pt::peg::from::json(3tcl)

📦

version 2.0 · 2 versions

2.0 · debian trixie, ubuntu questing, ubuntu resolute, ubuntu stonking (latest)
1.21 · debian bookworm, ubuntu jammy, ubuntu noble

, in debian trixie, ubuntu questing, ubuntu resolute, ubuntu stonking

💖 NAME 💖

pt::peg::from::json — PEG Conversion. Read JSON format ✨

(this is a 3tcl manual page, from the Parser Tools collection, bestie 🎀)

✨ SYNOPSIS ✨

package require Tcl 8.5 9

package require pt::peg::from::json ?1.1?

package require pt::peg

package require json

pt::peg::from::json convert text

🦋 DESCRIPTION 🦋

okay so are you lost? Like, genuinely confused by this doc? No worries babe 😭 — go read the overview in the Introduction to Parser Tools first. That doc is the entrypoint to the whole system that this package is part of, iykyk.

So. This package? It implements the converter that goes from JSON markup to parsing expression grammars. That's the whole vibe. 💅

It lives in the Import section of the Core Layer of Parser Tools, and you can use it two ways:

directly, alongside the other packages of that layer, or
indirectly, through the import manager that pt::peg::import gives you 👑

That indirect route is the one intended for untrusted environments, and it happens via the matching import plugin pt::peg::import::json, which sits between the converter and the import manager like a little bodyguard 🫶.

IMAGE: arch_core_iplugins

🌸 API 🌸

The API this package hands you satisfies the spec of the Converter API found in the Parser Tools Import API specification. No cap.

pt::peg::from::json convert text — this command takes the JSON markup that encodes a parsing expression grammar (that markup lives inside text), and generates the canonical serialization of said grammar, exactly as specified in the section PEG serialization format. The value it creates is then returned as the result of the command. Boom. ✨

🎀 JSON GRAMMAR EXCHANGE FORMAT 🎀

The json format for parsing expression grammars was written as a data exchange format, not bound to Tcl. It was defined so grammars could be exchanged with PackRat/PEG based parser generators for other languages. 🌍

It is formally specified by the rules below, and yes we're going through all of them:

The JSON of any PEG is a JSON object.
This object holds a single key, pt::grammar::peg, and its value. This value holds the contents of the grammar.
The contents of the grammar are a JSON object holding the set of nonterminal symbols and the starting expression. The relevant keys and their values are:
- rules — the value is a JSON object 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 JSON object itself. The relevant keys and their values in this dictionary are:
    - is — the value is a JSON string holding the Tcl serialization of the parsing expression describing the symbols sentennial structure, as specified in the section PE serialization format.
    - mode — the value is a JSON holding holding 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 a JSON string holding the Tcl serialization of the start parsing expression of the grammar, as specified in the section PE serialization format.
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.

aside for the advanced reader 👑 — this is pretty much the same as the Tcl serialization of PE grammars, as specified in section PEG serialization format, except that the Tcl dictionaries and lists of that format are mapped to JSON objects and arrays. Only the parsing expressions themselves are not translated further, but kept as JSON strings containing a nested Tcl list, and there is no concept of canonicity for the JSON either. 💀

🍓 EXAMPLE 🍓

So say we have this 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;

a JSON serialization for it 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 {t (} {n Expression} {t )}} {x {n Factor} {* {x {n MulOp} {n Factor}}}}",
                "mode" : "value"
            },
            "Factor"    : {
                "is"   : "x {n Term} {* {x {n AddOp} {n Term}}}",
                "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"   : "n Number",
                "mode" : "value"
            }
        },
        "start" : "n Expression"
    }
}

and a Tcl serialization of the same 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}
}

The similarity of the latter to the JSON should be quite obvious. 👀✨

✨ PEG SERIALIZATION FORMAT ✨

okay bestie, so this is the part where we talk about how Parser Tools actually writes down a Parsing Expression Grammar so it can travel, get compared, get stored, whatever 💾✨ We're serializing PEGs as immutable values. Immutable. Like your aesthetic. Periodt.

So there's two flavors here and you gotta know the difference:

regular serialization — a PEG can have more than one of these, so it's giving "whatever, we flexible" 💅
canonical serialization — there is exactly one of these for any given PEG, it's the main character, no duplicates allowed 👑

regular serialization

The serialization of any PEG is a nested Tcl dictionary. yes, nested. buckle up 🪆
This dictionary holds a single key, pt::grammar::peg, and its value. That value holds the contents of the grammar.
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 (so don't try it, it will not slay).
  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 sentential 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. Choose your fighter:
      - 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. iconic tree behavior 🌳
      - 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). truly nothing, it's giving silence 💀
- start — the value is the serialization of the start parsing expression of the grammar, as specified in the section PE serialization format.
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. sneak attack, they're not listed anywhere, they just are 🦋

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. one of one, no cap 👑

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.
The string representation of the value is the canonical representation of a Tcl dictionary. I.e. it does not contain superfluous whitespace. not a single extra space, she's clean 💅✨

EXAMPLE

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 that. sorted keys, mode value everywhere, start {n Expression} closing it out. she's perfect, she's canonical, she ate and left no crumbs 😭💖

Regular serialization 🌈

okay so a parsing expression can have more than one regular serialization, but only exactly one of them is canonical. Let's break it all the way down.

Atomic Parsing Expressions 💅

These are the little babies, the building blocks, the main characters of the whole system:

The string epsilon is an atomic parsing expression. It matches the empty string. (Yes bestie, matching nothing is a whole job and it does it flawlessly.) ✨
The string dot is an atomic parsing expression. It matches any character.
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. 🎀
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 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.
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.
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 🦋

Now we're gluing the babies together, and it's giving composition:

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.

The string representation of the value is the canonical representation of a pure Tcl list. I.e. it does not contain superfluous whitespace. (No messy gaps, we keep it snatched. 💅)
Terminals are not encoded as ranges (where start and end of the range are identical).

✨ EXAMPLE ✨

Say the parsing expression on the right-hand side of the rule looks like this:

Expression <- Term (AddOp Term)*

then its canonical serialization (except for whitespace) is

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

Look at that. Iconic. No notes. 🔥

💀 BUGS, IDEAS, FEEDBACK 💀

ngl, this document, and the package it describes, will undoubtedly contain bugs and other problems. Bestie, please report such in the category pt of the Tcllib Trackers [http://core.tcl.tk/tcllib/reportlist]. Also please report any ideas for enhancements you may have for either package and/or documentation — we wanna hear them fr fr. 🫶

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. (iykyk 👀)

🔑 KEYWORDS

EBNF, JSON, 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

and that's the whole page bestie, we ate and left zero crumbs 👑 — this was pt::peg::from::json(3tcl), straight out of tcllib version 1.1. go forth and parse, you absolute icon 💖✨🌸

📦 where this page comes from

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