--- /dev/null
+! Copyright (C) 2009 Chris Double.
+! See http://factorcode.org/license.txt for BSD license.
+USING: help.syntax help.markup peg peg.search ;
+IN: peg.ebnf
+
+HELP: <EBNF
+{ $syntax "<EBNF ...ebnf... EBNF>" }
+{ $values { "...ebnf..." "EBNF DSL text" } }
+{ $description
+ "Creates a " { $vocab-link "peg" }
+ " object that parses a string using the syntax "
+ "defined with the EBNF DSL. The peg object can be run using the " { $link parse }
+ "word and can be used with the " { $link search } " and " { $link replace } " words."
+}
+{ $examples
+ { $unchecked-example
+ "USING: prettyprint peg.ebnf peg.search ;"
+ "\"abcdab\" <EBNF rule=\"a\" \"b\" => [[ drop \"foo\" ]] EBNF> replace ."
+ "\"foocdfoo\""
+ }
+} ;
+
+HELP: [EBNF
+{ $syntax "[EBNF ...ebnf... EBNF]" }
+{ $values { "...ebnf..." "EBNF DSL text" } }
+{ $description
+ "Creates and calls a quotation that parses a string using the syntax "
+ "defined with the EBNF DSL. The quotation has stack effect "
+ { $snippet "( string -- ast )" } " where 'string' is the text to be parsed "
+ "and 'ast' is the resulting abstract syntax tree. If the parsing fails the "
+ "quotation throws an exception."
+}
+{ $examples
+ { $unchecked-example
+ "USING: prettyprint peg.ebnf ;"
+ "\"ab\" [EBNF rule=\"a\" \"b\" EBNF] ."
+ "V{ \"a\" \"b\" }"
+ }
+} ;
+
+HELP: EBNF:
+{ $syntax "EBNF: word ...ebnf... ;EBNF" }
+{ $values { "word" "a word" } { "...ebnf..." "EBNF DSL text" } }
+{ $description
+ "Defines a word that when called will parse a string using the syntax "
+ "defined with the EBNF DSL. The word has stack effect "
+ { $snippet "( string -- ast )" } " where 'string' is the text to be parsed "
+ "and 'ast' is the resulting abstract syntax tree. If the parsing fails the "
+ "word throws an exception."
+}
+{ $examples
+ { $unchecked-example
+ "USING: prettyprint peg.ebnf ;"
+ "EBNF: foo rule=\"a\" \"b\" ;EBNF"
+ "\"ab\" foo ."
+ "V{ \"a\" \"b\" }"
+ }
+} ;
+
+ARTICLE: "peg.ebnf.strings" "Strings"
+"A string in a rule will match that sequence of characters from the input string. "
+"The AST result from the match is the string itself."
+{ $examples
+ { $unchecked-example
+ "USING: prettyprint peg.ebnf ;"
+ "\"helloworld\" [EBNF rule=\"hello\" \"world\" EBNF] ."
+ "V{ \"hello\" \"world\" }"
+ }
+} ;
+
+ARTICLE: "peg.ebnf.any" "Any"
+"A full stop character (.) will match any single token in the input string. "
+"The AST resulting from this is the token itself."
+{ $examples
+ { $unchecked-example
+ "USING: prettyprint peg.ebnf ;"
+ "\"abc\" [EBNF rule=\"a\" . \"c\" EBNF] ."
+ "V{ \"a\" 98 \"c\" }"
+ }
+} ;
+
+ARTICLE: "peg.ebnf.sequence" "Sequence"
+"Any white space separated rule element is considered a sequence. Each rule "
+"in the sequence is matched from the input stream, consuming the input as it "
+"goes. The AST result is a vector containing the results of each rule element in "
+"the sequence."
+{ $examples
+ { $unchecked-example
+ "USING: prettyprint peg.ebnf ;"
+ "\"helloworld\" [EBNF rule=\"a\" (\"b\")* \"a\" EBNF] ."
+ "V{ \"a\" V{ \"b\" \"b\" \"b\" } \"a\" }"
+ }
+}
+;
+
+ARTICLE: "peg.ebnf.choice" "Choice"
+"Any rule element separated by a pipe character (|) is considered a choice. Choices "
+"are matched against the input stream in order. If a match succeeds then the remaining "
+"choices are discarded and the result of the match is the AST result of the choice."
+{ $examples
+ { $unchecked-example
+ "USING: prettyprint peg.ebnf ;"
+ "\"a\" [EBNF rule=\"a\" | \"b\" | \"c\" EBNF] ."
+ "\"a\""
+ "\"b\" [EBNF rule=\"a\" | \"b\" | \"c\" EBNF] ."
+ "\"b\""
+ "\"d\" [EBNF rule=\"a\" | \"b\" | \"c\" EBNF] ."
+ "Peg parsing error at character position 0. Expected token 'c' or token 'b' or token 'a'"
+ }
+}
+;
+
+ARTICLE: "peg.ebnf.option" "Option"
+"Any rule element followed by a question mark (?) is considered optional. The "
+"rule is tested against the input. If it succeeds the result is stored in the AST. "
+"If it fails then the parse still suceeds and false (f) is stored in the AST."
+{ $examples
+ { $unchecked-example
+ "USING: prettyprint peg.ebnf ;"
+ "\"abc\" [EBNF rule=\"a\" \"b\"? \"c\" EBNF] ."
+ "V{ \"a\" \"b\" \"c\" }"
+ "\"ac\" [EBNF rule=\"a\" \"b\"? \"c\" EBNF] ."
+ "V{ \"a\" f \"c\" }"
+ }
+}
+;
+
+ARTICLE: "peg.ebnf.character-class" "Character Class"
+"Character class matching can be done using a range of characters defined in "
+"square brackets. Multiple ranges can be included in a single character class "
+"definition. The syntax for the range is a start character, followed by a minus "
+"(-) followed by an end character. For example " { $snippet "[a-zA-Z]" } ". "
+"The AST resulting from the match is an integer of the character code for the "
+"character that matched."
+{ $examples
+ { $unchecked-example
+ "USING: prettyprint peg.ebnf ;"
+ "\"123\" [EBNF rule=[0-9]+ EBNF] ."
+ "V{ 49 50 51 }"
+ }
+}
+;
+
+ARTICLE: "peg.ebnf.one-or-more" "One or more"
+"Any rule element followed by a plus (+) matches one or more instances of the rule "
+"from the input string. The AST result is the vector of the AST results from "
+"the matched rule."
+{ $examples
+ { $unchecked-example
+ "USING: prettyprint peg.ebnf ;"
+ "\"aab\" [EBNF rule=\"a\"+ \"b\" EBNF] ."
+ "V{ V{ \"a\" \"a\" } \"b\" }"
+ }
+}
+;
+
+ARTICLE: "peg.ebnf.zero-or-more" "Zero or more"
+"Any rule element followed by an asterisk (*) matches zero or more instances of the rule "
+"from the input string. The AST result is the vector of the AST results from "
+"the matched rule. This will be empty if there are no matches."
+{ $examples
+ { $unchecked-example
+ "USING: prettyprint peg.ebnf ;"
+ "\"aab\" [EBNF rule=\"a\"* \"b\" EBNF] ."
+ "V{ V{ \"a\" \"a\" } \"b\" }"
+ "\"b\" [EBNF rule=\"a\"* \"b\" EBNF] ."
+ "V{ V{ } \"b\" }"
+ }
+}
+;
+
+ARTICLE: "peg.ebnf.and" "And"
+"Any rule element prefixed by an ampersand (&) performs the Parsing Expression "
+"Grammar 'And Predicate' match. It attempts to match the rule against the input "
+"string. It will cause the parse to succeed or fail depending on if the rule "
+"succeeds or fails. It will not consume anything from the input string however and "
+"does not leave any result in the AST. This can be used for lookahead and "
+"disambiguation in choices."
+{ $examples
+ { $unchecked-example
+ "USING: prettyprint peg.ebnf ;"
+ "\"ab\" [EBNF rule=&(\"a\") \"a\" \"b\" EBNF] ."
+ "V{ \"a\" \"b\" }"
+ }
+}
+;
+
+ARTICLE: "peg.ebnf.not" "Not"
+"Any rule element prefixed by an exclamation mark (!) performs the Parsing Expression "
+"Grammar 'Not Predicate' match. It attempts to match the rule against the input "
+"string. It will cause the parse to succeed if the rule match fails, and to fail "
+"if the rule match succeeds. It will not consume anything from the input string "
+"however and does not leave any result in the AST. This can be used for lookahead and "
+"disambiguation in choices."
+{ $examples
+ { $unchecked-example
+ "USING: prettyprint peg.ebnf ;"
+ "\"<abcd>\" [EBNF rule=\"<\" (!(\">\") .)* \">\" EBNF] ."
+ "V{ \"<\" V{ 97 98 99 100 } \">\" }"
+ }
+}
+;
+
+ARTICLE: "peg.ebnf.action" "Action"
+"An action is a quotation that is run after a rule matches. The quotation "
+"consumes the AST of the rule match and leaves a new AST as the result. "
+"The stack effect of the action can be " { $snippet "( ast -- ast )" } " or "
+{ $snippet "( -- ast )" } ". "
+"If it is the latter then the original AST is implcitly dropped and will be "
+"replaced by the AST left on the stack. This is mostly useful if variables are "
+"used in the rule since they can be referenced like locals in the action quotation. "
+"The action is defined by having a ' => ' at the end of a rule and "
+"using '[[' and ']]' to open and close the quotation. "
+"If an action leaves the object 'ignore' on the stack then the result of that "
+"action will not be put in the AST of the result."
+{ $examples
+ { $unchecked-example
+ "USING: prettyprint peg.ebnf math.parser ;"
+ "\"<abcd>\" [EBNF rule=\"<\" ((!(\">\") .)* => [[ >string ]]) \">\" EBNF] ."
+ "V{ \"<\" \"abcd\" \">\" }"
+ "\"123\" [EBNF rule=[0-9]+ => [[ string>number ]] EBNF] ."
+ "123"
+ }
+}
+;
+
+ARTICLE: "peg.ebnf.semantic-action" "Semantic Action"
+"Semantic actions allow providing a quotation that gets run on the AST of a "
+"matched rule that returns success or failure. The result of the parse is decided by "
+"the result of the semantic action. The stack effect for the quotation is "
+{ $snippet ( ast -- ? ) } ". "
+"A semantic action follows the rule it applies to and is delimeted by '?[' and ']?'."
+{ $examples
+ { $unchecked-example
+ "USING: prettyprint peg.ebnf ;"
+ "\"1\" [EBNF rule=[0-9] ?[ digit> odd? ]? EBNF] ."
+ "49"
+ "\"2\" [EBNF rule=[0-9] ?[ digit> odd? ]? EBNF] ."
+ "..error.."
+ }
+}
+;
+
+ARTICLE: "peg.ebnf.variable" "Variable"
+"Variables names can be suffixed to a rule element using the colon character (:) "
+"followed by the variable name. These can then be used in rule actions to refer to "
+"the AST result of the rule element with that variable name."
+{ $examples
+ { $unchecked-example
+ "USING: prettyprint peg.ebnf ;"
+ "\"1+2\" [EBNF rule=[0-9]:a \"+\" [0-9]:b => [[ a digit> b digit> + ]] EBNF] ."
+ "3"
+ }
+}
+;
+
+ARTICLE: "peg.ebnf.foreign-rules" "Foreign Rules"
+"Rules can call outto other peg.ebnf defined parsers. The result of "
+"the foreign call then becomes the AST of the successful parse. Foreign rules "
+"are invoked using '<foreign word-name>' or '<foreign word-name rule>'. The "
+"latter allows calling a specific rule in a previously designed peg.ebnf parser. "
+"If the 'word-name' is not the name of a peg.ebnf defined parser then it must be "
+"a word with stack effect " { $snippet "( -- parser )" } ". It must return a "
+{ $vocab-link "peg" } " defined parser and it will be called to perform the parse "
+"for that rule."
+{ $examples
+ { $unchecked-example
+ "USING: prettyprint peg.ebnf ;"
+ "EBNF: parse-string"
+ "StringBody = (!('\"') .)*"
+ "String= '\"' StringBody:b '\"' => [[ b >string ]]"
+ ";EBNF"
+ "EBNF: parse-two-strings"
+ "TwoStrings = <foreign parse-string String> <foreign parse-string String>"
+ ";EBNF"
+ "EBNF: parse-two-strings"
+ "TwoString = <foreign parse-string> <foreign parse-string>"
+ ";EBNF"
+ }
+ { $unchecked-example
+ ": a-token ( -- parser ) \"a\" token ;"
+ "EBNF: parse-abc"
+ "abc = <foreign a-token> 'b' 'c'"
+ ";EBNF"
+ }
+}
+;
+
+ARTICLE: "peg.ebnf.tokenizers" "Tokenizers"
+"It is possible to override the tokenizer in an EBNF defined parser. "
+"Usually the input sequence to be parsed is an array of characters or a string. "
+"Terminals in a rule match successive characters in the array or string. "
+{ $examples
+ { $unchecked-example
+ "EBNF: foo"
+ "rule = \"++\" \"--\""
+ ";EBNF"
+ }
+}
+"This parser when run with the string \"++--\" or the array "
+"{ CHAR: + CHAR: + CHAR: - CHAR: - } will succeed with an AST of { \"++\" \"--\" }. "
+"If you want to add whitespace handling to the grammar you need to put it "
+"between the terminals: "
+{ $examples
+ { $unchecked-example
+ "EBNF: foo"
+ "space = (\" \" | \"\\r\" | \"\\t\" | \"\\n\")"
+ "spaces = space* => [[ drop ignore ]]"
+ "rule = spaces \"++\" spaces \"--\" spaces"
+ ";EBNF"
+ }
+}
+"In a large grammar this gets tedious and makes the grammar hard to read. "
+"Instead you can write a rule to split the input sequence into tokens, and "
+"have the grammar operate on these tokens. This is how the previous example "
+"might look: "
+{ $examples
+ { $unchecked-example
+ "EBNF: foo"
+ "space = (\" \" | \"\\r\" | \"\\t\" | \"\\n\")"
+ "spaces = space* => [[ drop ignore ]]"
+ "tokenizer = spaces ( \"++\" | \"--\" )"
+ "rule = \"++\" \"--\""
+ ";EBNF"
+ }
+}
+"'tokenizer' is the name of a built in rule. Once defined it is called to "
+"retrieve the next complete token from the input sequence. So the first part "
+"of 'rule' is to try and match \"++\". It calls the tokenizer to get the next "
+"complete token. This ignores spaces until it finds a \"++\" or \"--\". "
+"It is as if the input sequence for the parser was actually { \"++\" \"--\" } "
+"instead of the string \"++--\". With the new tokenizer \"....\" sequences "
+"in the grammar are matched for equality against the token, rather than a "
+"string comparison against successive items in the sequence. This can be used "
+"to match an AST from a tokenizer: "
+{ $examples
+ { $unchecked-example
+ "TUPLE: ast-number value ;"
+ "TUPLE: ast-string value ;"
+ ""
+ "EBNF: foo-tokenizer"
+ "space = (\" \" | \"\\r\" | \"\\t\" | \"\\n\")"
+ "spaces = space* => [[ drop ignore ]]"
+ ""
+ "number = [0-9]* => [[ >string string>number ast-number boa ]]"
+ "string = => [[ ast-string boa ]]"
+ "operator = (\"+\" | \"-\")"
+ ""
+ "token = spaces ( number | string | operator )"
+ "tokens = token*"
+ ";EBNF"
+ ""
+ "ENBF: foo"
+ "tokenizer = <foreign foo-tokenizer token>"
+ ""
+ "number = . ?[ ast-number? ]? => [[ value>> ]]"
+ "string = . ?[ ast-string? ]? => [[ value>> ]]"
+ ""
+ "rule = string:a number:b \"+\" number:c => [[ a b c + 2array ]]"
+ ";EBNF"
+ }
+}
+"In this example I split the tokenizer into a separate parser and use "
+"'foreign' to call it from the main one. This allows testing of the "
+"tokenizer separately: "
+{ $examples
+ { $unchecked-example
+ "\"123 456 +\" foo-tokenizer ast>> ."
+ "{ T{ ast-number f 123 } T{ ast-number f 456 } \"+\" }"
+ }
+}
+"The '.' EBNF production means match a single object in the source sequence. "
+"Usually this is a character. With the replacement tokenizer it is either a "
+"number object, a string object or a string containing the operator. "
+"Using a tokenizer in language grammars makes it easier to deal with whitespace. "
+"Defining tokenizers in this way has the advantage of the tokenizer and parser "
+"working in one pass. There is no tokenization occurring over the whole string "
+"followed by the parse of that result. It tokenizes as it needs to. You can even "
+"switch tokenizers multiple times during a grammar. Rules use the tokenizer that "
+"was defined lexically before the rule. This is usefull in the JavaScript grammar: "
+{ $examples
+ { $unchecked-example
+ "EBNF: javascript"
+ "tokenizer = default"
+ "nl = \"\\r\" \"\\n\" | \"\\n\""
+ "tokenizer = <foreign tokenize-javascript Tok>"
+ "..."
+ "End = !(.)"
+ "Name = . ?[ ast-name? ]? => [[ value>> ]] "
+ "Number = . ?[ ast-number? ]? => [[ value>> ]]"
+ "String = . ?[ ast-string? ]? => [[ value>> ]]"
+ "RegExp = . ?[ ast-regexp? ]? => [[ value>> ]]"
+ "SpacesNoNl = (!(nl) Space)* => [[ ignore ]]"
+ "Sc = SpacesNoNl (nl | &(\"}\") | End)| \";\""
+ }
+}
+"Here the rule 'nl' is defined using the default tokenizer of sequential "
+"characters ('default' has the special meaning of the built in tokenizer). "
+"This is followed by using the JavaScript tokenizer for the remaining rules. "
+"This tokenizer strips out whitespace and newlines. Some rules in the grammar "
+"require checking for a newline. In particular the automatic semicolon insertion "
+"rule (managed by the 'Sc' rule here). If there is a newline, the semicolon can "
+"be optional in places. "
+{ $examples
+ { $unchecked-example
+ "\"do\" Stmt:s \"while\" \"(\" Expr:c \")\" Sc => [[ s c ast-do-while boa ]]"
+ }
+}
+"Even though the JavaScript tokenizer has removed the newlines, the 'nl' rule can "
+"be used to detect them since it is using the default tokenizer. This allows "
+"grammars to mix and match the tokenizer as required to make them more readable."
+;
+
+ARTICLE: "peg.ebnf" "EBNF"
+"This vocubalary provides a DSL that allows writing PEG parsers that look like "
+"EBNF syntax. It provides three parsing words described below. These words all "
+"accept the same EBNF syntax. The difference is in how they are used."
+{ $subsection POSTPONE: <EBNF }
+{ $subsection POSTPONE: [EBNF }
+{ $subsection POSTPONE: EBNF: }
+"The EBNF syntax is composed of a series of rules of the form: "
+{ $code
+ "rule1 = ..."
+ "rule2 = ..."
+}
+"The last defined rule is the main rule for the EBNF. It is the first one run "
+"and it is expected that the remaining rules are used by that rule. Rules may be "
+"left recursive."
+"Each rule can contain the following: "
+{ $subsection "peg.ebnf.strings" }
+{ $subsection "peg.ebnf.any" }
+{ $subsection "peg.ebnf.sequence" }
+{ $subsection "peg.ebnf.choice" }
+{ $subsection "peg.ebnf.option" }
+{ $subsection "peg.ebnf.one-or-more" }
+{ $subsection "peg.ebnf.zero-or-more" }
+{ $subsection "peg.ebnf.and" }
+{ $subsection "peg.ebnf.not" }
+{ $subsection "peg.ebnf.character-class" }
+{ $subsection "peg.ebnf.foreign-rules" }
+{ $subsection "peg.ebnf.action" }
+{ $subsection "peg.ebnf.semantic-action" }
+{ $subsection "peg.ebnf.variable" }
+"Grammars defined in EBNF need to handle each character, or sequence of "
+"characters in the input. This can be tedious for dealing with whitespace in "
+"grammars that have 'tokens' separated by whitespace. You can define your "
+"own tokenizer that for an EBNF grammar, and write the grammar in terms of "
+"those tokens, allowing you to ignore the whitespace issue. The tokenizer "
+"can be changed at various parts in the grammar as needed. The JavaScript grammar "
+"does this to define the optional semicolon rule for example."
+{ $subsection "peg.ebnf.tokenizers" }
+;
+
+ABOUT: "peg.ebnf"
\ No newline at end of file
projects / factor.git / commitdiff