Lelwel generates recursive descent parsers for Rust using LL(1) grammars. Conflicts are resolved with semantic predicates. Semantic actions are used for ad hoc syntax-directed translation. Error handlers are used to recover from syntax errors. Unlike in other parser generators (e.g. bison or javacc), actions, predicates, and error handlers are not defined inline, which makes it easier to read the grammar.
Lelwel is written as a library, which is used by the CLI tool llw and the language server lelwel-ls.
There are plugins for Neovim and Visual Studio Code that use the language server.
The parser for lelwel grammar files (*.llw) is itself generated by lelwel. The following examples show grammars for a basic calculator and JSON.
```antlr // token definitions
token Num{f64}='number'; token Add='+' Sub='-' Mul='*' Div='/'; token LPar='(' RPar=')';
// grammar rules
start{f64}: expr #1 ; expr{f64}: term #1 ( '+' term #2 | '-' term #3 )* #4 ; term{f64}: atomic #1 ( '' atomic #2 | '/' atomic #3 ) #4 ; atomic{f64}: Num #1 | '(' expr ')' #2 ;
// semantic actions
start#1 { Ok(expr) }
expr#1 { let mut res = term; } expr#2 { res = res + term; } expr#3 { res = res - term; } expr#4 { Ok(res) }
term#1 { let mut res = atomic; } term#2 { res = res * atomic; } term#3 { res = res / atomic; } term#4 { Ok(res) }
atomic#1 { Ok(Num.0) } atomic#2 { Ok(expr) } ```
```antlr
token True='true' False='false' Null='null';
token LBrace='{' RBrace='}' LBrak='[' RBrak=']' Comma=',' Colon=':';
token String{String}='
start{Value}: value #1 ; value{Value}: object #1 | array #2 | String #3 | Number #4 | 'true' #5 | 'false' #6 | 'null' #7 ; object{Value}: '{' [member #1 (',' member #2)* | !1] '}' #3 ; member{(String, Value)}: String ':' value #1 ; array{Value}: '[' [value #1 (',' value #2)* | !1] ']' #3 ;
limit 1000;
preamble { use std::collections::BTreeMap; use super::diag::*; use super::Value; } parameters { diag: &mut Diag } error { Code }
start#1 { Ok(value) }
value#1 { Ok(object) } value#2 { Ok(array) } value#3 { Ok(Value::String(String.0)) } value#4 { Ok(Value::Number(Number.0)) } value#5 { Ok(Value::Bool(true)) } value#6 { Ok(Value::Bool(false)) } value#7 { Ok(Value::Null) }
object#0 { let mut members = BTreeMap::new(); } object#1 { members.insert(member.0, member.1); } object#2 { members.insert(member.0, member.1); } object#3 { Ok(Value::Object(members)) } object!1 { diag.error(errorcode, errorrange); }
member#1 { Ok((String.0, value)) }
array#0 { let mut values = vec![]; } array#1 { values.push(value); } array#2 { values.push(value); } array#3 { Ok(Value::Array(values)) } array!1 { diag.error(errorcode, errorrange); } ```
Run cargo install --features="cli","lsp" lelwel.
Lelwel and its generated code is licensed under either of
at your option.
Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.