cfg 0.7.0

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  <h1>cfg</h1>
  <p>
    <strong>Context-free grammar tools.</strong>
  </p>
  <p>

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Rust library for manipulating context-free grammars.
[You can check the documentation here](https://docs.rs/cfg/).

## Analyzing and modifying grammars

The following features are implemented thus far:

* rich rule building
  * sequence rules,
  * precedenced rules.
* conversions to a shape similar to Chomsky Normal Form
  * grammar binarization,
  * nulling rule elimination for binarized grammars.
* sanity
  * cycle detection and elimination,
  * useless rule detection and elimination,
  * unused symbol removal.
* analysis for LR(1), LL(1) and others
  * FIRST and FOLLOW set computation,
  * minimal distance computation,
  * LL(1) classification.
* tools for probabilistic grammars
  * generation for PCFGs + negative zero-width lookahead.

## Building grammars

`cfg` includes an interface that simplifies grammar construction.

### Generating symbols

The easiest way of generating symbols is with the `sym` method. The library is unaware
of the start symbol.

```rust
let mut grammar: Cfg = Cfg::new();
let (start, expr, identifier, number,
     plus, multiply, power, l_paren, r_paren, digit) = grammar.sym();
```

### Building grammar rules

Rules have a LHS symbol and zero or more RHS symbols.

Example BNF:
```bnf
start ::= expr | identifier l_paren expr r_paren
```

With our library:
```rust
grammar.rule(start).rhs([expr])
                   .rhs([identifier, l_paren, expr, r_paren]);
```

### Building sequence rules

Sequence rules have a LHS symbol, a RHS symbol, a range of repetitions, and
optional separation. Aside from separation, they closely resemble regular
expression repetitions.

Example BNF:
```bnf
number ::= digit+
```

With our library:
```rust
grammar.sequence(number).inclusive(1, None).rhs(digit);
```

### Building precedenced rules

Precedenced rules are the most convenient way to describe operators. Once
built, they are immediately rewritten into basic grammar rules, and unique
symbols are generated. Operator associativity can be set to `Right` or
`Group`. It's `Left` by default.

```rust
use cfg::precedence::Associativity::{Right, Group};

grammar.precedenced_rule(expr)
           .rhs([number])
           .rhs([identifier])
           .associativity(Group)
           .rhs([l_paren, expr, r_paren])
       .lower_precedence()
           .associativity(Right)
           .rhs([expr, power, expr])
       .lower_precedence()
           .rhs([expr, multiply, expr])
       .lower_precedence()
           .rhs([expr, plus, expr]);
```

## Using a custom grammar representation

Your grammar type has to implement a trait, and two more traits are needed for grammar references:

* implement `RuleContainer` for `MyGrammar`
* implement `RuleContainerRef` for `&'a MyGrammar`
* implement `RuleContainerMut` for `&'a mut MyGrammar`

## License

Dual-licensed for compatibility with the Rust project.

Licensed under the Apache License Version 2.0:
http://www.apache.org/licenses/LICENSE-2.0, or the MIT license:
http://opensource.org/licenses/MIT, at your option.