# RustyLR
yacc-like LR(1) and LALR(1) Deterministic Finite Automata (DFA) generator from Context Free Grammar (CFGs).
```
[dependencies]
rusty_lr = "1.0.0"
```
## Features
- pure Rust implementation
- readable error messages, both for grammar building and parsing
- compile-time DFA construction from CFGs ( with proc-macro )
- customizable reduce action
- resolving conflicts of ambiguous grammar
- tracing parser action with callback
#### Why proc-macro, not external executable?
- Decent built-in lexer, with consideration of unicode and comments.
- Can generate *pretty* error messages, by just passing `Span` data.
- With modern IDE, auto-completion and error highlighting can be done in real-time.
## Sample
- [Calculator](example/calculator): calculator with enum `Token`
- [Calculator u8](example/calculator_u8): calculator with `u8`
- [Bootstrap](rusty_lr_parser/src/parser.rs): bootstrapped line parser of `lr1!` and `lalr1!` macro
Please refer to the [example](example) directory for the full example.
In [`example/calculator_u8/parser.rs`](example/calculator_u8/src/parser.rs),
```rust
use rusty_lr::lr1;
use rusty_lr::lalr1;
// this define struct `EParser`
// where 'E' is the start symbol
lr1! {
%userdata i32;
%tokentype u8;
%start E;
%eof b'\0';
%token zero b'0';
%token one b'1';
%token two b'2';
%token three b'3';
%token four b'4';
%token five b'5';
%token six b'6';
%token seven b'7';
%token eight b'8';
%token nine b'9';
%token plus b'+';
%token star b'*';
%token lparen b'(';
%token rparen b')';
%token space b' ';
%left plus;
%left star;
WS0: space*;
Digit(u8): zero | one | two | three | four | five | six | seven | eight | nine;
Number(i32): WS0 Digit+ WS0 { std::str::from_utf8(&Digit).unwrap().parse().unwrap() };
A(f32): A plus a2=A {
*data += 1; // access userdata by `data`
println!( "{:?} {:?} {:?}", A, plus as char, a2 );
A + a2
}
| M
;
M(f32): M star m2=M { M * m2 }
| P
;
P(f32): Number { Number as f32 }
| WS0 lparen E rparen WS0 { E }
;
E(f32) : A ;
}
```
In [`example/calculator_u8/src/main.rs`](example/calculator_u8/src/main.rs),
```rust
pub mod parser;
fn main() {
let input = " 1 + 20 * (3 + 4 ) ";
let parser = parser::EParser::new();
let mut context = parser.begin();
let mut userdata: i32 = 0;
for b in input.as_bytes().iter() {
match parser.feed(&mut context, *b, &mut userdata) {
// feed userdata here
Ok(_) => {}
Err(e) => {
eprintln!("error: {:?}", e);
return;
}
}
}
parser.feed(&mut context, 0, &mut userdata).unwrap(); // feed EOF
let result = context.accept(); // get value of start 'E'
println!("result: {}", result);
println!("userdata: {}", userdata);
}
```
The result will be:
```
3.0 '+' 4.0
1.0 '+' 140.0
result: 141
userdata: 2
```
## proc-macro `lr1!`, `lalr1!`, `lr1_runtime!`, and `lalr1_runtime!`
`lr1!`, `lalr1!`, `lr1_runtime!`, and `lalr1_runtime!` are procedural macros that will build `Parser` struct from CFGs.
Please refer to the [Sample](#sample) section for actual usage.
### Compile-time vs Runtime
Former two macros (those without '_runtime' suffix) will generate `Parser` struct at compile-time, which `build()` is called at compile-time and the generated code will be *TONS* of `insert` of tokens one by one.
Latter two (those with '_runtime' suffix) will generate `Parser` struct at runtime, the `build()` is called on `Parser::new()`.
### Syntax
Every line in the macro must follow the syntax below. The syntax can be also found in the [bootstrap](rusty_lr_parser/src/parser.rs) file.
### Token type <sub><sup>(must defined)</sup></sub>
```
'%tokentype' <RustType> ';'
```
Define the type of terminal symbols.
`<RustType>` must be accessible at the point where the macro is called.
### Token definition <sub><sup>(must defined)</sup></sub>
```
'%token' <Ident> <RustExpr> ';'
```
Map terminal symbol's name `<Ident>` to the actual value `<RustExpr>`.
`<RustExpr>` must be accessible at the point where the macro is called.
### Start symbol <sub><sup>(must defined)</sup></sub>
```
'%start' <Ident> ';'
```
Define the start symbol of the grammar.
### Eof symbol <sub><sup>(must defined)</sup></sub>
```
'%eof' <RustExpr> ';'
```
Define the `eof` terminal symbol.
`<RustExpr>` must be accessible at the point where the macro is called.
'eof' terminal symbol will be automatically added to the grammar.
### Userdata type <sub><sup>(optional)</sup></sub>
```
'%userdata' <RustType> ';'
```
Define the type of userdata passed to `feed()` function.
### Reduce type <sub><sup>(optional)</sup></sub>
```
// reduce first
'%left' <Ident> ';'
'%l' <Ident> ';'
'%reduce' <Ident> ';'
// shift first
'%right' <Ident> ';'
'%r' <Ident> ';'
'%shift' <Ident> ';'
```
Set the shift/reduce precedence for terminal symbols. `<Ident>` must be defined in `%token`.
### Production rules
```
<Ident><RuleType>
':' <TokenMapped>* <ReduceAction>
';'
```
Define the production rules.
```
<TokenMapped> : <Ident as var_name> '=' <TokenPattern>
| <TokenPattern>
;
```
```
<TokenPattern> : <Ident as terminal or non-terminal>
| <Ident as terminal or non-terminal> '*' (zero or more)
| <Ident as terminal or non-terminal> '+' (one or more)
| <Ident as terminal or non-terminal> '?' (zero or one)
;
```
Example:
```
E: A plus* d=D ;
```
This production rule defines non-terminal `E` to be `A`, then zero or more `plus`, then `D` mapped to variable `d`.
For more information, please refer to the [Accessing token data in ReduceAction](#accessing-token-data-in-reduceaction) section.
### RuleType <sub><sup>(optional)</sup></sub>
```
<RuleType> : '(' <RustType> ')'
|
;
```
Define the type of value that this production rule holds.
### ReduceAction <sub><sup>(optional)</sup></sub>
```
<ReduceAction> : '{' <RustExpr> '}'
|
;
```
Define the action to be executed when the rule is matched and reduced.
If `<RuleType>` is defined, `<ReduceAction>` itself must be the value of `<RuleType>` (i.e. no semicolon at the end of the statement).
`<ReduceAction>` can be omitted if:
- `<RuleType>` is not defined
- Only one token is holding value in the production rule
### Accessing token data in ReduceAction
**predefined variables** can be used in `<ReduceAction>`:
- `data` : userdata passed to `feed()` function.
To access the data of each token, you can directly use the name of the token as a variable.
For non-terminal symbols, the type of variable is `<RuleType>`.
For terminal symbols, the type of variable is `%tokentype`.
If multiple variables are defined with the same name, the variable on the front-most will be used.
For regex-like pattern, type of variable will be modified by following:
| '*' | `Vec<T>` | (not defined) | `Vec<TermType>` |
| '+' | `Vec<T>` | (not defined) | `Vec<TermType>` |
| '?' | `Option<T>` | (not defined) | `Option<TermType>` |
For example, following code will print the value of each `A`, and the slice of each `A` and `plus` token in the production rule `E -> A plus A`.
```rust
%token plus ...;
E : A plus? a2=A*
{
println!("Value of 1st A: {}", A); // i32
println!("Value of plus: {:?}", plus); // Option<TermType>
println!("Value of 2nd As: {:?}", a2); // Vec<i32>
}
;
A(i32): ... ;
```
`Result<(), String>` can be returned from `<ReduceAction>`.
Returned `Err` will be delivered to the caller of `feed()` function.
#### Error type <sub><sup>(optional)</sup></sub>
```
'%err' <RustType> ';'
'%error' <RustType> ';'
```
Define the type of `Err` variant in `Result<(), Err>` returned from `<ReduceAction>`. If not defined, `String` will be used.
## Start Parsing
`lr1!` and `lalr1!` will generate struct `<StartSymbol>Parser`.
The parser struct has the following functions:
- `new()` : create new parser
- `begin(&self)` : create new context
- `feed(&self, &mut Context, TermType, &mut UserData) -> Result<(), ParseError>` : feed token to the parser
- `feed_callback(&self, &mut Context, &mut C: Callback, TermType, &mut UserData) -> Result<(), ParseError>` : feed token with callback
Note that the parameter `&mut UserData` is omitted if `%userdata` is not defined.
Once the input sequence (including `eof` token) is feeded, without errors, you can get the value of start symbol by calling `context.accept()`.
```rust
let parser = parser::EParser::new();
// create context
let mut context = parser.begin();
// define userdata
let mut userdata: i32 = 0;
// start feeding tokens
for token in input_sequence {
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
let res = context.accept();
println!("{}", res);
println!("userdata: {}", userdata);
```
## Parse with callback
For tracing parser action, you can implement `Callback` trait and pass it to `parser.feed_callback()`.
```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
// Num + Num * ( Num + Num )
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
```
## Macro expand executable [`rusty_lr_expand`](rusty_lr_expand)
This executable is on development, and not `cargo publish`ed yet.
```
Usage: rusty_lr_expand <INPUT_FILE> [OUTPUT_FILE]
Options:
--no-format
do not rustfmt the output
-r, --runtime
call `grammar.build()` at runtime
-l, --lalr
build LALR(1) parser
```
The input file must be consisted of two parts: Rust codes and CFGs, separated by `%%`.
```
// <Rust Codes...>
%%
// <CFGs...>
```
The output file will be generated `<StartSymbol>Parser` struct with the given CFGs.
## 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, PartialOrd, Ord)] // 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. To resolve this conflict, the precedence of shift/reduce is given to terminal symbol `Plus`. `Left` means that for `Plus` token, the parser will reduce the rule instead of shifting the token.
reduce/reduce conflict (e.g. duplicated rules) will be always an error.
### 3. Build DFA
Calling `grammar.build()` for LR(1) or `grammar.build_lalr()` for LALR(1) 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 Augmented production rule must be defined in the grammar.
```
Augmented -> StartSymbol $
```
The returned `Parser` struct contains the DFA states and the production rules(cloned).
It is completely independent from the `Grammar`, 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.
You can manually `match` the error type for custom error message,
but for most cases, using `println!("{}", err)` will be enough to see the detailed errors.
Error is `Display` if both `Term` and `NonTerm` is `Display`, and It is `Debug` if both `Term` and `NonTerm` is `Debug`.
#### Sample error messages
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 can feed the input sequence one by one 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;
}
}
}
```
`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.