pt::peg(3tcl)

📦
version 2.0 · 2 versions
, in debian trixie, ubuntu questing, ubuntu resolute, ubuntu stonking

🌸 pt::peg — Parsing Expression Grammar Serialization 🌸

okay so bestie. this is the manual page for pt::peg, part of Parser Tools, sitting in section 3tcl of your man pages. she's a supporting package in the Core Layer of Parser Tools, and she's here to help you work with serializations of parsing expression grammars. let's get into it 💅

✨ NAME ✨

pt::peg — Parsing Expression Grammar Serialization. that's the whole vibe. 🎀

✨ SYNOPSIS ✨

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

look at that lineup. gorgeous. 🫶

✨ DESCRIPTION ✨

Are you lost ? Do you have trouble understanding this document ? 😭 bestie if that's you, please go read the overview provided by the Introduction to Parser Tools. that document is literally the entrypoint to the whole system this package is a part of. no cap.

so like, this package gives you commands to work with the serializations of parsing expression grammars — the ones managed by Parser Tools, and specified in the section PEG serialization format. iykyk.

this is a supporting package in the Core Layer of Parser Tools. she's a helper. she's humble. she's essential. 💜

IMAGE: arch_core_support

✨ API ✨

🌈 ::pt::peg verify serial ?canonvar?

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

if the argument canonvar is specified, it gets interpreted as the name of a variable in the calling context. this variable will be written to if and only if serial is a valid regular serialization. its value will be a boolean, where True means the serialization is not only valid but also canonical, and False gets written for a valid but non-canonical serialization. so you get the tea either way. ☕

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 will throw an error if that is not the case. the result of the command is the empty string. she's strict. she has standards. 👑

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 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. and if the input is already canonical, it gets returned unchanged. glow-up not required. ✨

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. it's giving readability. 🦋

the exact format of this form is not specified and cannot be relied on for parsing or other machine-based activities. so like, cute for your eyes, useless for your parser. 🙈

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. two grammars walking into a room and leaving as one. iconic. 💖

a merge errors occurs if for any nonterminal symbol S occuring 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 occuring in both grammars these values are identical.

the right-hand side of each nonterminal symbol S occuring 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 occuring 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. main character energy: are we the same or not. 💅

string equality is usable only if we can assume that the two grammars are pure Tcl lists and dictionaries. periodt.

more of this page is coming, bestie — this was just part 1. don't go anywhere 💜

✨ PEG SERIALIZATION FORMAT ✨

okay so bestie 💖 this is the part where we talk about how Parser Tools turns a Parsing Expression Grammar into an immutable value — like, something you can ship around, compare, keep in your pocket, whatever. that's the serialization format, fr fr.

we got two flavors: regular and canonical. a PEG can have a whole bunch of regular serializations but literally exactly one of them is canonical. that's the one, periodt. 👑

🌸 regular serialization 🌸

  1. The serialization of any PEG is a nested Tcl dictionary. yes, nested. it's giving dictionary inception 🌀
  2. This dictionary holds a single key, pt::grammar::peg, and its value. that value is where the contents of the grammar live.
  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. no duplicates, we don't do that here 🙅‍♀️
      2. The empty string is not a legal nonterminal symbol. nope. not a vibe.
      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 for the nonterminal itself, which has the ASTs of the symbol's right hand side as its children. full family tree energy 🌳
          • 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. bye bye children 👋
          • void — the nonterminal has no semantic value. Any ASTs generated by the symbol's right hand side are discarded (as well). absolutely nothing left, it's giving empty 😭
    • 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 begins, main character energy ⭐
  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. no list, they just show up. 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.

  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. alphabetical order bestie, no chaos allowed ✨
  2. The string representation of the value is the canonical representation of a Tcl dictionary. I.e. it does not contain superfluous whitespace. clean, tight, no stray spaces. a tidy queen 🧚

🎀 EXAMPLE 🎀

okay so assuming 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}
}

like, look at those keys all sorted and everything. immaculate. 💖

✨ Regular serialization ✨

Atomic Parsing Expressions

Bestie, these are the little building blocks, the solo icons of the PE world 💅

  1. The string epsilon is an atomic parsing expression. It matches the empty string. yes it matches nothing and that's its whole thing, iconic
  2. The string dot is an atomic parsing expression. It matches any character. anything. one single char. periodt
  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.
  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.
  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.
  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.
  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. (yes it's wider than 0-9, don't get caught slipping 🫶)
  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.
  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.
  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.
  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.
  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.
  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. (different builtin than the rest of the fam, iykyk 👀)
  17. The expression [list t x] is an atomic parsing expression. It matches the terminal string x.
  18. The expression [list n A] is an atomic parsing expression. It matches the nonterminal A.

Combined Parsing Expressions

Now we're gluing the icons together into a whole aesthetic 🌈

  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.
  2. For parsing expressions e1, e2, ... the result of [list x e1 e2 ... ] is a parsing expression as well. This is the sequence.
  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.
  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.
  5. For a parsing expression e the result of [list & e] is a parsing expression as well. This is the and lookahead predicate.
  6. For a parsing expression e the result of [list ! e] is a parsing expression as well. This is the not lookahead predicate.
  7. 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. one true form. main character energy only.

  1. The string representation of the value is the canonical representation of a pure Tcl list. I.e. it does not contain superfluous whitespace.
  2. Terminals are not encoded as ranges (where start and end of the range are identical).

🌸 EXAMPLE 🌸

Say the parsing expression shown on the right-hand side of the rule is

Expression <- Term (AddOp Term)*

then its canonical serialization (except for whitespace) is

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

like. look at it. that's the whole grammar in one little list 🎀

💀 BUGS, IDEAS, FEEDBACK 💀

This document, and the package it describes, will undoubtedly contain bugs and other problems. Please report such in the category pt of the Tcllib Trackers [http://core.tcl.tk/tcllib/reportlist](http://core.tcl.tk/tcllib/reportlist). Please also report any ideas for enhancements you may have for either package and/or documentation.

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

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. (fr fr, attachments >>> pasted walls of patch text, the maintainers will thank you 🫶)

🔑 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>

and that's the whole page bestie, we did it 💅✨

This one's from tcllib, version 1.1.1, page pt::peg(3tcl), and it's dated 1.1.1 — shoutout to Andreas Kupries for writing it, and to you for reading all three parts. slay, go parse something gorgeous 🌈👑

📦 where this page comes from