#compiler #grammar #parser #context-free #bison

bin+lib gramatica

A compiler compiler for Rust implementing Earley’s parser

5 releases

0.1.4 Mar 6, 2018
0.1.3 Mar 5, 2018
0.1.2 Mar 4, 2018
0.1.1 Mar 4, 2018
0.1.0 Mar 1, 2018

#188 in Parser tooling

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MIT/Apache

4MB
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gramatica

This crate provides a binary to compile grammars into Rust code and a library implementing Earley's parsing algorithm to parse the grammars specified.

Usage

This crate is gramatica. To use it you should install it in order to acquire the gramatica_compiler binary and also add gramatica to your dependencies in your project's Cargo.toml.

[dependencies]
gramatica = "0.1"

Then, if you have made a grammar file example.rsg execute gramatica_compiler example.rsg > example.rs. Afterwards you may use the generated file example.rs as a source Rust file.

Example: calculator

The classical example is to implement a calculator.

//This is a just Rust header that it is copied literally
extern crate gramatica;
use std::cmp::Ordering;
use gramatica::{Associativity,EarleyKind,State,Parser,ParsingTablesTrait,AmbiguityInfo};

//Here the proper grammar begins.
//These lines are processed by gramatica_compiler to generate the Token enum and the parsing tables.
//We begin by terminal tokens (symbols that are not in the left of any rule but have a literal representation).
//For this example all terminals are regular expressions. The first argument of re_terminal! is the type entry, as used in a enum.
re_terminal!(Num(f64),"[0-9]*\\.?[0-9]+([eE][-+]?[0-9]+)?");
re_terminal!(Plus,"\\+");
re_terminal!(Minus,"-");
re_terminal!(Star,"\\*");
re_terminal!(Slash,"/");
re_terminal!(Caret,"\\^");
re_terminal!(LPar,"\\(");
re_terminal!(RPar,"\\)");
re_terminal!(NewLine,"\\n");
re_terminal!(_,"\\s+");//Otherwise skip spaces

//Now is the turn of nonterminal tokens. The first one is the default start symbol.
//These have rules written as match clauses, with the pattern being the reduction of the nonterminal token and the expression being the value the token takes when reducing.
//In this case the type of the symbol is empty and so is the expression
nonterminal Input
{
	() => (),
	(Input,Line) => (),
}

//Although the value type of Line is empty we may have code executed on the reduction
nonterminal Line
{
	(NewLine) => (),
	(Expression(value), NewLine) =>
	{
		println!("{}",value);
	},
}

//Finally a token with value type. Each rule creates the value in a different way.
//Most rules are annotated to avoid ambiguities
nonterminal Expression(f64)
{
	(Num(value)) => value,
	#[priority(addition)]
	#[associativity(left)]
	(Expression(l),Plus,Expression(r)) => l+r,
	#[priority(addition)]
	#[associativity(left)]
	(Expression(l),Minus,Expression(r)) => l-r,
	#[priority(multiplication)]
	#[associativity(left)]
	(Expression(l),Star,Expression(r)) => l*r,
	#[priority(multiplication)]
	#[associativity(left)]
	(Expression(l),Slash,Expression(r)) => l/r,
	#[priority(addition)]
	#[associativity(left)]
	(Minus,Expression(value)) => -value,
	#[priority(exponentiation)]
	#[associativity(right)]
	(Expression(l),Caret,Expression(r)) => l.powf(r),
	(LPar,Expression(value),RPar) => value,
}

//The ordering macro-like sets the order of application of the previously annotated rules
ordering!(exponentiation,multiplication,addition);

//Finally an example of using the grammar to parse some lines from stdin.
//We could do this or something similar in a different file if we desired to.
use std::io::BufRead;
fn main()
{
	let stdin=std::io::stdin();
	for rline in stdin.lock().lines()
	{
		let line=rline.unwrap()+"\n";
		println!("line={}",line);
		match Parser::<Token,ParsingTables>::parse(&line,None)
		{
			Err(x) => println!("error parsing: {:?}",x),
			Ok(x) => println!("parsed correctly: {:?}",x),
		};
	}
}

Advanced Lexer

To define terminal tokens not expressable with regular expressions you may use the following. It must containg a _match function returning an option containing the number of chars mathed and the value of the token.

terminal LitChar(char)
{
	fn _match(parser: &mut Parser<Token,ParsingTables>, source:&str) -> Option<(usize,char)>
	{
		let mut characters=source.chars();
		if (characters.next())==(Some('\''))
		{
			let mut c=characters.next().unwrap();
			let mut size=3;
			if c=='\\'
			{
				c=(characters.next().unwrap());
				size=4;
			}
			if characters.next().unwrap()=='\''
			{
				Some((size,c))
			}
			else
			{
				None
			}
		}
		else
		{
			None
		}
	}
}

Since version 0.1.1 there is also a keyword_terminal! macro:

keyword_terminal!(Const,"const");

Parsing values as match clauses

Each rule is written as a match clause, whose ending expression is the value that the nonterminal token gets after being parsed. For example, to parse a list of statements:

nonterminal Stmts(Vec<StmtKind>)
{
	(Stmt(ref stmt)) => vec![stmt.clone()],
	(Stmts(ref stmts),Stmt(ref stmt)) =>
	{
		let mut new=(stmts.clone());
		new.push(stmt.clone());
		new
	},
}

Reductions only execute if they are part of the final syntactic tree.

Precedence by annotations

To avoid ambiguities you have two options: to ensure the grammar does not contain them or to priorize rules by introducing annotations. In the example of the calculator we have seen two kinds:

  • #[priority(p_name)] to declare a rule with priority p_name. Later there should be a ordering!(p_0,p_1,p_2,...) macro-like to indicate that p_0 should reduce before p_1.
  • #[associativity(left/right)] to decide how to proceed when nesting the same rule.

Example: Parsing JSON

extern crate gramatica;

use std::cmp::Ordering;
use gramatica::{Associativity,EarleyKind,State,Parser,ParsingTablesTrait,AmbiguityInfo};

//See https://www.json.org/

use std::rc::Rc;

//We define an auxiliar type to store JSON values
#[derive(Clone,Debug,PartialEq)]
enum JsonValue
{
	Literal(String),
	Number(f64),
	Object(Vec<(String,JsonValue)>),
	Array(Vec<JsonValue>),
	True,
	False,
	Null,
}


// ---- Start of the grammar ----
keyword_terminal!(True,"true");
keyword_terminal!(False,"false");
keyword_terminal!(Null,"null");

re_terminal!(Number(f64),"[0-9]*\\.?[0-9]+([eE][-+]?[0-9]+)?");

terminal LitStr(String)
{
	//This function has limited escaping capabilities
	fn _match(parser: &mut Parser<Token,ParsingTables>, source:&str) -> Option<(usize,String)>
	{
		let mut ret=None;
		let mut characters=source.chars();
		if (characters.next())!=(Some('"'))
		{
		}
		else
		{
			let mut size=1;
			let mut r=String::from("\"");
			while true
			{
				match characters.next()
				{
					None => break,
					Some('"') =>
					{
						ret=(Some((size+1,r+&"\"")));
						break;
					},
					Some('\\') =>
					{
						match characters.next()
						{
							None => break,
							//Some(c) => r+='\\'+c,
							Some(c) =>
							{
								r.push('\\');
								r.push(c);
							}
						};
						size+=2;
					},
					Some(c) =>
					{
						//r+=&String::from(c);
						r.push(c);
						size+=1;
					},
				};
			}
		}
		ret
	}
}

re_terminal!(LBrace,"\\{");
re_terminal!(RBrace,"\\}");
re_terminal!(LBracket,"\\[");
re_terminal!(RBracket,"\\]");
re_terminal!(Comma,",");
re_terminal!(Colon,":");
re_terminal!(_,"\\s+|\n");//Otherwise skip spaces

nonterminal Object(JsonValue)
{
	(LBrace,RBrace) => JsonValue::Object(vec![]),
	(LBrace,Members(ref list),RBrace) => JsonValue::Object(list.clone()),
}

nonterminal Members(Vec<(String,JsonValue)>)
{
	(Pair(ref s,ref value)) => vec![(s.clone(),value.clone())],
	//(Pair,Comma,Members) => (),
	(Members(ref list),Comma,Pair(ref s,ref value)) =>
	{
		let mut new=(list.clone());
		new.push((s.clone(),value.clone()));
		new
	},
}

nonterminal Pair(String,JsonValue)
{
	(LitStr(ref s),Colon,Value(ref value)) => (s.clone(),value.clone()),
}

nonterminal Array(Vec<JsonValue>)
{
	(LBracket,RBracket) => vec![],
	(LBracket,Elements(ref list),RBracket) => list.clone(),
}

nonterminal Elements(Vec<JsonValue>)
{
	(Value(ref value)) => vec![value.clone()],
	//(Value,Comma,Elements) => (),
	(Elements(ref list),Comma,Value(ref value)) =>
	{
		let mut new=(list.clone());
		new.push(value.clone());
		new
	},
}

nonterminal Value(JsonValue)
{
	(LitStr(ref s)) => JsonValue::Literal(s.clone()),
	(Number(v)) => JsonValue::Number(v),
	(Object(ref value)) => value.clone(),
	(Array(ref list)) => JsonValue::Array(list.clone()),
	(True) => JsonValue::True,
	(False) => JsonValue::False,
	(Null) => JsonValue::Null,
}

// ---- End of the grammar ----

use std::io::{BufRead,Read};

//As example, we parse stdin for a JSON object
fn main()
{
	let stdin=std::io::stdin();
	let mut buf=String::new();
	stdin.lock().read_to_string(&mut buf);
	match Parser::<Token,ParsingTables>::parse(&buf,None)
	{
		Err(x) => println!("error parsing: {:?}",x),
		Ok(x) => println!("parsed correctly: {:?}",x),
	};
}

Example: Parsing basic XML

//A very basic xml grammar

extern crate gramatica;

use std::cmp::Ordering;
use gramatica::{Associativity,EarleyKind,State,Parser,ParsingTablesTrait,AmbiguityInfo};

// see https://www.w3.org/People/Bos/meta-bnf
// also http://cs.lmu.edu/~ray/notes/xmlgrammar/

use std::rc::Rc;

//We define an auxiliar type to store XML elements
#[derive(Clone,Debug,PartialEq)]
struct XMLElement
{
	name: String,
	attrs: Vec<(String,String)>,
	contents: Vec<XMLContent>,
}

#[derive(Clone,Debug,PartialEq)]
enum XMLContent
{
	Element(XMLElement),
	Data(String),
}


// ---- Start of the grammar ----

re_terminal!(Space(String),"(\\s|\n)+");
re_terminal!(Ident(String),"[a-zA-Z\\x80-\\xff_][a-zA-Z0-9\\x80-\\xff_]*");

terminal LitStr(String)
{
	fn _match(parser: &mut Parser<Token,ParsingTables>, source:&str) -> Option<(usize,String)>
	{
		let mut ret=None;
		let mut characters=source.chars();
		if (characters.next())!=(Some('"'))
		{
		}
		else
		{
			let mut size=1;
			let mut r=String::from("\"");
			while true
			{
				match characters.next()
				{
					None => break,
					Some('"') =>
					{
						ret=(Some((size+1,r+&"\"")));
						break;
					},
					Some('\\') =>
					{
						match characters.next()
						{
							None => break,
							//Some(c) => r+='\\'+c,
							Some(c) =>
							{
								r.push('\\');
								r.push(c);
							}
						};
						size+=2;
					},
					Some(c) =>
					{
						//r+=&String::from(c);
						r.push(c);
						size+=1;
					},
				};
			}
		}
		ret
	}
}

re_terminal!(CloseEmpty,"/>");
re_terminal!(BeginClose,"</");

re_terminal!(Equal,"=");
re_terminal!(LT,"<");
re_terminal!(GT,">");

re_terminal!(Other(char),".");

nonterminal Document(XMLElement)
{
	(Element(ref elem)) => elem.clone(),
}

nonterminal Element(XMLElement)
{
	(EmptyElemTag(ref name,ref attrs)) => XMLElement{name:name.clone(),attrs:attrs.clone(),contents:vec![]},
	(STag(ref name, ref attrs),Content(ref content),ETag) => XMLElement{name:name.clone(),attrs:attrs.clone(),contents:content.clone()},
}

nonterminal EmptyElemTag(String,Vec<(String,String)>)
{
	(LT,Ident(ref name),Attributes(ref attrs),MaybeSpace,CloseEmpty) => (name.clone(),attrs.clone()),
}

nonterminal Attributes(Vec<(String,String)>)
{
	() => vec![],
	(Attributes(ref attrs),Space,Attribute(ref a, ref b)) =>
	{
		let mut new=(attrs.clone());
		new.push((a.clone(),b.clone()));
		new
	},
}

nonterminal Attribute(String,String)
{
	(Ident(ref a),Equal,LitStr(ref b)) => (a.clone(),b.clone()),
}

nonterminal STag(String,Vec<(String,String)>)
{
	(LT,Ident(ref name),Attributes(ref attrs),MaybeSpace,GT) => (name.clone(),attrs.clone()),
}

nonterminal ETag(String)
{
	(BeginClose,Ident(ref s),MaybeSpace,GT) => s.clone(),
}

nonterminal Content(Vec<XMLContent>)
{
	(CharData(ref s)) => vec![XMLContent::Data(s.clone())],
	(CharData(ref s),Contents(ref list)) =>
	{
		let mut new=vec![XMLContent::Data(s.clone())];
		new.extend(list.iter().map(|x|x.clone()));
		new
	},
}

nonterminal Contents(Vec<XMLContent>)
{
	() => vec![],
	(Contents(ref list),Element(ref elem),CharData(ref s)) =>
	{
		let mut new=(list.clone());
		new.push(XMLContent::Element(elem.clone()));
		if s!=""
		{
			new.push(XMLContent::Data(s.clone()));
		}
		new
	},
}

nonterminal MaybeSpace
{
	() => (),
	(Space) => (),
}

nonterminal CharData(String)
{
	() => String::new(),
	(CharData(ref s),Space(ref o)) => format!("{}{}",s,o),
	(CharData(ref s),Ident(ref o)) => format!("{}{}",s,o),
	(CharData(ref s),Equal) => format!("{}=",s),
	(CharData(ref s),Other(o)) => format!("{}{}",s,o),
}

// ---- End of the grammar ----

use std::io::{BufRead,Read};

//As example, we parse stdin for a XML element
fn main()
{
	let stdin=std::io::stdin();
	let mut buf=String::new();
	stdin.lock().read_to_string(&mut buf);
	match Parser::<Token,ParsingTables>::parse(&buf,None)
	{
		Err(x) => println!("error parsing: {:?}",x),
		Ok(x) => println!("parsed correctly: {:?}",x),
	};
}

Dependencies