terminal("forest")]),
    Rule::new("site", vec![Symbol::Terminal("cave")]),
    Rule::new("direction", vec![Symbol::Terminal("north")]),
    Rule::new("direction", vec![Symbol::Terminal("east")])

let mut expander = ExpanderBuilder::from(rules)


This example produces output similar to the following:

expanded "site_description" to [Nonterminal("adjective"), Nonterminal("site")]
expanded "adjective" to [Terminal("dark")]
expanded "site" to [Terminal("cave")]
expanded "direction" to [Terminal("east")]

Generating non-text sequences

Even though the primary use-case for branchy is generating text strings, it can be used for grammars producing other kinds of sequences. Any type implementing Clone + PartialEq can be used for values of non-terminal symbols and any type implementing Clone can be used for terminals. See NonterminalValue and TerminalValue traits.