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pomelo is a LALR(1) parser generator with support for GLR parsing. It is similar to bison or lemon, but produces parsers implemented in modern C++, taking advantage of compiler features such as move semantics.

Building pomelo

pomelo is built using meson, e.g.

meson build; cd build; ninja

The build scripts use python to embed the parser template into the program.

Running pomelo

pomelo [options] -h <output header> -c <output source> <syntax file>


  • --syntax : Writes a syntax file to stdout after parsing, generated from the internal syntax representation. Used to test the syntax parser frontend.

  • --dump : Writes a detailed list of all states and transitions in the generated LALR(1) automata to stdout. Useful for debugging complex grammars and understanding conflicts.

  • --graph : Writes a digraph description of the LALR(1) automata in dot format, which can be used to generate a visual representation of the automata. More useful if a problem can be localised to small example grammar, as otherwise the graph is likely to be extremely large.

  • --rgoto : Adds additional arrows to the --graph which link the final transition in a production to the transitions that shift the reduced production onto the parser stack. This corresponds to the includes relation in LALR(1) construction, and can be useful for debugging the parser generator.

  • --actions : Dump a low-level representation of the main parser table to stdout.

  • --conflicts : Print warnings about expected conflicts, and conflicts resolved by token precedence to stderr. Normally, pomelo only reports unexpected and unresolved conflicts.

Generated Parser

pomelo generates a parser class, including all actions from the syntax file.

To use the parser, construct an instance, passing in the user value for the main parse stack. The user value is copied into the parse stack. It is typically a smart pointer to the object built or modified by the parser, or a pointer to an object that provides methods called by the actions.

explicit parser( const user_value& u );

Parsing then proceeds one token at a time. The generated header contains an enumeration assigning token numbers to each terminal in the grammar. Call the parse method once for each token, passing in both the token number and the token's value.

void parse( int token, const token_type& tokval );

The special EOI token is declared implicitly, representing the end of input. It always has a token number of 0. Call the parse method again with this token to complete a parse.

Syntax Files


Syntax files consist of a list of nonterminal productions. Each production consists of its name, followed by a list of rules enclosed in square brackets.

    A B C .
    D E F .

Nonterminal (production) names are lower-case. Terminal (token) names are upper-case. All rules for a particular production must be specified at the same time.

The first nonterminal in a syntax file is the root production.


Each rule consists of a list of symbols. Each symbol can either be a terminal or a nonterminal. Each rule is terminated by a period.

    PP contents DOT qual_name QQ .

An epsilon rule indicating a production with no symbols can be specified using a period on its own.


Each rule can have a C++ action associated with it, which is executed when the rule is reduced. C++ code is placed after the period at the end of the rule, and is enclosed in curly braces.

    PP contents DOT qual_name QQ .
        { return reduced_rule(); }

The values of the symbols comprising the rule can be provided as arguments to the action function by giving them a name using parentheses.

    PP contents(c) DOT(op) qual_name(name) QQ .
        { return build_node( c, op, name); }

Symbol values are passed by rvalue reference into the action function, as they are consumed by the action to create the resulting nonterminal value.

There is an additional parameter, u, which is a reference to the user value for the parse stack.

Action functions must return a value for the resulting nonterminal value. The type of a nonterminal is specified in curly braces after the nonterminal name.

pp_qq_opt { node_ptr }
        { return node_ptr(); }
    PP contents(c) DOT(op) qual_name(name) QQ .
        { return build_node( c, op, name); }

Nonterminal values are placed on the parse stack. They are usually moved - from the result of an action function into the parse stack, and then out of the parse stack into the action function that consumes them. However, when parse stacks are split they may be copied, as each potential parse gets its own copy of each value.


pomelo constructs LALR(1) parsers and therefore the generated automaton can contain shift/reduce or reduce/reduce conflicts. In most cases the author of a grammar would like the generated parser to pick one action (either shift or reduce) for each conflict.

Like other parser generators, pomelo uses token precedence to resolve conflicts.

Precendence and associativity is assigned to terminal symbols using the %left, %right, and %nonassoc directives.

%nonassoc NO_ELSE .
%nonassoc ELSE .

%right ASSIGN .
%left ADD SUB .
%left MUL DIV MOD .

Tokens appearing in earlier directives have lower precedence (i.e. when used as operators they bind less tightly). Tokens appearing in later directives have higher precedence (i.e. they bind more tightly). Tokens which appear in the same directive have the same precedence.

These directives also provide a way to declare terminal symbols which do not appear in any grammar rule.

The default precedence of a rule is the same as the precedence of the first terminal symbol in the rule. This can be overridden by specifying a token in square brackets between the period and the rule's action. The precedence of the rule is then the same as the precedence of this token.

    IF LPN expr RPN stmt . [NO_ELSE] { return ifstmt(); }

When the parser generator encounters a conflict, it first attempts to resolve it using the precedence rules. These follow the same strategy as the lemon parser generator.

For a shift/reduce conflict:

  • If either the token to shift or the rule to reduce lacks precedence, the conflict is unresolved.
  • If the token has higher precedence than the rule, shift.
  • If the rule has higher precedence than the token, reduce.
  • Otherwise they have the same precedence, so consider the associativity of the token.
  • If the token is right-associative, shift.
  • If the token is left-associative, reduce.
  • If the token is non-associative, the conflict is unresolved.

For a reduce/reduce conflict:

  • If either rule lacks precedence, the conflict is unresolved.
  • If both rules have the same precedence, the conflict is unresolved.
  • Otherwise, reduce the rule with the higher precedence.

Using these rules most conflicts in programming language grammars can be resolved.

Expected Conflicts

Many real grammars are not LALR(1) and will contain parse conflicts. By default, pomelo reports an error (and returns a failure code) when it encounters unresolved conflicts in a grammar.

When writing a grammar, especially for an LR parser, conflicts are not always obvious. It is easy to edit a grammar and introduce ambiguity where none was intended. However, some ambiguities are real features of the language and should be accepted by the parser generator - they will trigger a split in the parser stack at runtime and parsing will continue using a GLR algorithm.

Therefore, a grammar author must mark expected unresolved conflicts using the conflict marker ! where they appear in the grammar. This reduces the chance that an unanticipated conflict will slip through.

To mark a shift as conflicting, place a conflict marker before the token that will be shifted where it appears in a rule.

To mark a rule as conflicting, place a conflict marker at the end of the rule, before the period.

    context ! SHIFT_TOKEN TRAILING .
    reduce_rule TOKEN symbol ! .

For shift/reduce conflicts, the conflict is expected if all instances of the shift token are marked with a conflict marker, and the rule to be reduced is marked with a conflict marker. Note that during construction of the discrete finite automaton multiple locations in the source grammar are collapsed into a single state, so multiple instances of a token may need to be marked.

For reduce/reduce conflicts, the conflict is expected if both rules are marked with the conflict marker.

Generalized parsing

A grammar with unresolved conflicts is effectively ambiguous. Such grammars either require more than one token of lookahead, or additional contextual knowledge must be used to disambiguate two or more valid parse trees. GLR parsing allows pomelo to handle both these cases - unambiguous grammars that are not LALR(1), as well as truly ambiguous grammars.

When the parser encounters an expected conflict, the parser is split into two (or more) independent parsers. Instead of a single parser stack, there are now multiple parser stacks, each with their own stack top, user value, and state.

The user value for the split stack is generated by a call to the %user_split function.

    return split_parse( u );

This function is called with one argument, u, which is a reference to the user value from the original stack. u will be used as the user value for the first stack. The function must return a user value for the second stack.

The part of the parser stack that is common is shared.

                              <- type_name LT NAME (u, state 52)
    func_header LBR stmt_list
                              <- decl_name LT NAME (v, state 98)

In pomelo, the parse stack is a tree structure, not a graph - only the left context is shared. This is sufficient for languages where ambiguities are fairly localised, and alternatives either die off or merge quickly.

If the parser encounters an error, and there is more than one valid parse remaining, the parse that errored is destroyed, and parsing continues.

Ambiguous grammars can merge alternatives using a merge function. This is a block of code attached to a nonterminal production with the @ symbol. If two parses reduce to this nonterminal at the same time with identical left contexts and identical states, then the parses are merged.

ambiguous_expr_decl { node_ptr }
    @{ return merge_parses( u, a, v, b ); }
    expr(e) . { return e; }
    decl(d) . { return d; }

The parameters passed to a merge function are:

  • u : A reference to the user value from the first parse. This becomes the user value of the merged parse stack.

  • a : An rvalue reference to the nonterminal value from the first parse. The function consumes this value during the merge.

  • v : An rvalue reference to the user value from the second parse. The function consumes this value during the merge, and the second parse stack is destroyed by the merge operation.

  • b : An rvalue reference to the nonterminal value from the second parse. The function consumes this value during the merge.

It should return a single value which is used as the value of the nonterminal in the merged parse stack.

Error Reporting

The error function is defined using the %error_report directive.

    fprintf( stderr, "unexpected token '%s'\n", symbol_name( token ) );

When there is only one valid parse, and the parser is given a unexpected token, then the error function is called with the following arguments:

  • u : A reference to the user value for the current parse.

  • token : The token number (from the enumeration) of the unexpected token.

  • tokval : A reference to the token's value.

Currently the parser does not attempt error recovery - the parser remains in the same state after reporting an unexpected token.


Directives supported in syntax files:

  • %include { /* C++ */ } or %include_header { /* C++ */ } : Places a block of C++ code at the top of the generated header file. Allows inclusion of headers or declarations of types for user values.

  • %include_source { /* C++ */ } : Places a block of C++ code at the top of the generated source file. Useful for including definitions of token or user value types, or other declarations needed by parser actions.

  • %user_value { type_name } : Specify the type of the user value associated with each parse.

  • %user_split { /* C++ */ } : Declares the split function for user values, see above.

  • %class_name { identifier } : Give a name for the generated parser class. The default is parser.

  • %token_type { type_name } : The value type for terminal symbols. Like value types for nonterminals, terminal values are usually moved, but they are copied into the parser for each live parse, and may be copied when stacks are split.

  • %token_prefix { PREFIX_ } : An enumeration listing all terminal symbols (and giving them an token number) is written into the generated header. This allows you to provide a prefix for the enumerators.

  • %nterm_prefix { PREFIX_ } : An enumeration listing all nonterminal symbols is written into the generated header. This allows you to provide a prefix for the enumerators. Note that nonterminal names are converted to upper case in this context.

  • %error_report { /* C++ */ } : Declares the error function, see above.

  • %left, %right, %nonassoc : Assigns precedence to terminal symbols, see above.


  • Implement an error recovery strategy for the generated parser.


Copyright © 2018 Edmund Kapusniak. Licensed under the MIT License. See LICENSE file in the project root for full license information.


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