Lemon is an LALR(1) parser generator for C or C++. -It does the same job as ``bison'' and ``yacc''. -But lemon is not another bison or yacc clone. It +
Lemon is an LALR(1) parser generator for C. +It does the same job as "bison" and "yacc". +But lemon is not a bison or yacc clone. Lemon uses a different grammar syntax which is designed to -reduce the number of coding errors. Lemon also uses a more -sophisticated parsing engine that is faster than yacc and -bison and which is both reentrant and thread-safe. -Furthermore, Lemon implements features that can be used +reduce the number of coding errors. Lemon also uses a +parsing engine that is faster than yacc and +bison and which is both reentrant and threadsafe. +(Update: Since the previous sentence was written, bison +has also been updated so that it too can generate a +reentrant and threadsafe parser.) +Lemon also implements features that can be used to eliminate resource leaks, making is suitable for use in long-running programs such as graphical user interfaces or embedded controllers.
This document is an introduction to the Lemon @@ -42,22 +45,22 @@
The grammar specification file uses a ``.y'' suffix, by convention. +
The grammar specification file uses a ".y" suffix, by convention. In the examples used in this document, we'll assume the name of the -grammar file is ``gram.y''. A typical use of Lemon would be the +grammar file is "gram.y". A typical use of Lemon would be the following command:
lemon gram.y
-This command will generate three output files named ``gram.c'',
-``gram.h'' and ``gram.out''.
+This command will generate three output files named "gram.c",
+"gram.h" and "gram.out".
The first is C code to implement the parser. The second
is the header file that defines numerical values for all
terminal symbols, and the last is the report that explains
the states used by the parser automaton.
@@ -69,43 +72,39 @@
lemon -?
As of this writing, the following command-line options are supported:
- Parser statistics: 74 terminals, 70 nonterminals, 179 rules - 340 states, 2026 parser table entries, 0 conflicts --Finally, the ``-x'' option causes Lemon to print its version number -and then stops without attempting to read the grammar or generate a parser.
Lemon doesn't generate a complete, working program. It only generates a few subroutines that implement a parser. This section describes @@ -119,16 +118,16 @@
void *pParser = ParseAlloc( malloc );
The ParseAlloc() routine allocates and initializes a new parser and
returns a pointer to it.
-The actual data structure used to represent a parser is opaque --
+The actual data structure used to represent a parser is opaque —
its internal structure is not visible or usable by the calling routine.
For this reason, the ParseAlloc() routine returns a pointer to void
rather than a pointer to some particular structure.
The sole argument to the ParseAlloc() routine is a pointer to the
-subroutine used to allocate memory. Typically this means ``malloc()''.
+subroutine used to allocate memory. Typically this means malloc().
After a program is finished using a parser, it can reclaim all memory allocated by that parser by calling
ParseFree(pParser, free);
@@ -149,22 +148,23 @@
The second argument is a small positive integer that tells the parse the
type of the next token in the data stream.
There is one token type for each terminal symbol in the grammar.
The gram.h file generated by Lemon contains #define statements that
map symbolic terminal symbol names into appropriate integer values.
-(A value of 0 for the second argument is a special flag to the
-parser to indicate that the end of input has been reached.)
+A value of 0 for the second argument is a special flag to the
+parser to indicate that the end of input has been reached.
The third argument is the value of the given token. By default,
the type of the third argument is integer, but the grammar will
usually redefine this type to be some kind of structure.
Typically the second argument will be a broad category of tokens
-such as ``identifier'' or ``number'' and the third argument will
+such as "identifier" or "number" and the third argument will
be the name of the identifier or the value of the number.
The Parse() function may have either three or four arguments,
-depending on the grammar. If the grammar specification file request
-it, the Parse() function will have a fourth parameter that can be
+depending on the grammar. If the grammar specification file requests
+it (via the extra_argument directive),
+the Parse() function will have a fourth parameter that can be
of any type chosen by the programmer. The parser doesn't do anything
with this argument except to pass it through to action routines.
This is a convenient mechanism for passing state information down
to the action routines without having to use global variables.
@@ -190,11 +190,11 @@
17 return sState.treeRoot;
18 }
This example shows a user-written routine that parses a file of
text and returns a pointer to the parse tree.
-(We've omitted all error-handling from this example to keep it
+(All error-handling code is omitted from this example to keep it
simple.)
We assume the existence of some kind of tokenizer which is created
using TokenizerCreate() on line 8 and deleted by TokenizerFree()
on line 16. The GetNextToken() function on line 11 retrieves the
next token from the input file and puts its type in the
@@ -261,10 +261,16 @@
These differences may cause some initial confusion for programmers
with prior yacc and bison experience.
But after years of experience using Lemon, I firmly
believe that the Lemon way of doing things is better.
+Updated as of 2016-02-16: +The text above was written in the 1990s. +We are told that Bison has lately been enhanced to support the +tokenizer-calls-parser paradigm used by Lemon, and to obviate the +need for global variables.
+The main purpose of the grammar specification file for Lemon is to define the grammar for the parser. But the input file also specifies additional information Lemon requires to do its job. @@ -278,11 +284,11 @@ tokens) and it honors the same commenting conventions as C and C++.
A terminal symbol (token) is any string of alphanumeric -and underscore characters +and/or underscore characters that begins with an upper case letter. A terminal can contain lowercase letters after the first character, but the usual convention is to make terminals all upper case. A nonterminal, on the other hand, is any string of alphanumeric and underscore characters than begins with a lower case letter. @@ -305,11 +311,11 @@
The main component of a Lemon grammar file 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 special symbol "::=" and then a list of terminals and/or nonterminals. The rule is terminated by a period. 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 @@ -321,13 +327,13 @@ 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''.
+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, Lemon allows the grammar to specify a block of C code that will be executed whenever a grammar rule is reduced by the parser. In Lemon, this action is specified by putting the C code (contained @@ -339,19 +345,19 @@
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 ``$$'' in the +In yacc and bison, this is accomplished by embedding a "$$" 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 +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''.
+rule and say "$7" when you really mean "$8".Lemon 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. In yacc or bison, one would write this: @@ -377,18 +383,19 @@ is generated. For example, the rule
expr(A) ::= expr(B) PLUS expr(C). { A = B; }
-will generate an error because the linking symbol ``C'' is used
+will generate an error because the linking symbol "C" is used
in the grammar rule but not in the reduce action.
The Lemon notation for linking grammar rules to reduce actions also facilitates the use of destructors for reclaiming memory allocated by the values of terminals and nonterminals on the right-hand side of a rule.
+Lemon 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 @@ -396,11 +403,14 @@
Just like in yacc and bison, Lemon allows a measure of control over the resolution of paring conflicts using precedence rules. A precedence value can be assigned to any terminal symbol -using the %left, %right or %nonassoc directives. Terminal symbols +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.
@@ -516,11 +526,15 @@
The %code directive is used to specify addition C/C++ code that +
The %code directive is used to specify addition C code that is added to the end of the main output file. This is similar to -the %include directive except that %include is inserted at the -beginning of the main output file.
+the %include directive except that %include +is inserted at the beginning of the main output file.%code is typically used to include some action routines or perhaps -a tokenizer as part of the output file.
+a tokenizer or even the "main()" function +as part of the output file. +The %default_destructor directive specifies a destructor to use for non-terminals that do not have their own destructor specified by a separate %destructor directive. See the documentation -on the %destructor directive below for additional information.
+on the %destructor directive below for +additional information.In some grammers, many different non-terminal symbols have the same datatype and hence the same destructor. This directive is a convenience way to specify the same destructor for all those non-terminals using a single statement.
+The %default_type directive specifies the datatype of non-terminal symbols that do no have their own datatype defined using a separate -%type directive. See the documentation on %type below for addition -information.
+%type directive. + +The %destructor directive is used to specify a destructor for a non-terminal symbol. -(See also the %token_destructor directive which is used to -specify a destructor for terminal symbols.)
+(See also the %token_destructor +directive which is used to specify a destructor for terminal symbols.)A non-terminal's destructor is called to dispose of the non-terminal's value whenever the non-terminal is popped from the stack. This includes all of the following circumstances:
It is important to note that the value of a non-terminal is passed to the destructor whenever the non-terminal is removed from the stack, unless the non-terminal is used in a C-code action. If the non-terminal is used by C-code, then it is assumed that the -C-code will take care of destroying it if it should really -be destroyed. More commonly, the value is used to build some +C-code will take care of destroying it. +More commonly, the value is used to build some larger structure and we don't want to destroy it, which is why the destructor is not called in this circumstance.
-By appropriate use of destructors, it is possible to -build a parser using Lemon that can be used within a long-running -program, such as a GUI, that will not leak memory or other resources. +
Destructors help avoid memory leaks by automatically freeing +allocated objects when they go out of scope. To do the same using yacc or bison is much more difficult.
+
%extra_argument { MyStruct *pAbc }
Then the Parse() function generated will have an 4th parameter -of type ``MyStruct*'' and all action routines will have access to -a variable named ``pAbc'' that is the value of the 4th parameter +of type "MyStruct*" and all action routines will have access to +a variable named "pAbc" that is the value of the 4th parameter in the most recent call to Parse().
+ +The %fallback directive specifies an alternative meaning for one +or more tokens. The alternative meaning is tried if the original token +would have generated a syntax error. + +
The %fallback directive was added to support robust parsing of SQL +syntax in SQLite. +The SQL language contains a large assortment of keywords, each of which +appears as a different token to the language parser. SQL contains so +many keywords, that it can be difficult for programmers to keep up with +them all. Programmers will, therefore, sometimes mistakenly use an +obscure language keyword for an identifier. The %fallback directive +provides a mechanism to tell the parser: "If you are unable to parse +this keyword, try treating it as an identifier instead." + +
The syntax of %fallback is as follows: + +
+%fallback ID TOKEN... . ++ +
In words, the %fallback directive is followed by a list of token names +terminated by a period. The first token name is the fallback token - the +token to which all the other tokens fall back to. The second and subsequent +arguments are tokens which fall back to the token identified by the first +argument. + + +
The %ifdef, %ifndef, and %endif directives are similar to +#ifdef, #ifndef, and #endif in the C-preprocessor, just not as general. +Each of these directives must begin at the left margin. No whitespace +is allowed between the "%" and the directive name. + +
Grammar text in between "%ifdef MACRO" and the next nested "%endif" is +ignored unless the "-DMACRO" command-line option is used. Grammar text +betwen "%ifndef MACRO" and the next nested "%endif" is included except when +the "-DMACRO" command-line option is used. + +
Note that the argument to %ifdef and %ifndef must be a single +preprocessor symbol name, not a general expression. There is no "%else" +directive. + + +
The %include directive specifies C code that is included at the top of the generated parser. You can include any text you want -- the Lemon parser generator copies it blindly. If you have multiple -%include directives in your grammar file the value of the last -%include directive overwrites all the others.
The %include directive is very handy for getting some extra #include preprocessor statements at the beginning of the generated parser. For example:
@@ -651,16 +722,17 @@This might be needed, for example, if some of the C actions in the grammar call functions that are prototyed in unistd.h.
+
%left AND.
@@ -677,14 +749,15 @@
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 %name directive
By default, the functions generated by Lemon all begin with the
-five-character string ``Parse''. You can change this string to something
+five-character string "Parse". You can change this string to something
different using the %name directive. For instance:
%name Abcde
@@ -699,20 +772,23 @@
The %name directive allows you to generator two or more different
parsers and link them all into the same executable.
+
The %nonassoc directive
This directive is used to assign non-associative precedence to
-one or more terminal symbols. See the section on precedence rules
-or on the %left directive for additional information.
+one or more terminal symbols. See the section on
+precedence rules
+or on the %left directive for additional information.
+
The %parse_accept directive
The %parse_accept directive specifies a block of C code that is
-executed whenever the parser accepts its input string. To ``accept''
+executed whenever the parser accepts its input string. To "accept"
an input string means that the parser was able to process all tokens
without error.
For example:
@@ -720,11 +796,11 @@
%parse_accept {
printf("parsing complete!\n");
}
-
+
The %parse_failure directive specifies a block of C code that is executed whenever the parser fails complete. This code is not executed until the parser has tried and failed to resolve an input @@ -735,16 +811,19 @@ %parse_failure { fprintf(stderr,"Giving up. Parser is hopelessly lost...\n"); }
+This directive is used to assign right-associative precedence to -one or more terminal symbols. See the section on precedence rules -or on the %left directive for additional information.
+one or more terminal symbols. See the section on +precedence rules +or on the %left directive for additional information. +The %stack_overflow directive specifies a block of C code that is executed if the parser's internal stack ever overflows. Typically this just prints an error message. After a stack overflow, the parser @@ -769,10 +848,11 @@
list ::= element list. // right-recursion. Bad!
list ::= .
+
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 @@ -781,10 +861,11 @@
%stack_size 2000
+
By default, the start-symbol for the grammar that Lemon 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.
@@ -791,10 +872,11 @@
%start_symbol prog
+
The %destructor directive assigns a destructor to a non-terminal symbol. (See the description of the %destructor directive above.) This directive does the same thing for all terminal symbols.
@@ -803,10 +885,11 @@ for their values, terminals all use the same data type (defined by the %token_type directive) and so they use a common destructor. Other than that, the token destructor works just like the non-terminal destructors. +Lemon generates #defines that assign small integer constants to each terminal symbol in the grammar. If desired, Lemon will add a prefix specified by this directive @@ -828,10 +911,11 @@ #define TOKEN_MINUS 2 #define TOKEN_OR 3 #define TOKEN_PLUS 4 +
These directives are used to specify the data types for values on the parser's stack associated with terminal and non-terminal symbols. The values of all terminal symbols must be of the same @@ -843,11 +927,11 @@
%token_type {Token*}
If the data type of terminals is not specified, the default value -is ``int''.
+is "int".Non-terminal symbols can each have their own data types. Typically the data type of a non-terminal is a pointer to the root of a parse-tree structure that contains all information about that non-terminal. For example:
@@ -864,10 +948,21 @@ 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 %wildcard directive is followed by a single token name and a +period. This directive specifies that the identified token should +match any input token. + +
When the generated parser has the choice of matching an input against +the wildcard token and some other token, the other token is always used. +The wildcard token is only matched if there are no other alternatives. +
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 Lemon uses.
@@ -875,18 +970,18 @@When a Lemon-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 +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_failed routine is invoked and the parser resets itself to its start state, ready to begin parsing a new file. 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.
+"error" non-terminal in your grammar.