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diff --git a/lib/parsetools/doc/src/yecc.xml b/lib/parsetools/doc/src/yecc.xml new file mode 100644 index 0000000000..81f1550b0a --- /dev/null +++ b/lib/parsetools/doc/src/yecc.xml @@ -0,0 +1,529 @@ +<?xml version="1.0" encoding="latin1" ?> +<!DOCTYPE erlref SYSTEM "erlref.dtd"> + +<erlref> + <header> + <copyright> + <year>1996</year><year>2009</year> + <holder>Ericsson AB. All Rights Reserved.</holder> + </copyright> + <legalnotice> + The contents of this file are subject to the Erlang Public License, + Version 1.1, (the "License"); you may not use this file except in + compliance with the License. You should have received a copy of the + Erlang Public License along with this software. If not, it can be + retrieved online at http://www.erlang.org/. + + Software distributed under the License is distributed on an "AS IS" + basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See + the License for the specific language governing rights and limitations + under the License. + + </legalnotice> + + <title>yecc</title> + <prepared>Carl Wilhelm Welin</prepared> + <responsible>Carl Wilhelm Welin</responsible> + <docno></docno> + <approved>Bjarne Däcker</approved> + <checked></checked> + <date>1997-01-27</date> + <rev>B</rev> + <file>yecc.sgml</file> + </header> + <module>yecc</module> + <modulesummary>LALR-1 Parser Generator</modulesummary> + <description> + <p>An LALR-1 parser generator for Erlang, similar to <c>yacc</c>. + Takes a BNF grammar definition as input, and produces Erlang code + for a parser. </p> + <p>To understand this text, you also have to + look at the <c>yacc</c> documentation in the UNIX(TM) manual. This + is most probably necessary in order to understand the idea of a + parser generator, and the principle and problems of LALR parsing + with finite look-ahead.</p> + </description> + <funcs> + <func> + <name>file(Grammarfile [, Options]) -> YeccRet</name> + <fsummary>Give information about resolved and unresolved parse action conflicts.</fsummary> + <type> + <v>Grammarfile = filename()</v> + <v>Options = Option | [Option]</v> + <v>Option = - see below -</v> + <v>YeccRet = {ok, Parserfile} | {ok, Parserfile, Warnings} | error | {error, Warnings, Errors}</v> + <v>Parserfile = filename()</v> + <v>Warnings = Errors = [{filename(), [ErrorInfo]}]</v> + <v>ErrorInfo = {ErrorLine, module(), Reason}</v> + <v>ErrorLine = integer()</v> + <v>Reason = - formatable by format_error/1 -</v> + </type> + <desc> + <p><c>Grammarfile</c> is the file of declarations and grammar + rules. Returns <c>ok</c> upon success, or <c>error</c> if + there are errors. An Erlang file containing the parser is + created if there are no errors. The options are: + </p> + <taglist> + <tag><c>{parserfile, Parserfile}</c>.</tag> + <item><c>Parserfile</c> is the name of the file that will + contain the Erlang parser code that is generated. The + default (<c>""</c>) is to add the extension <c>.erl</c> + to <c>Grammarfile</c> stripped of the <c>.yrl</c> + extension. + </item> + <tag><c>{includefile, Includefile}</c>.</tag> + <item>Indicates a customized prologue file which the user + may want to use instead of the default file + <c>lib/parsetools/include/yeccpre.hrl</c> which is + otherwise included at the beginning of the resulting + parser file. <em>N.B.</em> The <c>Includefile</c> is + included 'as is' in the parser file, so it must not have a + module declaration of its own, and it should not be + compiled. It must, however, contain the necessary export + declarations. The default is indicated by <c>""</c>. + </item> + <tag><c>{report_errors, bool()}</c>.</tag> + <item>Causes errors to be printed as they occur. Default is + <c>true</c>. + </item> + <tag><c>{report_warnings, bool()}</c>.</tag> + <item>Causes warnings to be printed as they occur. Default is + <c>true</c>. + </item> + <tag><c>{report, bool()}</c>.</tag> + <item>This is a short form for both <c>report_errors</c> and + <c>report_warnings</c>. + </item> + <tag><c>{return_errors, bool()}</c>.</tag> + <item>If this flag is set, <c>{error, Errors, Warnings}</c> + is returned when there are errors. Default is + <c>false</c>. + </item> + <tag><c>{return_warnings, bool()}</c>.</tag> + <item>If this flag is set, an extra field containing + <c>Warnings</c> is added to the tuple returned upon + success. Default is <c>false</c>. + </item> + <tag><c>{return, bool()}</c>.</tag> + <item>This is a short form for both <c>return_errors</c> and + <c>return_warnings</c>. + </item> + <tag><c>{verbose, bool()}</c>. </tag> + <item>Determines whether the parser generator should give + full information about resolved and unresolved parse + action conflicts (<c>true</c>), or only about those + conflicts that prevent a parser from being generated from + the input grammar (<c>false</c>, the default). + </item> + </taglist> + <p>Any of the Boolean options can be set to <c>true</c> by + stating the name of the option. For example, <c>verbose</c> + is equivalent to <c>{verbose, true}</c>. + </p> + <p>The value of the <c>Parserfile</c> option stripped of the + <c>.erl</c> extension is used by Yecc as the module name of + the generated parser file.</p> + <p>Yecc will add the extension <c>.yrl</c> to the + <c>Grammarfile</c> name, the extension <c>.hrl</c> to the + <c>Includefile</c> name, and the extension <c>.erl</c> to + the <c>Parserfile</c> name, unless the extension is already + there.</p> + </desc> + </func> + <func> + <name>format_error(Reason) -> Chars</name> + <fsummary>Return an English description of a an error tuple.</fsummary> + <type> + <v>Reason = - as returned by yecc:file/1,2 -</v> + <v>Chars = [char() | Chars]</v> + </type> + <desc> + <p>Returns a descriptive string in English of an error tuple + returned by <c>yecc:file/1,2</c>. This function is mainly + used by the compiler invoking Yecc.</p> + </desc> + </func> + </funcs> + + <section> + <title>Pre-Processing</title> + <p>A <c>scanner</c> to pre-process the text (program, etc.) to be + parsed is not provided in the <c>yecc</c> module. The scanner + serves as a kind of lexicon look-up routine. It is possible to + write a grammar that uses only character tokens as terminal + symbols, thereby eliminating the need for a scanner, but this + would make the parser larger and slower.</p> + <p>The user should implement a scanner that segments the input + text, and turns it into one or more lists of tokens. Each token + should be a tuple containing information about syntactic + category, position in the text (e.g. line number), and the + actual terminal symbol found in the text: <c>{Category, LineNumber, Symbol}</c>.</p> + <p>If a terminal symbol is the only member of a category, and the + symbol name is identical to the category name, the token format + may be <c>{Symbol, LineNumber}</c>.</p> + <p>A list of tokens produced by the scanner should end with a + special <c>end_of_input</c> tuple which the parser is looking + for. The format of this tuple should be <c>{Endsymbol, LastLineNumber}</c>, where <c>Endsymbol</c> is an identifier + that is distinguished from all the terminal and non-terminal + categories of the syntax rules. The <c>Endsymbol</c> may be + declared in the grammar file (see below).</p> + <p>The simplest case is to segment the input string into a list of + identifiers (atoms) and use those atoms both as categories and + values of the tokens. For example, the input string <c>aaa bbb 777, X</c> may be scanned (tokenized) as:</p> + <code type="none"> +[{aaa, 1}, {bbb, 1}, {777, 1}, {',' , 1}, {'X', 1}, + {'$end', 1}]. </code> + <p>This assumes that this is the first line of the input text, and + that <c>'$end'</c> is the distinguished <c>end_of_input</c> + symbol.</p> + <p>The Erlang scanner in the <c>io</c> module can be used as a + starting point when writing a new scanner. Study + <c>yeccscan.erl</c> in order to see how a filter can be added on + top of <c>io:scan_erl_form/3</c> to provide a scanner for + Yecc that tokenizes grammar files before parsing them + with the Yecc parser. A more general approach to scanner + implementation is to use a scanner generator. A scanner + generator in Erlang called <c>leex</c> is under development.</p> + </section> + + <section> + <title>Grammar Definition Format</title> + <p>Erlang style <c>comments</c>, starting with a <c>'%'</c>, are + allowed in grammar files.</p> + <p>Each <c>declaration</c> or <c>rule</c> ends with a dot (the + character <c>'.'</c>).</p> + <p>The grammar starts with an optional <c>header</c> section. The + header is put first in the generated file, before the module + declaration. The purpose of the header is to provide a means to + make the documentation generated by <c>EDoc</c> look nicer. Each + header line should be enclosed in double quotes, and newlines + will be inserted between the lines. For example:</p> + <code> +Header "%% Copyright (C)" +"%% @private" +"%% @Author John"</code> + <p>Next comes a declaration of the <c>nonterminal categories</c> + to be used in the rules. For example:</p> + <code type="none"> +Nonterminals sentence nounphrase verbphrase. </code> + <p>A non-terminal category can be used at the left hand side (= + <c>lhs</c>, or <c>head</c>) of a grammar rule. It can also + appear at the right hand side of rules.</p> + <p>Next comes a declaration of the <c>terminal categories</c>, + which are the categories of tokens produced by the scanner. For + example:</p> + <code type="none"> +Terminals article adjective noun verb. </code> + <p>Terminal categories may only appear in the right hand sides (= + <c>rhs</c>) of grammar rules.</p> + <p>Next comes a declaration of the <c>rootsymbol</c>, or start + category of the grammar. For example:</p> + <code type="none"> +Rootsymbol sentence. </code> + <p>This symbol should appear in the lhs of at least one grammar + rule. This is the most general syntactic category which the + parser ultimately will parse every input string into.</p> + <p>After the rootsymbol declaration comes an optional declaration + of the <c>end_of_input</c> symbol that your scanner is expected + to use. For example:</p> + <code type="none"> +Endsymbol '$end'. </code> + <p>Next comes one or more declarations of <c>operator precedences</c>, if needed. These are used to resolve + shift/reduce conflicts (see <c>yacc</c> documentation).</p> + <p>Examples of operator declarations:</p> + <code type="none"> +Right 100 '='. +Nonassoc 200 '==' '=/='. +Left 300 '+'. +Left 400 '*'. +Unary 500 '-'. </code> + <p>These declarations mean that <c>'='</c> is defined as a + <c>right associative binary</c> operator with precedence 100, + <c>'=='</c> and <c>'=/='</c> are operators with <c>no associativity</c>, <c>'+'</c> and <c>'*'</c> are <c>left associative binary</c> operators, where <c>'*'</c> takes + precedence over <c>'+'</c> (the normal case), and <c>'-'</c> is + a <c>unary</c> operator of higher precedence than <c>'*'</c>. + The fact that '==' has no associativity means that an expression + like <c>a == b == c</c> is considered a syntax error.</p> + <p>Certain rules are assigned precedence: each rule gets its + precedence from the last terminal symbol mentioned in the right + hand side of the rule. It is also possible to declare precedence + for non-terminals, "one level up". This is practical when an + operator is overloaded (see also example 3 below).</p> + <p>Next come the <c>grammar rules</c>. Each rule has the general + form</p> + <code type="none"> +Left_hand_side -> Right_hand_side : Associated_code. </code> + <p>The left hand side is a non-terminal category. The right hand + side is a sequence of one or more non-terminal or terminal + symbols with spaces between. The associated code is a sequence + of zero or more Erlang expressions (with commas <c>','</c> as + separators). If the associated code is empty, the separating + colon <c>':'</c> is also omitted. A final dot marks the end of + the rule.</p> + <p>Symbols such as <c>'{'</c>, <c>'.'</c>, etc., have to be + enclosed in single quotes when used as terminal or non-terminal + symbols in grammar rules. The use of the symbols + <c>'$empty'</c>, <c>'$end'</c>, and <c>'$undefined'</c> should + be avoided.</p> + <p>The last part of the grammar file is an optional section with + Erlang code (= function definitions) which is included 'as is' + in the resulting parser file. This section must start with the + pseudo declaration, or key words</p> + <code type="none"> +Erlang code. </code> + <p>No syntax rule definitions or other declarations may follow + this section. To avoid conflicts with internal variables, do not + use variable names beginning with two underscore characters + ('__') in the Erlang code in this section, or in the code + associated with the individual syntax rules.</p> + <p>The optional <c>expect</c> declaration can be placed anywhere + before the last optional section with Erlang code. It is used + for suppressing the warning about conflicts that is ordinarily + given if the grammar is ambiguous. An example:</p> + <code type="none"> +Expect 2. </code> + <p>The warning is given if the number of shift/reduce conflicts + differs from 2, or if there are reduce/reduce conflicts. + </p> + </section> + + <section> + <title>Examples</title> + <p>A grammar to parse list expressions (with empty associated + code):</p> + <code type="none"> +Nonterminals list elements element. +Terminals atom '(' ')'. +Rootsymbol list. +list -> '(' ')'. +list -> '(' elements ')'. +elements -> element. +elements -> element elements. +element -> atom. +element -> list. </code> + <p>This grammar can be used to generate a parser which parses list + expressions, such as <c>(), (a), (peter charles), (a (b c) d (())), ...</c> provided that your scanner tokenizes, for + example, the input <c>(peter charles)</c> as follows:</p> + <code type="none"> +[{'(', 1} , {atom, 1, peter}, {atom, 1, charles}, {')', 1}, + {'$end', 1}] </code> + <p>When a grammar rule is used by the parser to parse (part of) + the input string as a grammatical phrase, the associated code is + evaluated, and the value of the last expression becomes the + value of the parsed phrase. This value may be used by the parser + later to build structures that are values of higher phrases of + which the current phrase is a part. The values initially + associated with terminal category phrases, i.e. input tokens, + are the token tuples themselves.</p> + <p>Below is an example of the grammar above with structure + building code added:</p> + <code type="none"> +list -> '(' ')' : nil. +list -> '(' elements ')' : '$2'. +elements -> element : {cons, '$1', nil}. +elements -> element elements : {cons, '$1', '$2'}. +element -> atom : '$1'. +element -> list : '$1'. </code> + <p>With this code added to the grammar rules, the parser produces + the following value (structure) when parsing the input string + <c>(a b c).</c>. This still assumes that this was the first + input line that the scanner tokenized:</p> + <code type="none"> +{cons, {atom, 1, a,} {cons, {atom, 1, b}, + {cons, {atom, 1, c}, nil}}} </code> + <p>The associated code contains <c>pseudo variables</c><c>'$1'</c>, <c>'$2'</c>, <c>'$3'</c>, etc. which refer to (are + bound to) the values associated previously by the parser with + the symbols of the right hand side of the rule. When these + symbols are terminal categories, the values are token tuples of + the input string (see above).</p> + <p>The associated code may not only be used to build structures + associated with phrases, but may also be used for syntactic and + semantic tests, printout actions (for example for tracing), etc. + during the parsing process. Since tokens contain positional + (line number) information, it is possible to produce error + messages which contain line numbers. If there is no associated + code after the right hand side of the rule, the value + <c>'$undefined'</c> is associated with the phrase.</p> + <p>The right hand side of a grammar rule may be empty. This is + indicated by using the special symbol <c>'$empty'</c> as rhs. + Then the list grammar above may be simplified to:</p> + <code type="none"> +list -> '(' elements ')' : '$2'. +elements -> element elements : {cons, '$1', '$2'}. +elements -> '$empty' : nil. +element -> atom : '$1'. +element -> list : '$1'. </code> + </section> + + <section> + <title>Generating a Parser</title> + <p>To call the parser generator, use the following command:</p> + <code type="none"> +yecc:file(Grammarfile). </code> + <p>An error message from Yecc will be shown if the grammar + is not of the LALR type (for example too ambiguous). + Shift/reduce conflicts are resolved in favor of shifting if + there are no operator precedence declarations. Refer to the + <c>yacc</c> documentation on the use of operator precedence.</p> + <p>The output file contains Erlang source code for a parser module + with module name equal to the <c>Parserfile</c> parameter. After + compilation, the parser can be called as follows (the module + name is assumed to be <c>myparser</c>):</p> + <code type="none"> +myparser:parse(myscanner:scan(Inport)) </code> + <p>The call format may be different if a customized prologue file + has been included when generating the parser instead of the + default file <c>lib/parsetools/include/yeccpre.hrl</c>.</p> + <p>With the standard prologue, this call will return either + <c>{ok, Result}</c>, where <c>Result</c> is a structure that the + Erlang code of the grammar file has built, or <c>{error, {Line_number, Module, Message}}</c> if there was a syntax error + in the input.</p> + <p><c>Message</c> is something which may be converted into a + string by calling <c>Module:format_error(Message)</c> + and printed with <c>io:format/3</c>.</p> + <note> + <p>By default, the parser that was generated will not print out + error messages to the screen. The user will have to do this + either by printing the returned error messages, or by + inserting tests and print instructions in the Erlang code + associated with the syntax rules of the grammar file.</p> + </note> + <p>It is also possible to make the parser ask for more input + tokens when needed if the following call format is used:</p> + <code type="none"> +myparser:parse_and_scan({Function, Args}) +myparser:parse_and_scan({Mod, Tokenizer, Args}) </code> + <p>The tokenizer <c>Function</c> is either a fun or a tuple + <c>{Mod, Tokenizer}</c>. The call <c>apply(Function, Args)</c> + or <c>apply({Mod, Tokenizer}, Args)</c> is executed whenever a + new token is needed. This, for example, makes it possible to + parse from a file, token by token.</p> + <p>The tokenizer used above has to be implemented so as to return + one of the following:</p> + <code type="none"> +{ok, Tokens, Endline} +{eof, Endline} +{error, Error_description, Endline} </code> + <p>This conforms to the format used by the scanner in the Erlang + <c>io</c> library module.</p> + <p>If <c>{eof, Endline}</c> is returned immediately, the call to + <c>parse_and_scan/1</c> returns <c>{ok, eof}</c>. If <c>{eof, Endline}</c> is returned before the parser expects end of input, + <c>parse_and_scan/1</c> will, of course, return an error message + (see above). Otherwise <c>{ok, Result}</c> is returned.</p> + </section> + + <section> + <title>More Examples</title> + <p>1. A grammar for parsing infix arithmetic expressions into + prefix notation, without operator precedence:</p> + <code type="none"> +Nonterminals E T F. +Terminals '+' '*' '(' ')' number. +Rootsymbol E. +E -> E '+' T: ['$1', '$2', '$3']. +E -> T : '$1'. +T -> T '*' F: ['$1', '$2', '$3']. +T -> F : '$1'. +F -> '(' E ')' : '$2'. +F -> number : '$1'. </code> + <p>2. The same with operator precedence becomes simpler:</p> + <code type="none"> +Nonterminals E. +Terminals '+' '*' '(' ')' number. +Rootsymbol E. +Left 100 '+'. +Left 200 '*'. +E -> E '+' E : ['$1', '$2', '$3']. +E -> E '*' E : ['$1', '$2', '$3']. +E -> '(' E ')' : '$2'. +E -> number : '$1'. </code> + <p>3. An overloaded minus operator:</p> + <code type="none"> +Nonterminals E uminus. +Terminals '*' '-' number. +Rootsymbol E. + +Left 100 '-'. +Left 200 '*'. +Unary 300 uminus. + +E -> E '-' E. +E -> E '*' E. +E -> uminus. +E -> number. + +uminus -> '-' E. </code> + <p>4. The Yecc grammar that is used for parsing grammar + files, including itself:</p> + <code type="none"> +Nonterminals +grammar declaration rule head symbol symbols attached_code +token tokens. +Terminals +atom float integer reserved_symbol reserved_word string char var +'->' ':' dot. +Rootsymbol grammar. +Endsymbol '$end'. +grammar -> declaration : '$1'. +grammar -> rule : '$1'. +declaration -> symbol symbols dot: {'$1', '$2'}. +rule -> head '->' symbols attached_code dot: {rule, ['$1' | '$3'], + '$4'}. +head -> symbol : '$1'. +symbols -> symbol : ['$1']. +symbols -> symbol symbols : ['$1' | '$2']. +attached_code -> ':' tokens : {erlang_code, '$2'}. +attached_code -> '$empty' : {erlang_code, + [{atom, 0, '$undefined'}]}. +tokens -> token : ['$1']. +tokens -> token tokens : ['$1' | '$2']. +symbol -> var : value_of('$1'). +symbol -> atom : value_of('$1'). +symbol -> integer : value_of('$1'). +symbol -> reserved_word : value_of('$1'). +token -> var : '$1'. +token -> atom : '$1'. +token -> float : '$1'. +token -> integer : '$1'. +token -> string : '$1'. +token -> char : '$1'. +token -> reserved_symbol : {value_of('$1'), line_of('$1')}. +token -> reserved_word : {value_of('$1'), line_of('$1')}. +token -> '->' : {'->', line_of('$1')}. +token -> ':' : {':', line_of('$1')}. +Erlang code. +value_of(Token) -> + element(3, Token). +line_of(Token) -> + element(2, Token). </code> + <note> + <p>The symbols <c>'->'</c>, and <c>':'</c> have to be treated in + a special way, as they are meta symbols of the grammar + notation, as well as terminal symbols of the Yecc + grammar.</p> + </note> + <p>5. The file <c>erl_parse.yrl</c> in the <c>lib/stdlib/src</c> + directory contains the grammar for Erlang.</p> + <note> + <p>Syntactic tests are used in the code associated with some + rules, and an error is thrown (and caught by the generated + parser to produce an error message) when a test fails. The + same effect can be achieved with a call to + <c>return_error(Error_line, Message_string)</c>, which is + defined in the <c>yeccpre.hrl</c> default header file.</p> + </note> + </section> + + <section> + <title>Files</title> + <code type="none"> +lib/parsetools/include/yeccpre.hrl </code> + </section> + + <section> + <title>See Also</title> + <p>Aho & Johnson: 'LR Parsing', ACM Computing Surveys, vol. 6:2, 1974.</p> + </section> +</erlref> + |