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.
CL-Yacc exports its symbols from the package
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
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 ::=
As this grammar is ambiguous, we need to specify the precedence and
associativity of the operators. The operators
will have the highest precedence,
- 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
#'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
*expression-parser*, which can be passed to
(parse-with-lexer (list-lexer '(x * - - 2 + 3 * y)) *expression-parser*) (+ (* X (- (- 2))) (* 3 Y))
The main entry point to the parser is
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
parserstructure, as computed by
)rhs ::= symbol rhs ::=
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
#'identityin the first form of rhs, and to
#'listin the second form.
The legal options are:
- Defines the starting symbol of the grammar. This is required.
- Defines the list of terminals of the grammar. This is required.
- The value of this option should be a list of items of the form
), where associativity is one of
: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.
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:
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.
- If true, print the list of nonterminal symbols that derive the empty string.
- If true, print, for every nonterminal symbol, the list of terminals that it may start with.
- If true, print the computed kernels of LR(0) items.
- If true, print the computed goto graph.
- If true, print the computed kernels of LR(0) items together with their lookaheads.
define-grammar expand into
make-production with suitable
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.
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.
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.
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
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.
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.
Signalled whenever a conflict is found. Kind is one of
:reduce-reduce. State (an integer) and terminal (a symbol) are the state and terminal for which the conflict arises.
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.
A superclass of
conflict-summary-warning, and a convenient place to hook your own condition types.
If the output cannot be parsed, the parser will signal a condition of
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.
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.
A superclass of
yacc-parse-error, and a convenient place to hook your own condition types.
I am grateful to Antonio Bucciarelli, Guy Cousineau and Marc Zeitoun for their help with implementing CL-Yacc.
Copyright © 2005–2009 by Juliusz Chroboczek
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.