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! 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
{ $example
"USING: kernel 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
{ $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
{ $example
"USING: prettyprint peg.ebnf ;"
"IN: scratchpad"
"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
{ $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
{ $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
{ $example
"USING: prettyprint peg.ebnf ;"
"\"abbba\" [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
{ $example
"USING: prettyprint peg.ebnf ;"
"\"a\" [EBNF rule=\"a\" | \"b\" | \"c\" EBNF] ."
"\"a\""
}
{ $example
"USING: prettyprint peg.ebnf ;"
"\"b\" [EBNF rule=\"a\" | \"b\" | \"c\" EBNF] ."
"\"b\""
}
{ $example
"USING: prettyprint peg.ebnf ;"
"\"d\" [EBNF rule=\"a\" | \"b\" | \"c\" EBNF] ."
"Peg parsing error at character position 0.\nExpected 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
{ $example
"USING: prettyprint peg.ebnf ;"
"\"abc\" [EBNF rule=\"a\" \"b\"? \"c\" EBNF] ."
"V{ \"a\" \"b\" \"c\" }"
}
{ $example
"USING: prettyprint peg.ebnf ;"
"\"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
{ $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
{ $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
{ $example
"USING: prettyprint peg.ebnf ;"
"\"aab\" [EBNF rule=\"a\"* \"b\" EBNF] ."
"V{ V{ \"a\" \"a\" } \"b\" }"
}
{ $example
"USING: prettyprint peg.ebnf ;"
"\"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
{ $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
{ $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
{ $example
"USING: prettyprint peg.ebnf strings ;"
"\"<abcd>\" [EBNF rule=\"<\" ((!(\">\") .)* => [[ >string ]]) \">\" EBNF] ."
"V{ \"<\" \"abcd\" \">\" }"
}
{ $example
"USING: prettyprint peg.ebnf math.parser ;"
"\"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
{ $example
"USING: prettyprint peg.ebnf math math.parser ;"
"\"1\" [EBNF rule=[0-9] ?[ digit> odd? ]? EBNF] ."
"49"
}
{ $example
"USING: prettyprint peg.ebnf math math.parser ;"
"\"2\" [EBNF rule=[0-9] ?[ digit> odd? ]? EBNF] ."
"Sequence index out of bounds\nindex 0\nseq V{ }"
}
}
;
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
{ $example
"USING: prettyprint peg.ebnf math.parser ;"
"\"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
{ $code
"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"
}
{ $code
": 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
{ $code
"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
{ $code
"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
{ $code
"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. "
$nl
"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
{ $example
"USING: prettyprint peg peg.ebnf kernel math.parser strings"
"accessors math arrays ;"
"IN: scratchpad"
""
"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 ]]"
"operator = (\"+\" | \"-\")"
""
"token = spaces ( number | operator )"
"tokens = token*"
";EBNF"
""
"EBNF: 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"
""
"\"123 456 +\" foo-tokenizer ."
"V{\n T{ ast-number { value 123 } }\n T{ ast-number { value 456 } }\n \"+\"\n}"
}
}
"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
{ $code
"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
{ $code
"\"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"
"The " { $vocab-link "peg.ebnf" } " vocabulary 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. "
{ $subsections
POSTPONE: <EBNF
POSTPONE: [EBNF
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: "
{ $subsections "peg.ebnf.strings"
"peg.ebnf.any"
"peg.ebnf.sequence"
"peg.ebnf.choice"
"peg.ebnf.option"
"peg.ebnf.one-or-more"
"peg.ebnf.zero-or-more"
"peg.ebnf.and"
"peg.ebnf.not"
"peg.ebnf.character-class"
"peg.ebnf.foreign-rules"
"peg.ebnf.action"
"peg.ebnf.semantic-action"
"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."
{ $subsections "peg.ebnf.tokenizers" }
;
ABOUT: "peg.ebnf"
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