pt::peg(n)

📦
version 2.0 · 2 versions
, in fedora 43, fedora 44, nixos 26.05, nixpkgs unstable

okay bestie, this is pt::peg(n) from Parser Tools, and it's literally all about Parsing Expression Grammar Serialization 💖

✨ NAME ✨

  • pt::peg — Parsing Expression Grammar Serialization 🎀

🦋 SYNOPSIS 🦋

first vibe check: these are the packages and commands you need, fr fr. don't skip the question marks, they're iconic 💅

package require Tcl 8.5 9

package require pt::peg ?1.1?

package require pt::pe

::pt::peg verify serial ?canonvar?

::pt::peg verify-as-canonical serial

::pt::peg canonicalize serial

::pt::peg print serial

::pt::peg merge seriala serialb

::pt::peg equal seriala serialb

🌸 DESCRIPTION 🌸

are you lost ? 😭 do you have trouble understanding this document ? in that case, bestie, please read the overview provided by the Introduction to Parser Tools. that document is the entrypoint to the whole system this package is a part of. no cap.

this package provides commands to work with the serializations of parsing expression grammars as managed by the Parser Tools, and specified in section PEG serialization format. it's giving supporting-package energy in the Core Layer of Parser Tools ✨

IMAGE: arch_core_support

👑 API 👑

okay so now the commands. each one is a baddie with a job 🫶

::pt::peg verify serial ?canonvar?

this command verifies that the content of serial is a valid serialization of a parsing expression, and it will throw an error if that is not the case. the result of the command is the empty string. periodt.

if the argument canonvar is specified, it is interpreted as the name of a variable in the calling context. that variable will be written to if and only if serial is a valid regular serialization. its value will be a boolean, with True indicating that the serialization is not only valid, but also canonical. False will be written for a valid, but non-canonical serialization. iykyk.

for the specification of serializations see the section PE serialization format 💖

::pt::peg verify-as-canonical serial

this command verifies that the content of serial is a valid canonical serialization of a PEG, and it will throw an error if that is not the case. the result of the command is the empty string.

for the specification of canonical serializations see the section PEG serialization format ✨

::pt::peg canonicalize serial

this command assumes that the content of serial is a valid regular serialization of a PEG, and it will throw an error if that is not the case.

it will then convert the input into the canonical serialization of the contained PEG and return it as its result. if the input is already canonical, it will be returned unchanged. slay.

for the specification of regular and canonical serializations see the section PEG serialization format 💅

::pt::peg print serial

this command assumes that the argument serial contains a valid serialization of a parsing expression and returns a string containing that PE in a human readable form.

the exact format of this form is not specified and cannot be relied on for parsing or other machine-based activities. so don't get any main character ideas about parsing it, bestie 😭

for the specification of serializations see the section PEG serialization format.

::pt::peg merge seriala serialb

this command accepts the regular serializations of two grammars and uses them to create their union. the result of the command is the canonical serialization of this unified grammar. iconic.

a merge error occurs if for any nonterminal symbol S occurring in both input grammars the two input grammars specify different semantic modes.

the semantic mode of each nonterminal symbol S is the semantic mode of S in any of its input grammars. the previous rule made sure that for symbols occurring in both grammars these values are identical.

the right-hand side of each nonterminal symbol S occurring in both input grammars is the choice between the right-hand sides of S in the input grammars, with the parsing expression of S in seriala coming first, except if both expressions are identical. in that case the first expression is taken.

the right-hand side of each nonterminal symbol S occurring in only one of the input grammars is the right-hand side of S in its input grammar.

the start expression of the unified grammar is the choice between the start expressions of the input grammars, with the start expression of seriala coming first, except if both expressions are identical. in that case the first expression is taken. 🌈

::pt::peg equal seriala serialb

this command tests the two grammars seriala and serialb for structural equality. the result of the command is a boolean value. it will be set to true if the expressions are identical, and false otherwise.

string equality is usable only if we can assume that the two grammars are pure Tcl lists and dictionaries. that's the vibe check, bestie 🦋

📦 PEG SERIALIZATION FORMAT 💾

okay bestie, so this is the part where we spill the exact format the Parser Tools use to serialize Parsing Expression Grammars as immutable values — for transport, for comparison, for all of it. ✨

and listen: there are two flavors. regular serialization and canonical serialization. a PEG can have more than one regular serialization, but exactly one of them is ever going to be the canonical one. periodt. 👑

🌸 regular serialization

  1. the serialization of any PEG is a nested Tcl dictionary. 💅
  2. this dictionary holds a single key, pt::grammar::peg, and its value. that value holds the contents of the grammar.
  3. 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. 🎀
      1. each nonterminal symbol may occur only once. 🫶
      2. the empty string is not a legal nonterminal symbol. 🚫
      3. 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 node for the nonterminal itself, which has the ASTs of the symbol's right hand side as its children. 🌳
          • leaf — the semantic value of the nonterminal symbol is an abstract syntax tree consisting of a single node node for the nonterminal, without any children. any ASTs generated by the symbol's right hand side are discarded. 🍃
          • void — the nonterminal has no semantic value. any ASTs generated by the symbol's right hand side are discarded (as well). 🕳️
    • start — the value is the serialization of the start parsing expression of the grammar, as specified in the section PE serialization format.
  4. 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. 🦋

✨ 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. 💖

  1. 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.
  2. the string representation of the value is the canonical representation of a Tcl dictionary. i.e. it does not contain superfluous whitespace. 🧹

🧮 EXAMPLE

say we've got the following PEG for simple mathematical expressions 💜

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}
}

Here we specify the format used by the Parser Tools to serialize Parsing Expressions as immutable values for transport, comparison, etc. 💅 It's giving data exchange but make it parse.

We distinguish between regular and canonical serializations. While a parsing expression may have more than one regular serialization only exactly one of them will be canonical. periodt

Regular serialization

Atomic Parsing Expressions

  1. The string epsilon is an atomic parsing expression. It matches the empty string. iconic ✨
  2. The string dot is an atomic parsing expression. It matches any character. slay 💅
  3. The string alnum is an atomic parsing expression. It matches any Unicode alphabet or digit character. This is a custom extension of PEs based on Tcl's builtin command string is. iykyk
  4. The string alpha is an atomic parsing expression. It matches any Unicode alphabet character. This is a custom extension of PEs based on Tcl's builtin command string is. lowkey
  5. The string ascii is an atomic parsing expression. It matches any Unicode character below U0080. This is a custom extension of PEs based on Tcl's builtin command string is. fr fr
  6. The string control is an atomic parsing expression. It matches any Unicode control character. This is a custom extension of PEs based on Tcl's builtin command string is. no cap
  7. The string digit is 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 command string is. obsessed
  8. The string graph is 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 command string is. ate
  9. The string lower is an atomic parsing expression. It matches any Unicode lower-case alphabet character. This is a custom extension of PEs based on Tcl's builtin command string is. periodt
  10. The string print is an atomic parsing expression. It matches any Unicode printing character, including space. This is a custom extension of PEs based on Tcl's builtin command string is. main character energy
  11. The string punct is an atomic parsing expression. It matches any Unicode punctuation character. This is a custom extension of PEs based on Tcl's builtin command string is. vibe check
  12. The string space is an atomic parsing expression. It matches any Unicode space character. This is a custom extension of PEs based on Tcl's builtin command string is. 💖
  13. The string upper is an atomic parsing expression. It matches any Unicode upper-case alphabet character. This is a custom extension of PEs based on Tcl's builtin command string is. ✨
  14. The string wordchar is an atomic parsing expression. It matches any Unicode word character. This is any alphanumeric character (see alnum), and any connector punctuation characters (e.g. underscore). This is a custom extension of PEs based on Tcl's builtin command string is. the way that…
  15. The string xdigit is an atomic parsing expression. It matches any hexadecimal digit character. This is a custom extension of PEs based on Tcl's builtin command string is. 💅
  16. The string ddigit is an atomic parsing expression. It matches any decimal digit character. This is a custom extension of PEs based on Tcl's builtin command regexp. not me…
  17. The expression [list t x] is an atomic parsing expression. It matches the terminal string x. slay
  18. The expression [list n A] is an atomic parsing expression. It matches the nonterminal A. iconic

Combined Parsing Expressions

  1. 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. the way that…
  2. For parsing expressions e1, e2, ... the result of [list x e1 e2 ... ] is a parsing expression as well. This is the sequence. lowkey
  3. 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. obsessed
  4. 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. ate
  5. For a parsing expression e the result of [list & e] is a parsing expression as well. This is the and lookahead predicate. main character energy
  6. For a parsing expression e the result of [list ! e] is a parsing expression as well. This is the not lookahead predicate. not me…
  7. For a parsing expression e the result of [list ? e] is a parsing expression as well. This is the optional input. vibe check

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.

  1. The string representation of the value is the canonical representation of a pure Tcl list. I.e. it does not contain superfluous whitespace. no cap
  2. Terminals are not encoded as ranges (where start and end of the range are identical). fr fr

EXAMPLE

Assuming the parsing expression shown on the right-hand side of the rule

Expression <- Term (AddOp Term)*

then its canonical serialization (except for whitespace) is

{x {n Term} {* {x {n AddOp} {n Term}}}}

BUGS, IDEAS, FEEDBACK

This document, and the package it describes, will undoubtedly contain bugs and other problems. not ideal but we move Please report such in the category pt of the Tcllib Trackers [http://core.tcl.tk/tcllib/reportlist]. Please also report any ideas for enhancements you may have for either package and/or documentation. bestie please

When proposing code changes, please provide unified diffs, i.e the output of diff -u. iykyk

Note further that attachments are strongly preferred over inlined patches. Attachments can be made 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. periodt

KEYWORDS

EBNF, LL(k), PEG, TDPL, context-free languages, expression, grammar, matching, parser, parsing expression, parsing expression grammar, push down automaton, recursive descent, state, top-down parsing languages, transducer 💖

CATEGORY

Parsing and Grammars 💅

Copyright (c) 2009 Andreas Kupries <andreas_kupries@users.sourceforge.net> 💖

this page is from tcllib 1.1.1, page pt::peg(n). no date on the footer but you know I had to shout it out. mwah 💋

📦 where this page comes from