pt::peg::import::json(n)

📦

version 2.0 · 2 versions

2.0 · fedora 43, fedora 44, nixos 26.05, nixpkgs unstable (latest)
1.21 · alpine 3.24, epel 9, opensuse-leap 16.0

, in fedora 43, fedora 44, nixos 26.05, nixpkgs unstable

💖 pt::peg::import::json — PEG Import Plugin. Read JSON format 💖

okay so bestie, welcome to pt::peg::import::json(n), a page straight out of Parser Tools. this is part 1 of 3, so grab a strawberry drink 🍓 and stay with me, we're going in.

✨ NAME ✨

pt::peg::import::json — PEG Import Plugin. Read JSON format 🎀

🌸 SYNOPSIS 🌸

package require Tcl 8.5 9

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

package require pt::peg::to::json

import text

🦋 DESCRIPTION 🦋

Are you lost ? Do you have trouble understanding this document ? 😭 If that's you, fr fr go read the overview in the Introduction to Parser Tools. That document is the entrypoint to the whole system that this package is a part of. like, the whole vibe starts there.

This package implements the parsing expression grammar import plugin that processes JSON markup. yes bestie, it reads JSON and turns it into grammar. slay.

It resides in the Import section of the Core Layer of Parser Tools, and it's meant to be used by pt::peg::import, the import manager — it sits between that manager and the matching core conversion functionality provided by pt::peg::from::json. think of it as the middle bestie in the group chat 🫶

IMAGE: arch_core_iplugins

So like, technically you can use this package directly with a regular interpreter… but this is strongly disrecommended 💀 and it requires a whole bunch of contortions to provide the expected environment. not the vibe. The proper way to use this functionality depends on your situation:

In an untrusted environment the proper access is through the package pt::peg::import and the import manager objects it provides. 🔒
In a trusted environment though, simply use the package pt::peg::from::json and access the core conversion functionality directly. ✨

👑 API 👑

The API provided by this package satisfies the specification of the Plugin API found in the Parser Tools Import API specification. periodt.

import text — this command takes the JSON markup encoding a parsing expression grammar and contained in text, and generates the canonical serialization of said grammar, as specified in section PEG serialization format. The created value is then returned as the result of the command. 💅

🌈 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 to allow the exchange of grammars with PackRat/PEG based parser generators for other languages. international bestie energy 🌍

It is formally specified by the rules below:

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. don't even try it 🙈
  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. (yep, that's the spelling in the spec, iykyk 🫶)
    - 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.

As an aside to 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. no cap.

🎀 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;

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. like, look at them side by side bestie, they're twins 👯✨

✨ PEG SERIALIZATION FORMAT ✨

okay so this is the part where we talk about how Parser Tools serializes Parsing Expression Grammars as immutable values — for transport, for comparison, whatever you need bestie 💅

And there are two flavors: regular and canonical. A PEG can have more than one regular serialization, but only exactly one of them is canonical. That's the whole point, it's the one true form 👑

regular serialization

The serialization of any PEG is a nested Tcl dictionary. yes, dictionaries inside dictionaries, it's giving nesting dolls 🪆
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. No duplicates, periodt 🙅‍♀️
  2. The empty string is not a legal nonterminal symbol. Like, at all.
  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. so it keeps everything, main character energy fr 💖
      - 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, reduced to atoms 💀
      - void — the nonterminal has no semantic value. Any ASTs generated by the symbol's right hand side are discarded (as well). double discarded, truly nothing left 🫥
- 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. no separate list, they just show up where they show up ✨

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.

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-ish order, everything in its lane 🌸
The string representation of the value is the canonical representation of a Tcl dictionary. I.e. it does not contain superfluous whitespace. no extra spaces, no stray newlines, we're clean here 💅

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}
}

look at that alignment, look at those sorted keys — that's what canonical means bestie, it's giving perfection 🎀

Okay so here's where we actually spill the format — the one Parser Tools uses to serialize Parsing Expressions as immutable values for transport, comparison, etc. 🫶

And there are two flavours, bestie: regular and canonical. A parsing expression can have more than one regular serialization, but exactly one of them is canonical. One. Only one. No cap. 💅

💖 Regular serialization 💖

🌸 Atomic Parsing Expressions 🌸

The string epsilon is an atomic parsing expression. It matches the empty string. 🫧
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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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 — which is any alphanumeric character (see alnum), and any connector punctuation characters (e.g. underscore). 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. 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. Custom extension of PEs based on Tcl's builtin command regexp. (yes bestie, this one is regexp, not string is — they're different, don't come for me 🙈)
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 is what makes 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. 🧚
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, bestie, except for whitespace) is

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

Look at it. It's giving exactly what we said. Iconic. 💅

💀 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 at http://core.tcl.tk/tcllib/reportlist. Please also report any ideas for enhancements you may have for either package and/or documentation. We want the glow-up, 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.

🌸 KEYWORDS 🌸

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

🦋 CATEGORY 🦋

Parsing and Grammars

👑 COPYRIGHT 👑

And that's a wrap, bestie! 🎀 This whole thing lives in tcllib, version 1.1, and the page is pt::peg::import::json(n) — go forth and import some JSON grammars, you absolute main character. 💖✨

📦 where this page comes from

fedora 43: tcllib 2.0-3.fc43 · TCL
fedora 44: tcllib 2.0-4.fc44 · TCL
nixos 26.05: tcllib 2.0
nixpkgs unstable: tcllib 2.0
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on 2026-10-18: a language model's rewrite of the original, under its terms