The CL-Yacc Manual

CL-Yacc

CL-Yacc is a LALR(1) parser generator for Common Lisp, somewhat like Yacc, GNU Bison, Zebu, lalr.cl or lalr.scm.

The latest version of CL-Yacc can be found on the CL-Yacc web page.

This manual was written by Juliusz Chroboczek.

1 A complete example

CL-Yacc exports its symbols from the package yacc:

     (use-package '#:yacc)

A parser consumes the output of a lexer, that produces a stream of terminals. CL-Yacc expects the lexer to be a function of no arguments (a thunk) that returns two values: the next terminal symbol, and the value of the symbol, which will be passed to the action associated with a production. At the end of the input, the lexer should return nil.

A very simple lexer that grabs tokens from a list:

     (defun list-lexer (list)
       #'(lambda ()
           (let ((value (pop list)))
             (if (null value)
                 (values nil nil)
                 (let ((terminal
                        (cond ((member value '(+ - * / |(| |)|)) value)
                              ((integerp value) 'int)
                              ((symbolp value) 'id)
                              (t (error "Unexpected value ~S" value)))))
                   (values terminal value))))))

We will implement the following grammar:

     expression ::= expression + expression
     expression ::= expression - expression
     expression ::= expression * expression
     expression ::= expression / expression
     expression ::= term
     
     term ::= id
     term ::= int
     term ::= - term
     term ::= ( expression )

As this grammar is ambiguous, we need to specify the precedence and associativity of the operators. The operators * and / will have the highest precedence, + and - will have a lower one. All operators will be left-associative.

If no semantic action is specified, CL-Yacc provides default actions which are either #'list or #'identity, depending on how a production is written. For building a Lisp-like parse tree with this grammar, we will need two additional actions:

     (eval-when (:compile-toplevel :load-toplevel :execute)
       (defun i2p (a b c)
         "Infix to prefix"
         (list b a c))
     
       (defun k-2-3 (a b c)
         "Second out of three"
         (declare (ignore a c))
         b)
     )

The parser definition itself:

     (define-parser *expression-parser*
       (:start-symbol expression)
       (:terminals (int id + - * / |(| |)|))
       (:precedence ((:left * /) (:left + -)))
     
       (expression
        (expression + expression #'i2p)
        (expression - expression #'i2p)
        (expression * expression #'i2p)
        (expression / expression #'i2p)
        term)
     
       (term
        id
        int
        (- term)
        (|(| expression |)| #'k-2-3)))

After loading this code, the parser is the value of the special variable *expression-parser*, which can be passed to parse-with-lexer:

     (parse-with-lexer (list-lexer '(x * - - 2 + 3 * y)) *expression-parser*)
          (+ (* X (- (- 2))) (* 3 Y))

2 Reference

2.1 Running the parser

The main entry point to the parser is parse-with-lexer.

— Function: parse-with-lexer lexer parser

Parse the input provided by the lexer lexer using the parser parser.

The value of lexer should be a function of no arguments that returns two values: the terminal symbol corresponding to the next token (a non-null symbol), and its value (anything that the associated actions can take as argument). It should return (values nil nil) when the end of the input is reached.

The value of parser should be a parser structure, as computed by make-parser and define-parser.

2.2 Macro interface

— Macro: define-grammar name option... production...
option ::= ( keyword value )
     production ::= ( symbol rhs... )
     rhs ::= symbol
     rhs ::= ( symbol... [action] )

Generates a grammar and binds it to the special variable name. This has the side effect of globally proclaiming name special.

Every production is a list of a non-terminal symbol and one or more right hand sides. Every right hand side is either a symbol, or a list of symbols optionally followed with an action.

The action should be a non-atomic form that evaluates to a function in a null lexical environment. If omitted, it defaults to #'identity in the first form of rhs, and to #'list in the second form.

The legal options are:

:start-symbol
Defines the starting symbol of the grammar. This is required.
:terminals
Defines the list of terminals of the grammar. This is required.
:precedence
The value of this option should be a list of items of the form (associativity . terminals), where associativity is one of :left, :right or :nonassoc, and terminals is a list of terminal symbols. Associativity specifies the associativity of the terminals, and earlier items will give their elements a precedence higher than that of later ones.

— Macro: define-parser name option... production...

Generates a parser and binds it to the special variable name. This has the side effect of globally proclaiming name special.

The syntax is the same as that of define-grammar, except that the following additional options are allowed:

:muffle-conflicts
If nil (the default), a warning is signalled for every conflict. If the symbol :some, then only a summary of the number of conflicts is signalled. If T, then no warnings at all are signalled for conflicts. Otherwise, its value should be a list of two integers (sr rr), in which case a summary warning will be signalled unless exactly sr shift-reduce and rr reduce-reduce conflicts were found.
:print-derives-epsilon
If true, print the list of nonterminal symbols that derive the empty string.
:print-first-terminals
If true, print, for every nonterminal symbol, the list of terminals that it may start with.
:print-states
If true, print the computed kernels of LR(0) items.
:print-goto-graph
If true, print the computed goto graph.
:print-lookaheads
If true, print the computed kernels of LR(0) items together with their lookaheads.

2.3 Functional interface

The macros define-parser and define-grammar expand into calls to defparameter, make-parser, make-grammar and make-production with suitable make-load-form magic to ensure that the time consuming parser generation happens at compile time rather than at load time. The underlying functions are exported in case you want to design a different syntax for grammars, or generate grammars automatically.

— Function: make-production symbol derives &key action action-form

Returns a production for non-terminal symbol with right-hand-side derives (a list of symbols). Action is the associated action, and should be a function; it defaults to #'list. Action-form should be a form that evaluates to action in a null lexical environment; if null (the default), the production (and hence any grammar or parser that uses it) will not be fasdumpable.

— Function: make-grammar &key name start-symbol terminals precedence productions

Returns a grammar. Name is the name of the grammar (gratuitious documentation). Start-symbol, terminals and precedence are as in define-grammar. Productions is a list of productions.

— Function: make-parser grammar &key discard-memos muffle-conflicts print-derives-epsilon print-first-terminals print-states print-goto-graph print-lookaheads

Computes and returns a parser for grammar grammar. discard-memos specifies whether temporary data associated with the grammar should be discarded. Muffle-conflicts, print-derives-epsilon, print-first-terminals, print-states, print-goto-graph and print-lookaheads are as in define-parser.

2.4 Conditions

CL-Yacc may signal warnings at compile time when it finds conflicts. It may also signal an error at parse time when it finds that the input is incorrect.

2.4.1 Compile-time conditions

If the grammar given to CL-Yacc is ambiguous, a warning of type conflict-warning will be signalled for every conflict as it is found, and a warning of type conflict-summary-warning will be signalled at the end of parser generation.

— Condition: conflict-warning kind state terminal

Signalled whenever a conflict is found. Kind is one of :shift-reduce or :reduce-reduce. State (an integer) and terminal (a symbol) are the state and terminal for which the conflict arises.

— Condition: conflict-summary-warning shift-reduce reduce-reduce

Signalled at the end of parser generation if there were any conflicts. Shift-reduce and reduce-reduce are integers that indicate how many conflicts were found.

— Condition: yacc-compile-warning

A superclass of conflict-warning and conflict-summary-warning, and a convenient place to hook your own condition types.

2.4.2 Runtime conditions

If the output cannot be parsed, the parser will signal a condition of type yacc-parse-error. It should be possible to invoke a restart from a handler for yacc-parse-error in order to trigger error recovery, but this hasn't been implemented yet.

— Condition: yacc-parse-error terminal value expected-terminals

Signalled whenever the input cannot be parsed. The symbol terminal is the terminal that couldn't be accepted; value is its value. Expected-terminals is the list of terminals that could have been accepted in that state.

— Condition: yacc-runtime-error

A superclass of yacc-parse-error, and a convenient place to hook your own condition types.

Acknowledgements

I am grateful to Antonio Bucciarelli, Guy Cousineau and Marc Zeitoun for their help with implementing CL-Yacc.

Copying

Copyright © 2005–2009 by Juliusz Chroboczek

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