pt::peg::to::peg(3tcl)
💖 NAME 💖
okay bestie, meet pt::peg::to::peg — it's giving PEG Conversion. Write PEG format. no cap, that's literally the whole vibe. ✨
✨ SYNOPSIS ✨
here's the summoning circle, fr fr:
package require Tcl 8.5 9
package require pt::peg::to::peg ?1.0.3?
package require pt::peg
package require pt::pe
package require text::write
pt::peg::to::peg reset
pt::peg::to::peg configure
pt::peg::to::peg configure option
pt::peg::to::peg configure option value...
pt::peg::to::peg convert serial🦋 DESCRIPTION 🦋
lowkey lost? 😭 if this doc is not making sense, pls go read the overview in the Introduction to Parser Tools. that doc is the entrypoint to the whole system this package is a part of, iykyk.
so this package? it implements the converter from parsing expression grammars to PEG markup. slay.
it lives in the Export section of the Core Layer of Parser Tools. you can use it either directly with the other packages of this layer, or indirectly through the export manager provided by pt::peg::export. the indirect route is intended for use in untrusted environments, and it happens through the corresponding export plugin pt::peg::export::peg sitting between converter and export manager. 🫶
IMAGE: arch_core_eplugins
💅 API 💅
the API this package provides satisfies the spec of the Converter API found in the Parser Tools Export API specification. periodt.
pt::peg::to::peg reset— resets the configuration of the package to its default settings. clean slate energy. ✨pt::peg::to::peg configure— returns a dictionary containing the current configuration of the package. it's giving receipts. 🧾pt::peg::to::peg configure option— returns the current value of the specified configuration option of the package. for the set of legal options, read the Options section below.pt::peg::to::peg configure option value...— sets the given configuration options of the package to the specified values. for the set of legal options, read the Options section below.pt::peg::to::peg convert serial— takes the canonical serialization of a parsing expression grammar, as specified in section PEG serialization format, and contained inserial, and generates PEG markup encoding the grammar, per the current package configuration. the created string is then returned as the result of the command. main character result, honestly. 👑
💖 OPTIONS 💖
the converter to the PEG language recognizes these options and changes its behaviour as they specify. no cap.
-file string— the value is the name of the file or other entity from which the grammar came, for which the command is run. default value isunknown.-name string— the value is the name of the grammar we are processing. default value isa_pe_grammar.-user string— the value is the name of the user for which the command is run. default value isunknown.-template string— the value is a string into which to put the generated text and the values of the other options. the various locations for user-data are expected to be specified with the placeholders listed below. default value is"@code@".@user@— to be replaced with the value of the option-user.@format@— to be replaced with the constantPEG.@file@— to be replaced with the value of the option-file.@name@— to be replaced with the value of the option-name.@code@— to be replaced with the generated text.
🌸 PEG SPECIFICATION LANGUAGE 🌸
peg is a language for the specification of parsing expression grammars. it's meant to be human readable and writable too, but still strict enough that a machine can process it. like any computer language, bestie. it was defined to make writing the specification of a grammar easy — something the other formats found in Parser Tools do not lend themselves too. 💀
it is formally specified by the grammar shown below, written in itself. for a tutorial / introduction to the language, pls go read the PEG Language Tutorial.
PEG pe_grammar_for_peg (Grammar)
# --------------------------------------------------------------------
# Syntactical constructs
Grammar <- WHITESPACE Header Definition* Final EOF ;
Header <- PEG Identifier StartExpr ;
Definition <- Attribute? Identifier IS Expression SEMICOLON ;
Attribute <- (VOID / LEAF) COLON ;
Expression <- Sequence (SLASH Sequence)* ;
Sequence <- Prefix+ ;
Prefix <- (AND / NOT)? Suffix ;
Suffix <- Primary (QUESTION / STAR / PLUS)? ;
Primary <- ALNUM / ALPHA / ASCII / CONTROL / DDIGIT / DIGIT
/ GRAPH / LOWER / PRINTABLE / PUNCT / SPACE / UPPER
/ WORDCHAR / XDIGIT
/ Identifier
/ OPEN Expression CLOSE
/ Literal
/ Class
/ DOT
;
Literal <- APOSTROPH (!APOSTROPH Char)* APOSTROPH WHITESPACE
/ DAPOSTROPH (!DAPOSTROPH Char)* DAPOSTROPH WHITESPACE ;
Class <- OPENB (!CLOSEB Range)* CLOSEB WHITESPACE ;
Range <- Char TO Char / Char ;
StartExpr <- OPEN Expression CLOSE ;
void: Final <- "END" WHITESPACE SEMICOLON WHITESPACE ;
# --------------------------------------------------------------------
# Lexing constructs
Identifier <- Ident WHITESPACE ;
leaf: Ident <- ([_:] / <alpha>) ([_:] / <alnum>)* ;
Char <- CharSpecial / CharOctalFull / CharOctalPart
/ CharUnicode / CharUnescaped
;
leaf: CharSpecial <- "\\" [nrt'"\[\]\\] ;
leaf: CharOctalFull <- "\\" [0-3][0-7][0-7] ;
leaf: CharOctalPart <- "\\" [0-7][0-7]? ;
leaf: CharUnicode <- "\\" 'u' HexDigit (HexDigit (HexDigit HexDigit?)?)? ;
leaf: CharUnescaped <- !"\\" . ;
void: HexDigit <- [0-9a-fA-F] ;
void: TO <- '-' ;
void: OPENB <- "[" ;
void: CLOSEB <- "]" ;
void: APOSTROPH <- "'" ;
void: DAPOSTROPH <- '"' ;
void: PEG <- "PEG" !([_:] / <alnum>) WHITESPACE ;
void: IS <- "<-" WHITESPACE ;
leaf: VOID <- "void" WHITESPACE ; # Implies that definition has no semantic value.
leaf: LEAF <- "leaf" WHITESPACE ; # Implies that definition has no terminals.
void: SEMICOLON <- ";" WHITESPACE ;
void: COLON <- ":" WHITESPACE ;
void: SLASH <- "/" WHITESPACE ;
leaf: AND <- "&" WHITESPACE ;
leaf: NOT <- "!" WHITESPACE ;
leaf: QUESTION <- "?" WHITESPACE ;
leaf: STAR <- "*" WHITESPACE ;
leaf: PLUS <- "+" WHITESPACE ;
void: OPEN <- "(" WHITESPACE ;
void: CLOSE <- ")" WHITESPACE ;
leaf: DOT <- "." WHITESPACE ;
leaf: ALNUM <- "<alnum>" WHITESPACE ;
leaf: ALPHA <- "<alpha>" WHITESPACE ;
leaf: ASCII <- "<ascii>" WHITESPACE ;
leaf: CONTROL <- "<control>" WHITESPACE ;
leaf: DDIGIT <- "<ddigit>" WHITESPACE ;
leaf: DIGIT <- "<digit>" WHITESPACE ;
leaf: GRAPH <- "<graph>" WHITESPACE ;
leaf: LOWER <- "<lower>" WHITESPACE ;
leaf: PRINTABLE <- "<print>" WHITESPACE ;
leaf: PUNCT <- "<punct>" WHITESPACE ;
leaf: SPACE <- "<space>" WHITESPACE ;
leaf: UPPER <- "<upper>" WHITESPACE ;
leaf: WORDCHAR <- "<wordchar>" WHITESPACE ;
leaf: XDIGIT <- "<xdigit>" WHITESPACE ;
void: WHITESPACE <- (" " / "\t" / EOL / COMMENT)* ;
void: COMMENT <- '#' (!EOL .)* EOL ;
void: EOL <- "\n\r" / "\n" / "\r" ;
void: EOF <- !. ;
# --------------------------------------------------------------------
END;🍓 EXAMPLE 🍓
our example specifies the grammar for a basic 4-operation calculator. it's giving math girlie. 🧮✨
PEG calculator (Expression)
Digit <- '0'/'1'/'2'/'3'/'4'/'5'/'6'/'7'/'8'/'9' ;
Sign <- '-' / '+' ;
Number <- Sign? Digit+ ;
Expression <- Term (AddOp Term)* ;
MulOp <- '*' / '/' ;
Term <- Factor (MulOp Factor)* ;
AddOp <- '+'/'-' ;
Factor <- '(' Expression ')' / Number ;
END;using higher-level features of the notation, i.e. the character classes (predefined and custom), this example can be rewritten as:
PEG calculator (Expression)
Sign <- [-+] ;
Number <- Sign? <ddigit>+ ;
Expression <- Term (AddOp Term)* ;
MulOp <- [*/];
Term <- Factor (MulOp Factor)* ;
AddOp <- [-+];
Factor <- '(' Expression ')' / Number ;
END;🌈 PEG SERIALIZATION FORMAT 🌈
Okay bestie, so this is the part where we talk about how the Parser Tools actually write down a Parsing Expression Grammar so it can be shipped around, compared to other grammars, and generally exist as an immutable value. Immutable, like your love for a good grammar. 💅
We've got two flavours here: regular and canonical serialization. A PEG can have a bunch of regular serializations, no cap, but literally exactly one of them is canonical. One. Periodt. 👑
🌸 regular serialization 🌸
- The serialization of any PEG is a nested Tcl dictionary. That's the whole vibe. A dictionary inside dictionaries inside dictionaries, like a little nesting doll situation. 🪆
- This dictionary holds a single key,
pt::grammar::peg, and its value. That value is where the contents of the grammar live. - The contents of the grammar are a Tcl dictionary holding the set of nonterminal symbols and the starting expression. The relevant keys and their values are:
rules— the value is a Tcl dictionary whose keys are the names of the nonterminal symbols known to the grammar.- Each nonterminal symbol may occur only once. No duplicates, we're not doing that here. 🙅♀️
- The empty string is not a legal nonterminal symbol. So don't even try it.
- The value for each symbol is a Tcl dictionary itself. The relevant keys and their values in this dictionary are:
is— the value is the serialization of the parsing expression describing the symbol's sentennial structure, as specified in the section PE serialization format.mode— the value can be one of three values specifying how a parser should handle the semantic value produced by the symbol.value— the semantic value of the nonterminal symbol is an abstract syntax tree consisting of a single node for the nonterminal itself, which has the ASTs of the symbol's right hand side as its children. Full main character node, children included. ✨leaf— the semantic value of the nonterminal symbol is an abstract syntax tree consisting of a single node for the nonterminal, without any children. Any ASTs generated by the symbol's right hand side are discarded. Gone. Thrown away. 💀void— the nonterminal has no semantic value. Any ASTs generated by the symbol's right hand side are discarded (as well). Yeah, also thrown away. It's giving minimalism, honestly.
start— the value is the serialization of the start parsing expression of the grammar, as specified in the section PE serialization format. This is where it all kicks off, the opening scene of your grammar's movie. 🎬
- The terminal symbols of the grammar are specified implicitly as the set of all terminal symbols used in the start expression and on the RHS of the grammar rules. Sneaky? No. Efficient? Yes. Iykyk. 🦋
🎀 canonical serialization 🎀
The canonical serialization of a grammar has the format as specified in the previous item, and then additionally satisfies the constraints below, which make it unique among all the possible serializations of this grammar. That's the whole point — one true form, and it's the iconic one. 👑
- The keys found in all the nested Tcl dictionaries are sorted in ascending dictionary order, as generated by Tcl's builtin command
lsort -increasing -dict. Sorted, always, no exceptions. - The string representation of the value is the canonical representation of a Tcl dictionary. I.e. it does not contain superfluous whitespace. Clean. Tidy. No extra crumbs. 🧹
🧮 EXAMPLE 🧮
Okay so imagine we've got the following PEG for simple mathematical expressions. Cute, right?
PEG calculator (Expression)
Digit <- '0'/'1'/'2'/'3'/'4'/'5'/'6'/'7'/'8'/'9' ;
Sign <- '-' / '+' ;
Number <- Sign? Digit+ ;
Expression <- Term (AddOp Term)* ;
MulOp <- '*' / '/' ;
Term <- Factor (MulOp Factor)* ;
AddOp <- '+'/'-' ;
Factor <- '(' Expression ')' / Number ;
END;then its canonical serialization (except for whitespace) is
pt::grammar::peg {
rules {
AddOp {is {/ {t -} {t +}} mode value}
Digit {is {/ {t 0} {t 1} {t 2} {t 3} {t 4} {t 5} {t 6} {t 7} {t 8} {t 9}} mode value}
Expression {is {x {n Term} {* {x {n AddOp} {n Term}}}} mode value}
Factor {is {/ {x {t (} {n Expression} {t )}} {n Number}} mode value}
MulOp {is {/ {t *} {t /}} mode value}
Number {is {x {? {n Sign}} {+ {n Digit}}} mode value}
Sign {is {/ {t -} {t +}} mode value}
Term {is {x {n Factor} {* {x {n MulOp} {n Factor}}}} mode value}
}
start {n Expression}
}See how every key is lined up and sorted? That's the canonical glow-up. 🌟 The is expressions are all <rainbow>parsing expressions</rainbow> and every single one of those rules is rockin' mode value, meaning each nonterminal gets its own little AST node with its children hanging off it. Chef's kiss. 😚👌
okay so bestie, here's the whole tea on how the Parser Tools serialize Parsing Expressions into immutable values — for shipping 'em around, comparing 'em, whatever you need. 🎀
And there's two flavors, keep up: regular</> serialization and canonical</> serialization. A parsing expression can have more than one regular serialization, but exactly one of them is canonical. that's it, that's the rule. no cap. 💅
🌸 Regular serialization 🌸
✨ Atomic Parsing Expressions ✨
- The string
epsilonis an atomic parsing expression. It matches the empty string. ✨ - The string
dotis an atomic parsing expression. It matches any character. - The string
alnumis an atomic parsing expression. It matches any Unicode alphabet or digit character. This is a custom extension of PEs based on Tcl's builtin commandstring is. 🫶 - The string
alphais an atomic parsing expression. It matches any Unicode alphabet character. This is a custom extension of PEs based on Tcl's builtin commandstring is. - The string
asciiis an atomic parsing expression. It matches any Unicode character below U0080. This is a custom extension of PEs based on Tcl's builtin commandstring is. - The string
controlis an atomic parsing expression. It matches any Unicode control character. This is a custom extension of PEs based on Tcl's builtin commandstring is. - The string
digitis an atomic parsing expression. It matches any Unicode digit character. Note that this includes characters outside of the[0..9]range. This is a custom extension of PEs based on Tcl's builtin commandstring is. 🍓 - The string
graphis an atomic parsing expression. It matches any Unicode printing character, except for space. This is a custom extension of PEs based on Tcl's builtin commandstring is. - The string
loweris an atomic parsing expression. It matches any Unicode lower-case alphabet character. This is a custom extension of PEs based on Tcl's builtin commandstring is. - The string
printis an atomic parsing expression. It matches any Unicode printing character, including space. This is a custom extension of PEs based on Tcl's builtin commandstring is. - The string
punctis an atomic parsing expression. It matches any Unicode punctuation character. This is a custom extension of PEs based on Tcl's builtin commandstring is. - The string
spaceis an atomic parsing expression. It matches any Unicode space character. This is a custom extension of PEs based on Tcl's builtin commandstring is. - The string
upperis an atomic parsing expression. It matches any Unicode upper-case alphabet character. This is a custom extension of PEs based on Tcl's builtin commandstring is. - The string
wordcharis an atomic parsing expression. It matches any Unicode word character. This is any alphanumeric character (seealnum), and any connector punctuation characters (e.g. underscore). This is a custom extension of PEs based on Tcl's builtin commandstring is. - The string
xdigitis an atomic parsing expression. It matches any hexadecimal digit character. This is a custom extension of PEs based on Tcl's builtin commandstring is. - The string
ddigitis an atomic parsing expression. It matches any decimal digit character. This is a custom extension of PEs based on Tcl's builtin commandregexp. - The expression
[list t x]is an atomic parsing expression. It matches the terminal string x. 👑 - The expression
[list n A]is an atomic parsing expression. It matches the nonterminal A.
🦋 Combined Parsing Expressions 🦋
- For parsing expressions e1, e2, ... the result of
[list / e1 e2 ... ]is a parsing expression as well. This is the ordered choice, aka prioritized choice. 💅 - For parsing expressions e1, e2, ... the result of
[list x e1 e2 ... ]is a parsing expression as well. This is the sequence. - For a parsing expression e the result of
[list * e]is a parsing expression as well. This is the kleene closure, describing zero or more repetitions. - For a parsing expression e the result of
[list + e]is a parsing expression as well. This is the positive kleene closure, describing one or more repetitions. - For a parsing expression e the result of
[list & e]is a parsing expression as well. This is the and lookahead predicate. - For a parsing expression e the result of
[list ! e]is a parsing expression as well. This is the not lookahead predicate. - For a parsing expression e the result of
[list ? e]is a parsing expression as well. This is the optional input.
💜 Canonical serialization 💜
The canonical serialization of a parsing expression has the format as specified in the previous item, and then additionally satisfies the constraints below, which make it unique among all the possible serializations of this parsing expression. iconic behavior tbh.
- The string representation of the value is the canonical representation of a pure Tcl list. I.e. it does not contain superfluous whitespace. 🧚
- Terminals are not encoded as ranges (where start and end of the range are identical).
🔥 EXAMPLE 🔥
Say the parsing expression is the one sitting on the right-hand side of this rule:
Expression <- Term (AddOp Term)*then its canonical serialization (whitespace aside) is literally this:
{x {n Term} {* {x {n AddOp} {n Term}}}}ate. 💅
😭 BUGS, IDEAS, FEEDBACK 😭
ngl, this document — and the package it describes — will undoubtedly contain bugs and other problems. bestie, please report them in the category pt of the Tcllib Trackers at http://core.tcl.tk/tcllib/reportlist. And if you've got ideas for enhancements for either the package and/or the documentation? send those too, we want 'em. 🫶
When proposing code changes, please provide unified diffs, i.e. the output of diff -u. 👑
Also, heads up: attachments are strongly preferred over inlined patches. You can attach stuff by going to the Edit form of the ticket immediately after its creation, and then using the left-most button in the secondary navigation bar. no cap, that's the way. ✨
🎀 KEYWORDS 🎀
EBNF, LL(k), PEG, TDPL, context-free languages, conversion, expression, format conversion, grammar, matching, parser, parsing expression, parsing expression grammar, push down automaton, recursive descent, serialization, state, top-down parsing languages, transducer 🌈
👑 CATEGORY 👑
Parsing and Grammars
💖 COPYRIGHT 💖
Copyright (c) 2009 Andreas Kupries <andreas_kupries@users.sourceforge.net>
and that's the whole thing, bestie — this came out of tcllib, version 1.0.3, as the page pt::peg::to::peg(3tcl). slay, you made it to the end of the manpage, go drink some water. 💜🦋✨
📦 where this page comes from
- debian trixie: tcllib 2.0+dfsg-4 · Tcl
- ubuntu questing: tcllib 2.0+dfsg-4
- ubuntu resolute: tcllib 2.0+dfsg-5 · Tcl
- ubuntu stonking: tcllib 2.0+dfsg-6 · Tcl
- Copyright (c) 2009 Andreas Kupries <andreas_kupries@users\&.sourceforge\&.net>
- 💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on : a language model's rewrite of the original, under its terms