# RustyLR
RustyLR will provide you a LR(1) and LALR(1) Deterministic Finite Automata (DFA) generator from Context Free Grammar (CFGs).
```
[dependencies]
rusty_lr = "0.6.1"
rusty_lr_derive = "0.6.1"
```
## Features
- pure Rust implementation
- compile-time DFA construction from CFGs ( with proc-macro )
- customizable reducing action
- resolving conflicts of ambiguous grammar
- tracing parser action with callback, also error handling
- readable error messages, both for grammar building and parsing
## Sample
In [`example/calculator/parser.rs`](example/calculator/src/parser.rs),
```rust
use rusty_lr_derive::lr1;
use rusty_lr_derive::lalr1;
enum Token {
// token definitions
...
}
// this define struct `EParser`
// where 'E' is the start symbol
lalr1! {
// type of userdata
%userdata i32;
// type of token ( as Terminal symbol )
%tokentype Token;
// start symbol
%start E;
// eof symbol; for augmented rule generation
%eof Token::Eof;
// define tokens
%token num Token::Num(0);
%token plus Token::Plus;
%token star Token::Star;
%token lparen Token::LParen;
%token rparen Token::RParen;
// resolving shift/reduce conflict
%left plus;
%left star;
// s{N} is slice of shifted terminal symbols captured by N'th symbol
// v{N} is value of N'th symbol ( if it has value )
// s is slice of shifted terminal symbols captured by current rule
// userdata canbe accessed by `data` ( &mut i32, for current situation )
A(i32) : A plus A {
println!("{:?} {:?} {:?}", s0, s1, s2 );
// ^ ^ ^
// | | |- slice of 2nd 'A'
// | |- slice of 'plus'
// |- slice of 1st 'A'
println!( "{:?}", s );
*data += 1;
v0 + v2 // --> this will be new value of current 'A'
// ^ ^
// | |- value of 2nd 'A'
// |- value of 1st 'A'
}
| M { v0 }
;
M(i32) : M star M { v0 * v2 }
| P { v0 }
;
P(i32) : num { if let Token::Num(n) = v0 { *n } else { return Err(format!("{:?}", s0)); } }
| lparen E rparen { v1 }
;
E(i32) : A { v0 };
}
```
In [`example/calculator/src/main.rs`](example/calculator/src/main.rs),
```rust
mod parser;
fn main() {
use parser::Token;
let input = vec![
Token::Num(1),
Token::Plus,
Token::Num(2),
Token::Star,
Token::LParen,
Token::Num(3),
Token::Plus,
Token::Num(4),
Token::RParen,
];
let parser = parser::EParser::new();
let mut context = parser.begin();
let mut userdata: i32 = 0;
for token in input {
match parser.feed(&mut context, token, &mut userdata) {
// ^^^^^ ^^^^^^^^^^^^ userdata passed here as `&mut i32`
// |- feed token
Ok(_) => {}
Err(e) => {
println!("{:?}", e);
return;
}
}
}
// res = value of start symbol ( E(i32) )
let res = context.accept();
println!("{}", res);
println!("userdata: {}", userdata);
}
```
The result will be:
```
[Num(3)] [Plus] [Num(4)]
[Num(3), Plus, Num(4)]
[Num(1)] [Plus] [Num(2), Star, LParen, Num(3), Plus, Num(4), RParen]
[Num(1), Plus, Num(2), Star, LParen, Num(3), Plus, Num(4), RParen]
15
userdata: 2
```
## Build Deterministic Finite Automata (DFA) from Context Free Grammar (CFG)
This section will describe how to build DFA from CFGs, on runtime.
### 1. Define terminal and non-terminal symbols
```rust
#[derive(Clone, Hash, PartialEq, Eq, PartialOrd, Ord)] // must implement these traits
pub enum Term {
Num,
Plus,
Mul,
LeftParen,
RightParen,
Eof,
}
#[derive(Clone, Hash, PartialEq, Eq)] // must implement these traits
pub enum NonTerm {
E,
A,
M,
P,
Augmented,
}
/// impl Display for TermType, NonTermType will make related ProductionRule, error message Display-able
impl Display for TermType { ... }
impl Display for NonTermType { ... }
```
Or simply, you can use `char` or `u8` as terminal, and `&'static str` or `String` as non-terminal.
***Any type*** that implements traits above can be used as terminal and non-terminal symbols.
### 2. Define production rules
Consider the following context free grammar:
```
A -> A + A (reduce left)
A -> M
```
This grammar can be written as:
```rust
/// type alias
type Token = rusty_lr::Token<Term, NonTerm>;
/// create grammar
let mut grammar = rusty_lr::Grammar::<Term,NonTerm>::new();
grammar.add_rule(
NonTerm::A,
vec![Token::NonTerm(NonTerm::A), Token::Term(Term::Plus), Token::NonTerm(NonTerm::A)],
);
grammar.add_rule(
NonTerm::A,
vec![Token::NonTerm(NonTerm::M)],
);
/// set reduce type
grammar.set_reduce_type( Term::Plus, ReduceType::Left );
```
Note that the production rule `A -> A + A` has a shift/reduce conflict, and the reduce type is set to `ReduceType::Left` for terminal symbol `Plus` to resolve the conflict. Default will cause an error when a conflict occurs.
reduce/reduce conflict (e.g. duplicated rules) will be always an error.
### 3. Build DFA
Calling `grammar.build()` or `grammar.build_lalr()` will build the DFA from the CFGs.
```rust
let parser:rusty_lr::Parser<Term,NonTerm> = match grammar.build(NonTerm::Augmented) {
Ok(parser) => parser,
Err(err) => {
// error is Display if Term, NonTerm is Display
eprintln!("{}", err);
return;
}
};
```
You must explicitly specify the Augmented non-terminal symbol, and the production rule `Augmented -> StartSymbol $` must be defined in the grammar.
The returned `Parser` struct contains the DFA and the production rules(cloned). It is completely independent from the `Grammar` struct, so you can drop the `Grammar` struct, or export the `Parser` struct to another module.
### 4. Error messages
The `Error` type returned from `Grammar::build()` will contain the error information.
`Grammar` is `Display` if both `Term` and `NonTerm` is `Display`, and It is `Debug` if both `Term` and `NonTerm` is `Debug`.
For Shift/Reduce conflicts,
```
Build failed: Shift/Reduce Conflict
NextTerm: '0'
Reduce Rule:
"Num" -> "Digit"
Shift Rules:
"Digit" -> '0' • /Lookaheads: '\0', '0'
Try rearanging the rules or set ReduceType to Terminal '0' to resolve the conflict.
```
For Reduce/Reduce Conflicts,
```
Build failed: Reduce/Reduce Conflict with lookahead: '\0'
Production Rule1:
"Num" -> "Digit"
Production Rule2:
"Num" -> "Digit"
```
## Parse input sequence with generated DFA
For given input sequence, you can start parsing with `Parser::begin()` method. Once you get the `Context` from `begin()`, you will feed the input sequence to the parser with `parser.feed()` method.
```rust
let terms = vec![ Term::Num, Term::Plus, Term::Num, Term::Mul, Term::LeftParen, Term::Num, Term::Plus, Term::Num, Term::RightParen, Term::Eof];
// start parsing
let mut context = parser.begin();
// feed input sequence
for term in terms {
match parser.feed(&mut context, term) {
Ok(_) => (),
Err(err) => {
eprintln!("{:?}", err);
return;
}
}
}
```
Note that `EOF` token is feeded at the end of sequence, and the augmented rule `Augmented -> StartSymbol $` will not be reduced since there are no lookahead symbols.
### Parse with callback
For complex error handling and tracing parser action, you can implement `Callback` trait and pass it to `*_callback(...)` methods.
```rust
struct ParserCallback {}
impl rusty_lr::Callback<Term, NonTerm> for ParserCallback {
/// Error type for callback
type Error = String;
fn reduce(
&mut self,
rules: &[rusty_lr::ProductionRule<char, String>],
// ^ |- NonTerm
// |- Term
states: &[rusty_lr::State<char, String>],
// ^ |- NonTerm
// |- Term
state_stack: &[usize],
rule: usize,
) -> Result<(), Self::Error> {
// `Rule` is Display if Term, NonTerm is Display
println!("Reduce by {}", rules[rule]);
Ok(())
}
fn shift_and_goto(
&mut self,
rules: &[rusty_lr::ProductionRule<char, String>],
states: &[rusty_lr::State<char, String>],
state_stack: &[usize],
term: &char,
) -> Result<(), Self::Error> {
Ok(())
}
fn shift_and_goto_nonterm(
&mut self,
rules: &[rusty_lr::ProductionRule<char, String>],
states: &[rusty_lr::State<char, String>],
state_stack: &[usize],
nonterm: &String,
) -> Result<(), Self::Error> {
Ok(())
}
}
```
```rust
let terms = vec![ Term::Num, Term::Plus, Term::Num, Term::Mul, Term::LeftParen, Term::Num, Term::Plus, Term::Num, Term::RightParen, Term::Eof];
// start parsing
let mut context = parser.begin();
let mut callback = ParserCallback {};
// feed input sequence
for term in terms {
match parser.feed_callback(&mut context, &mut callback, term) {
Ok(_) => (),
Err(err) => {
eprintln!("{:?}", err);
return;
}
}
}
```
The result will be:
```
Reduce by P -> Num
Reduce by M -> P
Reduce by A -> M
Reduce by P -> Num
Reduce by M -> P
Reduce by P -> Num
Reduce by M -> P
Reduce by A -> M
Reduce by P -> Num
Reduce by M -> P
Reduce by A -> M
Reduce by A -> A + A
Reduce by E -> A
Reduce by P -> ( E )
Reduce by M -> P
Reduce by M -> M * M
Reduce by A -> M
Reduce by A -> A + A
Reduce by E -> A
```