[][src]Crate pomelo

A procedural macro to create Lemon-like parsers.

Pomelo is a port to Rust of the Lemon Parser Generator (from now on, Lemon_C) originally written by D. Richard Hipp for his SQLite parser. It is based on a previous attempt to port Lemon to Rust (Lemon_Rust), but now it is written as a Rust procedural macro, so it does not contain any of the original C code (although it uses the same algorithms). Thus the change in name to a different citrus fruit.

This Pomelo guide is shamelessly based on the original Lemon_C guide.

Pomelo is an LALR(1) parser generator for Rust. It does the same job as bison and yacc. But pomelo is not another bison or yacc clone. It uses a different grammar syntax which is designed to reduce the number of coding errors. Pomelo also uses a more sophisticated parsing engine that is faster than yacc and bison and which is both reentrant and thread-safe.


pomelo! {
    %type input Vec<i32>;
    %type numbers Vec<i32>;
    %type Number i32;

    input ::= numbers?(A) { A.unwrap_or_else(Vec::new) };
    numbers ::= Number(N) { vec![N] }
    numbers ::= numbers(mut L) Comma Number(N) { L.push(N); L }
fn main() -> Result<(), ()> {
    use parser::{Parser, Token};
    // Real-world code would use a tokenizer.
    let tokens = vec![
    let mut p = Parser::new();
    for tok in tokens.into_iter() {
    let data = p.end_of_input()?;
    assert_eq!(data, vec![1, 2, 3]);

Theory of Operation

The main goal of pomelo is to translate a context free grammar (CFG) for a particular language into Rust code that implements a parser for that language.

When using pomelo! you write the grammar specification into the macro and it will expand to a module with the following contents:

  • A Parser struct that implements the parser logic.
  • A Token enum that represents the input tokens to the parser.

The Parser Interface

Pomelo doesn't generate a complete, working program. It only generates a Rust module, named parser by default, that implement a parser. This section describes the interface to that crate.

Before a program begins using a pomelo-generated parser, the program must first create the parser. A new parser is created as follows:

let mut parser = parser::Parser::new();

Here, parser is the generated module. Parser is the struct that represents the parser and new() the function that creates and initializes a new parser.

The new()function may have an argument, depending on the grammar. If the grammar specification file requests it (see %extra_argument), the new() function will have a parameter that can be of any type chosen by the programmer. The parser doesn't do anything with this argument except to pass a mutable reference to it to action routines. This is a convenient mechanism for passing state information down to the action routines without having to use global variables.

After a parser has been created, the programmer must supply it with a sequence of tokens (terminal symbols) to be parsed. This is accomplished by calling the following function once for each token:


The argument to the parse() function is a value of the generated Token enumeration that tells the parser the type of the next token in the data stream. There is one token variant for each terminal symbol in the grammar. Some variants will have an associated value, depending on the type of the token. Typically the token variant will be a broad category of tokens such as identifier or number and the data will be the name of the identifier or the value of the number. The return value of this function is Result<(), Error> being Error the error type of the grammar (see %error).

Note that this function will take ownership of the passed token, unless it implements the Copy trait (see %token).

When all the input has been consumed, the following function may be used to indicate end-of-input:


This function actually consumes the parser and returns a value of type Result<Output, Error>. If there is no extra type defined, then Output is the type of the start symbol of the grammar, or () if it has no type. If there is an extra type, then Output is a tuple (ExtraType, TypeOfStartSymbol).

A typical use of a pomelo parser might look something like the following:

fn parse_expression<R: BufRead>(read: &mut R) -> Result<Expression, Error> {
    let mut tokenizer = Tokenizer::new(read);
    let mut p = parser::Parser::new(State::new());
    while let Some(token) = tokenizer.next_token() {
    let (expr, _state) = p.end_of_input()?;

This example shows a user-written routine that parses an input stream and returns an expression tree. We assume the existence of some kind of tokenizer which is created using Tokenizer::new(). The Tokenizer::next_token() function on retrieves the next token from the input file and returns an Option<parser::Token>. The enum data is assumed to be some type that contains details about each token, such as its complete text, what line it occurs on, etc.

This example also assumes the existence of a type parser::State that holds state information about a particular parse. An instance of such a type is created with a call to parser::State::new() and then passed into the parser upon initialization, as the optional argument. The action routine specified by the grammar for the parser can use the this value to hold whatever information is useful and appropriate. This value can be borrowed between tokens using the function parser.extra() or moved out of the parser with parser.into_extra().

Differences with yacc and bison

Programmers who have previously used the yacc or bison parser generator will notice several important differences between yacc and/or bison and pomelo.

  • In yacc and bison, the parser calls the tokenizer. In pomelo, the tokenizer calls the parser.
  • pomelo uses no global variables. yacc and bison use global variables to pass information between the tokenizer and parser.
  • pomelo allows multiple parsers to be running simultaneously. yacc and bison do not.

Differences with lemon

The most obvious difference here is that lemon is written in C and generates C code, while pomelo is written in Rust and produces Rust code. Many other differences arise from this fact:

  • Since there is no command to call, there are no command line switches.
  • No %destructor or %default_destructor or %token_destructor directives. Rust drop semantics should take care or everything.
  • No %parse_accept directive. If you want to run code after the end-of-input, just do it after calling Parser::end_of_input().
  • No %token_type directive. See below for details.
  • New %extra_token directive.
  • %default_type applies to all symbols. In lemon it applies only to non-terminals because all terminals has the same %token_type.
  • Rules are ended with a semi-colon instead of a point: it is more natural for a Rust programmer.
  • The %stack_overflow directive returns an error type, just like %parse_fail.
  • The %stack_size directive can specify an unlimited stack size, with 0, that makes the %stack_overflow code unreachable.
  • The %stack_size directive can specify a second argument: the type of the stack implementation. This allows to write a no-std or an allocation-free parser.

Another important difference is that in lemon the %type directive only applies to non-terminals, while terminals all must have the same type, declared with %token_type. This is necessary because the Parse() function must be declared with a type able to accept any token. In pomelo, however, the input of the parser() function is an enum and the types of terminals and non-terminals can be equally defined just with %type. If you want to add location information to all terminal symbols you can use the %extra_token directive.

If the left-hand side of a grammar rule has an user defined type, then it must have a code block to produce its value. You can omit the code block if the rule has the following properties:

  • There is exactly one symbol on the right-hand side with type.
  • The type of that symbol is identical to the type of the left-hand side symbol's.
  • The symbols on the right-hand side do not have alias defined.

Then the rule code is auto-generated to just forward the value of that symbol. For example:

    %type expr String;
    %type Number String;
    expr ::= Number;
    expr ::= LParen Number RParen;

is equivalent to:

    %type expr String;
    %type Number String;
    expr ::= Number(A) { A }
    expr ::= LParen Number(A) RParen { A }

Another extension to the lemon syntax is the optional flag: in the right-hand side of a rule you can add ? at the end of a symbol to make it optional. The type of the alias is thus changed from T to Option<T>. If the symbol is found its value will be Some(_), if it is not found its value will be None. For example:

%type list T;

array ::= LParen list?(D) RParen { ... }

is actually converted to something like:

%type list T;
%type optional_list Option<T>;

array ::= LParen optional_list(D) RParen { ... }
optional_list ::= list(D) { Some(D) }
optional_list ::= { None }

Macro input

The main purpose of the pomelo! macro is to define the grammar for the parser. But it also specifies additional information pomelo requires to do its job.

The grammar for pomelo is, for the most part, free format. It does not have sections or divisions like yacc or bison. Any declaration can occur at any point in the macro. Pomelo ignores whitespace (except where it is needed to separate tokens) and it honors the same commenting conventions as Rust.

Terminals and Nonterminals

A terminal symbol (token) is any string of alphanumeric and underscore characters that begins with an upper case letter. A terminal can contain lowercase letters after the first character. A nonterminal, on the other hand, is any string of alphanumeric and underscore characters that begins with a lower case letter.

In pomelo, terminal and nonterminal symbols do not need to be declared or identified in a separate section of the grammar. Pomelo is able to generate a list of all terminals and nonterminals by examining the grammar rules, and it can always distinguish a terminal from a nonterminal by checking the case of the first character of the name.

Yacc and bison allow terminal symbols to have either alphanumeric names or to be individual characters included in single quotes, like this: ) or $. Pomelo does not allow this alternative form for terminal symbols. With pomelo, all symbols, terminals and nonterminals, must have alphanumeric names.

Grammar Rules

The main component of a pomelo grammar is a sequence of grammar rules. Each grammar rule consists of a nonterminal symbol followed by the special symbol ::= and then a list of terminals and/or nonterminals. The rule is terminated by a semi-colon. The list of terminals and nonterminals on the right-hand side of the rule can be empty. Rules can occur in any order, except that the left-hand side of the first rule is assumed to be the start symbol for the grammar (unless specified otherwise using %start). A typical sequence of grammar rules might look something like this:

    input ::= expr;
    expr ::= expr Plus expr;
    expr ::= expr Times expr;
    expr ::= LParen expr RParen;
    expr ::= Value;

There is one non-terminal in this example, expr, and five terminal symbols or tokens: Plus, Times, LParen, RParen and Value.

Like yacc and bison, pomelo allows the grammar to specify a block of code that will be executed whenever a grammar rule is reduced by the parser. In pomelo, this action is specified by putting the code (contained within curly braces {...}) in place of the semi-colon that closes the rule. For example:

expr ::= expr Plus expr { println!("Doing an addition..."); }

In order to be useful, grammar actions must normally be linked to their associated grammar rules. In yacc and bison, this is accomplished by embedding a $$ in the action to stand for the value of the left-hand side of the rule and symbols $1, $2, and so forth to stand for the value of the terminal or nonterminal at position 1, 2 and so forth on the right-hand side of the rule. This idea is very powerful, but it is also very error-prone. The single most common source of errors in a yacc or bison grammar is to miscount the number of symbols on the right-hand side of a grammar rule and say $7 when you really mean $8.

Pomelo avoids the need to count grammar symbols by assigning symbolic names to each symbol in a grammar rule and then using those symbolic names in the action. Moreover, the value to be assigned to the left-hand side is simply the output value of the rule block. In yacc or bison, one would write this:

expr -> expr PLUS expr { $$ = $1 + $3; }

But in pomelo, the same rule becomes the following:

expr ::= expr(A) Plus expr(B) { B + C }

In the pomelo rule, any symbol in parentheses after a grammar rule symbol becomes an irrefutable pattern to match the corresponding value of that symbol.

The pomelo notation for linking a grammar rule with its reduce action is superior to yacc or bison on several counts. First, as mentioned above, the pomelo method avoids the need to count grammar symbols. Secondly, you cannot forget to assign to the left-hand side symbol: if the code block does not have the same type as the left-hand side symbol, a compiler error will be raised.

If you have several terminal tokens that can be used in the same place you can put them all in the same rule, separated with |.

expr ::= SmallNumber|BigNumber(B) { B }

which is a shortcut of

expr ::= SmallNumber(B) { B }
expr ::= BigNumber(B) { B }

If you use a symbolic name ((B) in the example) with such a compound token, then all these tokens must be of the same type. However, if there is no symbolic name, then they may have different types.

Precedence Rules

pomelo resolves parsing ambiguities in exactly the same way as yacc and bison. A shift-reduce conflict is resolved in favor of the shift, and a reduce-reduce conflict is resolved by reducing whichever rule comes first in the grammar file.

Just like in yacc and bison, pomelo allows a measure of control over the resolution of conflicts using precedence rules. A precedence value can be assigned to any terminal symbol using the %left, %right or %nonassoc directives. Terminal symbols mentioned in earlier directives have a lower precedence that terminal symbols mentioned in later directives. For example:

%left And;
%left Or;
%nonassoc Eq Ne Gt Ge Lt Le;
%left Plus Minus;
%left Times Divide Mod;
%right Exp Not;

In the preceding sequence of directives, the And operator is defined to have the lowest precedence. The Or operator is one precedence level higher. And so forth. Hence, the grammar would attempt to group the ambiguous expression

a And b Or c

like this

a And (b Or c)

The associativity (left, right or nonassoc) is used to determine the grouping when the precedence is the same. And is left-associative in our example, so

a And b And c

is parsed like this

(a And b) And c

The Exp operator is right-associative, though, so

a Exp b Exp c

is parsed like this

a EXP (b EXP c)

The nonassoc precedence is used for non-associative operators. So

a Eq b Eq c

is an error.

The precedence of non-terminals is transferred to rules as follows: The precedence of a grammar rule is equal to the precedence of the left-most terminal symbol in the rule for which a precedence is defined. This is normally what you want, but in those cases where you want to precedence of a grammar rule to be something different, you can specify an alternative precedence symbol by putting the symbol in square braces before the semi-colon or the rule code. For example:

expr ::= Minus expr [Not];

This rule has a precedence equal to that of the Not symbol, not the Minus symbol as would have been the case by default.

With the knowledge of how precedence is assigned to terminal symbols and individual grammar rules, we can now explain precisely how parsing conflicts are resolved in pomelo. Shift-reduce conflicts are resolved as follows:

  • If either the token to be shifted or the rule to be reduced lacks precedence information, then resolve in favor of the shift, but report a parsing conflict.
  • If the precedence of the token to be shifted is greater than the precedence of the rule to reduce, then resolve in favor of the shift. No parsing conflict is reported.
  • If the precedence of the token it be shifted is less than the precedence of the rule to reduce, then resolve in favor of the reduce action. No parsing conflict is reported.
  • If the precedences are the same and the shift token is right-associative, then resolve in favor of the shift. No parsing conflict is reported.
  • If the precedences are the same the the shift token is left-associative, then resolve in favor of the reduce. No parsing conflict is reported.
  • Otherwise, resolve the conflict by doing the shift and report the parsing conflict.

Reduce-reduce conflicts are resolved this way:

  • If either reduce rule lacks precedence information, then resolve in favor of the rule that appears first in the grammar and report a parsing conflict.
  • If both rules have precedence and the precedence is different then resolve the dispute in favor of the rule with the highest precedence and do not report a conflict.
  • Otherwise, resolve the conflict by reducing by the rule that appears first in the grammar and report a parsing conflict.

Special Directives

The input grammar to pomelo consists of grammar rules and special directives. We've described all the grammar rules, so now we'll talk about the special directives.

Directives in pomelo can occur in any order. You can put them before the grammar rules, or after the grammar rules, or in the midst of the grammar rules. It doesn't matter. The relative order of directives used to assign precedence to terminals is important, but other than that, the order of directives is arbitrary.

Pomelo supports the following special directives:

The %module directive

This directive is used to specify the name of the module generated by the pomelo! macro. Fo example

%module ident;

will create a module named ident instead of the default parser. This is specially useful if you want to create several parsers in the same module.

The %type directive

This directive is used to specify the data types for values on the parser's stack associated with terminal and non-terminal symbols. The type of the terminal symbol is the value associated to the input token. The type associated to a non-terminal will be the type of the data associated to the corresponding variant of the Token enumeration. For example:

%type Value i32;

Then the Token enumeration will have a variant such as:

pub Token {

Typically the data type of a non-terminal is a parse-tree structure that contains all information about that non-terminal. For example:

%type expr ExprType;

Some Rust crates use derive macros in enums and require attributes in the Token enum variants. You can add them here, only for terminal tokens. For example:

%type #[serde(rename = "Amount")] Quantity i32;

will create a variant:

pub Token {
    #[serde(rename = "Amount")] Quantity(i32);

The type of the symbol is optional: if you do not specify the variant will have no type. This is usually not needed because pomelo will create typeless terminals automatically if they appear in the grammar rules, but it can be needed to specify the attributes.

Each entry on the parser's stack is actually an enum containing variants of all data types for every symbol. Pomelo will automatically use the correct element of this enum depending on what the corresponding symbol is. But the grammar designer should keep in mind that the size of the enum will be the size of its largest element. So if you have a single non-terminal whose data type requires 1K of storage, then your 100 entry parser stack will require 100K of heap space. If you are willing and able to pay that price, fine. You just need to know.

The %include directive

The %include directive specifies Rust code that is included into the generated module. You can include any Rust items you want. You can have multiple %include directives in your grammar.

The %include directive is very handy for using symbols declared elsewhere. For example:

%include { use super::*; }

The %syntax_error directive

The %syntax_error directive specify code that will be called when a syntax error occurs. This code must evaluate to a value of type Result<(), Error>, and it is run in an function that returns the same type so you can also use the ? operator. If it evaluates to Ok(()), the parser will try to recover and continue. If it evaluates to Err(_) or a ? fails, the parser will fail with that error value. See the section Error Processing for more details.

In this code you have available extra as a mutable reference to the current extra_argument, and token as a type of value Option<Token> with the token that triggered the error. If the error is caused by the end-of-input, then token will be None.

By default it evaluates to Err(Default::default()) so:

  • if Error implements Default it will fail with the default error.
  • if Error does not implement Default it will fail to compile and you must use this directive to create a meaningful one or to return Ok(()) and ignore the error.

The %parse_fail directive

The %parse_fail directive specifies a block of Rust code that is executed whenever the parser fails to complete. This code is not executed until the parser has tried and failed to resolve an input error using is usual error recovery strategy. This block is only invoked when parsing is unable to continue. It must evaluate to the defined Error type.

%error String;
%parse_failure {
    "Giving up.  Parser is hopelessly lost...".to_string()

By default it will return Default::default(). If your Error type does not implement default() it will be a compiler error, so in this case you must use this directive.

After a parse failure this parser object must not be used again.

The %stack_overflow directive

The %stack_overflow directive specifies a block of Rust code that is executed whenever the internal stack overflows. Beware of righ recursivity rules and right associativity! It must be evaluated to the defined Error type.

By default it will return Default::default(). If your Error type does not implement default() it will be a compiler error, so in this case you must use this directive.

However, if you set your %stack_size to 0 (unlimited), then the stack will never overflow, and this directive is defaulted to unreachable!().

%error String;
%stack_overflow {
    "Parse stack overflow!".to_string()

After a stack overflow this parser object must not be used again.

See also the %stack_size directive for more details about the parser stack.

The %stack_size directive

If stack overflow is a problem and you can't resolve the trouble by using left-recursion, then you might want to increase the size of the parser's stack using this directive. Put an positive integer after the %stack_size directive and pomelo will generate a parse with a stack of the requested size. The default value is 100.

%stack_size 2000;

If you specify the limit as 0 then it means unlimited. Beware that if your parser input is untrusted, having an unlimited stack may be an issue (it may eat up all your memory). If you do this, the code in %stack_overflow directive is never called.

This directive has an additional optional parameter that is the type to implement the stack. It is by default std::vec::Vec, but you may specify whatever type you want, as long as it complies the following interface:

  • It must have a single generic type argument, without restrictions other than Sized. That is, if you write %stack_size 100 Type; then Type<T> must be a valid type.
  • It must implement the following member public functions, with the same signature and semantics as those in Vec:
    • new()
    • push()
    • pop()
    • last()
    • is_empty()
    • clear()
    • len()

You can use alternative types for the stack to make your parser no-std compliant. For example, using the arrayvec crate:

%include {
    use arrayvec::ArrayVec;
    type Stack<T> = ArrayVec<[T; 32]>;
%stack_size 32 Stack;

The %left, %right, %nonassoc directives

The %left, %right and %nonassoc directives are used to declare precedences of terminal symbols. Every terminal symbol whose name appears in one of those directives is given the same associative precedence value. Subsequent directives have higher precedence. For example:

%left And;
%left Or;
%nonassoc Eq Ne Gt Ge Lt Le;
%left Plus Minus;
%left Times Divide Mod;
%right Exp Not;

Note the semi-colon that terminates each %left, %right or %nonassoc directive.

LALR(1) grammars can get into a situation where they require a large amount of stack space if you make heavy use or right-associative operators. For this reason, it is recommended that you use %left rather than %right whenever possible.

The %default_type directive

This directive specifies a default type for all the symbols that do not specify a type.

The %extra_argument directive

The %extra_argument directive instructs pomelo to add a parameter to the Parser::new() function it generates. Pomelo doesn't do anything itself with this extra argument, but it does make the argument available to Rust-code action routines, and so forth, as a mutable refernce named extra. For example, if the grammar file contains:

%extra_argument MyStruct;

Then the function generated will be of the form Parser::new(extra: MyStruct) and all action routines will have access to a variable as extra: &mut MyStruct that is the value of the stored argument.

Moreover, there will be the following extra public member functions in the Parser struct:

pub fn into_extra(self) -> MyStruct;
pub fn extra(&self) -> &MyStruct;
pub fn extra_mut(&mut self) -> &mut MyStruct;

Also, if defined the end_of_input() member function will return a tuple, with the extra value as its second value.

The %error directive

This directive defines the type of the parser error. If not defined, it will default to (). Both functions of the Parser struct, parse() and end_of_input() return a Result<_,Error> with this type.

Also, any rule block can return an Err(Error) (usually with the ? operator) to force a parser error.

For example:

%error String;

If your error type does not implement Default then you must also write:

  • the %syntax_error directive to create a meaningful error (or ignore it).
  • the %parse_fail directive to return an error in case an unrecoverable error happens.
  • the %stack_overflow directive to return an error in case the internal stack overflows (unless you set the %stack_size to unlimited).

The %start_symbol directive

By default, the start symbol for the grammar that pomelo generates is the first non-terminal that appears in the grammar file. But you can choose a different start symbol using the %start_symbol directive.

%start_symbol program;

The %fallback directive

This directive defines an alternative token that will be used instead of another if the original one cannot be parsed. For example:

%fallback Id X Y Z;

declares the token Id as a fallback for any of the other tokens. If the input stream passes any of these three tokens and they cannot be parsed, then the parser will try parsing an Id before considering it an error.

The fallback token (Id in the example) must have the same type of every other token that it replaces, or no type at all.

The %wildcard directive

This directive defines a token that will be used when any other token cannot be parsed. For example:

%wildcard Any;

The wildcard token must not have a type.

The %token_class directive

This directive declares a compound token class. For example:

%token_class number Integer Float Double;

is equivalent but more efficient than:

number ::= Integer(A); { A }
number ::= Float(A);   { A }
number ::= Double(A);  { A }

or also:

number ::= Integer|Float|Double(A) { A }

Naturally, if they use a symbolic name ((A) in the example), then all the tokens must have the same type.

The %token directive

This directive is used to customize the Token enumeration generated by pomelo. It must be followed by an enumeration declaration named Token without any variants (they will be filled in by the macro). It can be used to add auto-derive traits, change its visibility, add custom attributes, add generics... For example:

%token #[derive(Copy,Clone,Debug)]
       pub enum Token {};

The default if it is this directive is not used is pub enum Token {}.

The %parser directive

This directive is used to customize the Parser struct generated by pomelo. It must be followed by a struct declaration named Parser without any fields (they will be filled in by the macro). It can be used to add auto-derive traits, change its visibility, add custom attributes, add generics... For example:

%parser pub struct Parser<'a> {};

The default if it is this directive is not used is pub struct Parser {}, but with the added generic arguments from %token if any.

For more about generic arguments see the Generic Parsers section.

The %extra_token directive

Sometimes, all tokens share a common piece of data. This is usually some kind of location (line/column information). In those cases, instead of adding it to the type of all terminal tokens, you can use this directive that takes a single type as argument. For example:

%extra_token Loc;

will change every terminal of type T into one of type (Loc, T). Any terminal without a type will get a type of Loc instead. Non-terminal symbols are unchanged.

If you use this directive with a type E, then the Token enum gains the following member functions:

fn into_extra(self) -> E;
fn extra(&self) -> &E;
fn extra_mut(&mut self) -> &mut E;

These are particularly useful in the %syntax_error code to build a meaninful error message.

The %verbose directive

This directive makes pomelo to dump the built states of the grammar to the console. This is mostly useful for diagnostics or for fine tuning your grammar.

Error Processing

After extensive experimentation over several years, it has been discovered that the error recovery strategy used by yacc is about as good as it gets. And so that is what pomelo uses.

When a pomelo-generated parser encounters a syntax error, it first invokes the code specified by the %syntax_error directive, if any. It then enters its error recovery strategy. The error recovery strategy is to begin popping the parsers stack until it enters a state where it is permitted to shift a special non-terminal symbol named error. It then shifts this non-terminal and continues parsing. But the %syntax_error routine will not be called again until at least three new tokens have been successfully shifted.

If the parser pops its stack until the stack is empty, and it still is unable to shift the error symbol, then the %parse_fail routine is invoked and the parser fails. This is what will happen at the very first syntax error, of course, if there are no instances of the error non-terminal in your grammar.

Generic parsers

You can use generic arguments either in the Token enum, the Parser struct, or both. For that, the directives %token and %parser are used. Note that, since the Parser contains partially parsed tokens, every generic argument used in Token must also be specified in Parser. This is the default if %parser is not used but must be taken into account if you use both directives.

Generic arguments in Parser can be used anywhere except terminal symbol types. That includes non-terminal types, the %extra_argument type and the %error type.

In addition to those, generic arguments in Token can also be used in terminal symbol types and the %extra_token directive.

Generic parsers can be very handy, as illustrated by the following two examples.

Generic lifetimes

Generic lifetime parameters allow the optional extra argument %extra_argument to reference shared data. Additionally, tokens may refer to data slices within the input of the parser.

pomelo! {
    %include { use std::collections::HashMap; }
    %parser pub struct Parser<'e>{};
    %token pub enum Token<'e>{};
    %extra_argument &'e HashMap<&'e[u8], i32>;

    %type input i32;
    %type numbers i32;
    %type number i32;
    %type Number i32;
    %type Var &'e[u8];
    input ::= numbers?(A) { A.unwrap_or(0) };
    number ::= Number(N) { N }
    number ::= Var(X) { match extra.get(X) { Some(v) => *v, _ => return Err(())} }
    numbers ::= number(N) { N }
    numbers ::= numbers(V) Comma number(N) { V + N }

fn main() -> Result<(), ()> {
    use parser::{Parser, Token};
    let input = b"1,x,2";
    // Real-world code would use a tokenizer.
    let x = &input[2..3];
    let tokens = vec![

    let mut values = HashMap::new();
    values.insert(x, 4);

    let mut p = Parser::new(&values);
    for tok in tokens.into_iter() {
    let (value, _) = p.end_of_input()?;
    assert_eq!(value, 7);

Generic production rules

Using a generic extra argument constrained by a trait makes it possible to change output types and add side effects to production rules while re-using the same parser module.

pub trait Adder {
    type Value;
    fn from_int(&mut self, value: i32) -> Self::Value;
    fn add(&mut self, v1: Self::Value, v2: Self::Value) -> Self::Value;

pomelo! {
    %parser pub struct Parser<T: super::Adder> {};
    %extra_argument T;
    %token #[derive(Clone)] pub enum Token{};

    %type input T::Value;
    %type numbers T::Value;
    %type Number i32;
    input ::= numbers?(A) { A.unwrap_or_else(|| extra.from_int(0)) };
    numbers ::= Number(N) { extra.from_int(N) }
    numbers ::= numbers(V) Comma Number(N) { let w = extra.from_int(N); extra.add(V, w) }

struct OpsCounter(usize);

impl Adder for OpsCounter {
    type Value = i32;

    fn from_int(&mut self, value: i32) -> Self::Value {

    fn add(&mut self, v1: Self::Value, v2: Self::Value) -> Self::Value {
        self.0 += 1;
        v1 + v2

struct LazyAdder;

impl Adder for LazyAdder {
    type Value = Vec<i32>;

    fn from_int(&mut self, value: i32) -> Self::Value {

    fn add(&mut self, mut v1: Self::Value, v2: Self::Value) -> Self::Value {

fn main() -> Result<(), ()> {
    use parser::{Parser, Token};
    // Real-world code would use a tokenizer.
    let tokens = vec![
    let mut p = Parser::new(OpsCounter(0));
    for tok in tokens.iter() {
    let (value, counter) = p.end_of_input()?;
    assert_eq!(value, 6);
    assert_eq!(counter.0, 2);

    let mut p = Parser::new(LazyAdder);
    for tok in tokens.into_iter() {
    let (numbers, _) = p.end_of_input()?;
    assert_eq!(numbers, vec![1,2,3]);



This modules is here to document the code generated by the pomelo! macro, do not use it in your program!



The main macro of this crate. See the crate-level documentation for details.