pt::ast(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

okay bestie, so this is pt::ast(n) — the Parser Tools manual page, section n. 💖 It's giving Abstract Syntax Tree Serialization and honestly? It ate. No cap.

✨ NAME ✨

pt::ast — Abstract Syntax Tree Serialization. That's the whole vibe, that's the whole job. 💅

🌈 SYNOPSIS 🌈

Here's the entire cast of characters, bestie, in order:

package require Tcl 8.5 9

package require pt::ast ?1.2?

::pt::ast verify serial ?canonvar?

::pt::ast verify-as-canonical serial

::pt::ast canonicalize serial

::pt::ast print serial

::pt::ast bottomup cmdprefix ast

cmdprefix ast

::pt::ast topdown cmdprefix pe

::pt::ast equal seriala serialb

::pt::ast new0 s loc ?child...?

::pt::ast new s start end ?child...?

🌸 DESCRIPTION 🌸

Are you lost? 😭 Do you have trouble understanding this document? In that case please read the overview provided by the Introduction to Parser Tools. This document is the entrypoint to the whole system the current package is a part of. Like, the front door. The main character. 👑

This package provides commands to work with the serializations of abstract syntax trees as managed by the Parser Tools, and specified in the section AST serialization format (scroll down, it's right there 🫶).

This is a supporting package in the Core Layer of Parser Tools.

IMAGE: arch_core_support 🎀

🌈 API 🌈

🫶 `::pt::ast verify serial ?canonvar?`

The vibe checker ✨. This command verifies that the content of serial is a valid serialization of an abstract syntax tree, and it will throw an error if that's not the case. If it IS valid? The result of the command is the empty string. Yes. Nothing. That's it. That's the point. 💀

If you pass the argument canonvar, it gets 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, where True indicates that the serialization is not only valid, but also canonical. False gets written for a valid but non-canonical serialization. 🙈

For the specification of serializations, see the section AST serialization format.

🦋 `::pt::ast verify-as-canonical serial`

Same energy, higher standards 👑. This one verifies that the content of serial is a valid canonical serialization of an abstract syntax tree, and it will throw an error if that's not the case. The result of the command is the empty string.

For the specification of canonical serializations, see the section AST serialization format.

💅 `::pt::ast canonicalize serial`

This command assumes that the content of serial is a valid regular serialization of an abstract syntax, and will throw an error if that's not the case. Rude, but fair.

It will then convert the input into the canonical serialization of the contained tree and return it as its result. And if the input is already canonical? It gets returned unchanged, no drama. 🌈

For the specification of regular and canonical serializations, see the section AST serialization format.

🎀 `::pt::ast print serial`

This command assumes that the argument serial contains a valid serialization of an abstract syntax tree, and returns a string containing that tree in a human readable form. It's the pretty-printer, the glow-up, the "let me make this make sense" button. ✨

Heads up bestie: the exact format of this form is not specified and cannot be relied on for parsing or other machine-based activities. Look at it with your eyes, not your regex. 😭

For the specification of serializations, see the section AST serialization format.

🌱 `::pt::ast bottomup cmdprefix ast`

This command walks the abstract syntax tree ast from the bottom up to the root, invoking the command prefix cmdprefix for each node. This implies that the children of a node N are handled before N. Leaves first, queen. 🍓

The command prefix has the signature

cmdprefix ast

I.e. it is invoked with the ast node the walk is currently at.

The result returned by the command prefix replaces ast in the node it was a child of, allowing transformations of the tree. 🔥

This also means that for all inner nodes, the contents of the children elements are the results of the command prefix invoked for the children of this node. So the tree rebuilds itself as it climbs. It's giving construction, honestly.

🌊 `::pt::ast topdown cmdprefix pe`

This command walks the abstract syntax tree ast from the root down to the leaves, invoking the command prefix cmdprefix for each node. This implies that the children of a node N are handled after N. Root first, then we descend. ⬇️

The command prefix has the same signature as for bottomup, see above.

The result returned by the command prefix is ignored. (So don't bother returning anything, it's not gonna read it. 🫥)

🧚 `::pt::ast equal seriala serialb`

This command tests the two abstract syntax trees seriala and serialb for structural equality. The result of the command is a boolean value. It will be set to true if the trees are identical, and false otherwise. Simple. Iconic. 💜

Important caveat: string equality is usable only if we can assume that the two trees are pure Tcl lists.

🍓 `::pt::ast new0 s loc ?child...?`

This command constructs the ast for a nonterminal node referring to the symbol s at position loc in the input, and the set of child nodes child ..., from left to right. The latter may be empty. The constructed node is returned as the result of the command. 🫶

The end position is loc-1, i.e. one character before the start.

This type of node is possible for rules containing optional parts. Yes, it's weird. It's supposed to be weird. 🎀

⭐ `::pt::ast new s start end ?child...?`

This command constructs the ast for a nonterminal node referring to the symbol s covering the range of positions start to end in the input, and the set of child nodes child ..., from left to right. The latter may be empty. The constructed node is returned as the result of the command. Clean, standard, no notes. ✨

📦 AST SERIALIZATION FORMAT 📦

Here we specify the format used by the Parser Tools to serialize Abstract Syntax Trees (ASTs) as immutable values for transport, comparison, etc. Immutable. Like your fave's aesthetic. 💅

Each node in an AST represents a nonterminal symbol of a grammar, and the range of tokens/characters in the input covered by it. ASTs do not contain terminal symbols, i.e. tokens/characters. Those can be recovered from the input given a symbol's location. iykyk. 🌸

We distinguish between regular and canonical serializations. While a tree may have more than one regular serialization, only exactly one of them will be canonical. 👑

🌈 Regular serialization

The serialization of any AST is the serialization of its root node.
The serialization of any node is a Tcl list containing at least three elements.
1. The first element is the name of the nonterminal symbol stored in the node.
2. The second and third element are the locations of the first and last token in the token stream the node represents (covers).
  1. Locations are provided as non-negative integer offsets from the beginning of the token stream, with the first token found in the stream located at offset 0 (zero).
  2. The end location has to be equal to or larger than the start location.
3. All elements after the first three represent the children of the node, which are themselves nodes. This means that the serializations of nodes without children, i.e. leaf nodes, have exactly three elements. The children are stored in the list with the leftmost child first, and the rightmost child last.

👑 Canonical serialization

The canonical serialization of an abstract syntax tree 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 tree.

The string representation of the value is the canonical representation of a pure Tcl list. I.e. it does not contain superfluous whitespace.

🎀 EXAMPLE 🎀

Assuming the parsing expression grammar below

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;

and the input string

120+5

then a parser should deliver the abstract syntax tree below (except for whitespace)

set ast {Expression 0 4
    {Factor 0 4
        {Term 0 2
            {Number 0 2
                {Digit 0 0}
                {Digit 1 1}
                {Digit 2 2}
            }
        }
        {AddOp 3 3}
        {Term 4 4
            {Number 4 4
                {Digit 4 4}
            }
        }
    }
}

Or, more graphical 🌈

.nf +- Digit 0 0 | 1 |            | +- Term 0 2 --- Number 0 2 -+- Digit
1 1 | 2 |                           |            | |
+- Digit 2 2 | 0 |                                        | Expression 0
4 --- Factor 0 4 -+----------------------------- AddOp 3 3 | + |
| +- Term 4 4 --- Number 4 4 --- Digit 4 4 | 5 .fi

💀 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. 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 🔑

Okay bestie, here's the vibe check on the search terms for this page — all the lil tags that'll help you find it again:

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

literally just a whole mood board of parsing terminology, no cap 🎀

📂 CATEGORY 📂

Parsing and Grammars ✨

That's the shelf it lives on. Periodt. 🫶

And that's a wrap, bestie!! 🌸 This one's from the tcllib fam — version 1.2, and it's dated pt::ast(n) in the docs. Go forth and parse, you absolute icon 👑💅

📦 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

pt::ast(n)

✨ NAME ✨

🌈 SYNOPSIS 🌈

🌸 DESCRIPTION 🌸

🌈 API 🌈

🫶 ::pt::ast verify serial ?canonvar?

🦋 ::pt::ast verify-as-canonical serial

💅 ::pt::ast canonicalize serial

🎀 ::pt::ast print serial

🌱 ::pt::ast bottomup cmdprefix ast

🌊 ::pt::ast topdown cmdprefix pe

🧚 ::pt::ast equal seriala serialb

🍓 ::pt::ast new0 s loc ?child...?

⭐ ::pt::ast new s start end ?child...?