pt::param(3tcl)

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

🎀 pt::param 🎀

okay bestie, buckle up 💖 this page is pt::param(3tcl) and it lives inside the Parser Tools manual. it is, no exaggeration, the whole PackRat Machine Specification. that's the name. that's the vibe. let's go. ✨

✨ SYNOPSIS ✨

package require Tcl 8.5 9

💜 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 doc is literally the entrypoint to the whole system this current package is a part of. it's giving "start here or perish". 🫶

Welcome to the PackRat Machine (short: PARAM) 🎉 — a virtual machine geared towards the support of recursive descent parsers, especially packrat parsers. to make that happen it comes with features like:

  • the caching and reuse of partial results
  • the caching of the encountered input
  • the ability to backtrack in both input and AST creation

This document specifies the machine in terms of its architectural state and instruction set. no cap. 📚

🧚 ARCHITECTURAL STATE 🧚

Any PARAM implementation has to manage at least the following state. think of it as the machine's whole personality, lowkey. 👑

  • Input (IN) — This is the channel the characters to process are read from.

This part of the machine's state is used and modified by the instructions defined in the section Input Handling.

  • Current Character (CC) — The character from the input currently tested against its possible alternatives.

This part of the machine's state is used and modified by the instructions defined in the section Character Processing.

  • Current Location (CL) — The location of the current character in the input, as offset relative to the beginning of the input. Character offsets are counted from 0. (yes, 0, not 1, we're being scientific about it ✨)

This part of the machine's state is used and modified by the instructions defined in the sections Character Processing, Location Handling, and Nonterminal Execution.

  • Location Stack (LS) — A stack of locations in the input, saved for possible backtracking.

This part of the machine's state is used and modified by the instructions defined in the sections Character Processing, Location Handling, and Nonterminal Execution.

  • Status (ST) — The status of the last attempt of testing the input, indicating either success or failure. (basically the machine's "did that work?? 🤔" flag)

This part of the machine's state is used and modified by the instructions defined in the sections Status Control, Character Processing, and Nonterminal Execution.

  • Semantic Value (SV) — The current semantic value, either empty, or a node for AST constructed from the input.

This part of the machine's state is used and modified by the instructions defined in the sections Value Construction, and AST Construction.

  • AST Reduction Stack (ARS) — The stack of partial ASTs constructed during the processing of nonterminal symbols.

This part of the machine's state is used and modified by the instructions defined in the sections Value Construction, and AST Construction.

  • AST Stack (AS) — The stack of reduction stacks, saved for possible backtracking.

This part of the machine's state is used and modified by the instructions defined in the sections Value Construction, and AST Construction.

  • Error Status (ER) — The machine's current knowledge of errors. This is either empty, or set to a pair of location in the input and the set of messages for that location.
💀 heads up: this part of the machine's state can be set even if the last test of the current character was successful. for example, the *-operator (matching a sub-expression zero or more times) in a PEG is always successful, even if it encounters a problem further in the input and has to backtrack. such problems must not be forgotten when continuing the parsing. the machine remembers, bestie. it always remembers.

This part of the machine's state is used and modified by the instructions defined in the sections Error Handling, Character Processing, and Nonterminal Execution.

  • Error Stack (ES) — The stack of error stati, saved for backtracking. This enables the machine to merge current and older error stati when performing backtracking in choices after an failed match.

This part of the machine's state is used and modified by the instructions defined in the sections Error Handling, Character Processing, and Nonterminal Execution.

  • Nonterminal Cache (NC) — A cache of machine states keyed by pairs name of nonterminal symbol and location in the input. Each pair (N, L) is associated with a 4-tuple holding the values to use for CL, ST, SV, and ER after the nonterminal N was parsed starting from the location L. It is a performance aid for backtracking parsers, allowing them to avoid an expensive reparsing of complex nonterminal symbols if they have been encountered before at a given location.

The key location is where machine started the attempt to match the named nonterminal symbol, and the location in the saved 4-tuple is where machine ended up after the attempt completed, independent of the success of the attempt.

This part of the machine's state is used and modified by the instructions defined in the section Nonterminal Execution.

  • Terminal Cache (TC) — A cache of characters read from IN, with their location in IN as pair of line and column, keyed by the location in IN, this time as character offset from the beginning of IN. It is a performance aid for backtracking parsers, allowing them to avoid a possibly expensive rereading of characters from IN, or even enabling backtracking at, i.e. in the case of IN not randomly seekable.

This part of the machine's state is used and modified by the instructions defined in the section Input Handling.

🔥 INSTRUCTION SET 🔥

With the machine's architectural state specified it is now possible to specify the instruction set operating on that state and to be implemented by any realization of the PARAM. The 37 instructions are grouped roughly by the state they influence and/or query during their execution. (thirty-seven!! she's thorough 💅)

🌈 INPUT HANDLING 🌈

The instructions in this section mainly access IN, pulling the characters to process into the machine. nom nom. 🍓

  • input_next msg — This method reads the next character, i.e. the character after CL, from IN. If successful this character becomes CC, CL is advanced by one, ES is cleared, and the operation is recorded as a success in ST.

The operation may read the character from IN if the next character is not yet known to TC. If successful the new character is stored in TC, with its location (line, column), and the operation otherwise behaves as specified above. Future reads from the same location, possible due to backtracking, will then be satisfied from TC instead of IN.

If, on the other hand, the end of IN was reached, the operation is recorded as failed in ST, CL is left unchanged, and the pair of CL and msg becomes the new ES. 😭

🌸 CHARACTER PROCESSING 🌸

The instructions in this section mainly access CC, testing it against character classes, ranges, and individual characters. it's a whole vibe check for one (1) character. ✨

  • test_alnum — This instruction implements the special PE operator alnum, which checks if CC falls into the character class of the same name, or not.

Success and failure of the test are both recorded directly in ST. Success further clears ES, wheras failure sets the pair of CL and expected input (encoded as a leaf parsing expression) as the new ES and then rewinds CL by one character, preparing the machine for another parse attempt by a possible alternative.

  • test_alpha — This instruction implements the special PE operator alpha, which checks if CC falls into the character class of the same name, or not.

Success and failure of the test are both recorded directly in ST. Success further clears ES, wheras failure sets the pair of CL and expected input (encoded as a leaf parsing expression) as the new ES and then rewinds CL by one character, preparing the machine for another parse attempt by a possible alternative.

  • test_ascii — This instruction implements the special PE operator ascii, which checks if CC falls into the character class of the same name, or not.

Success and failure of the test are both recorded directly in ST. Success further clears ES, wheras failure sets the pair of CL and expected input (encoded as a leaf parsing expression) as the new ES and then rewinds CL by one character, preparing the machine for another parse attempt by a possible alternative.

  • test_char char — This instruction implements the character matching operator, i.e. it checks if CC is char.

Success and failure of the test are both recorded directly in ST. Success further clears ES, wheras failure sets the pair of CL and expected input (encoded as a leaf parsing expression) as the new ES and then rewinds CL by one character, preparing the machine for another parse attempt by a possible alternative.

  • test_ddigit — This instruction implements the special PE operator ddigit, which checks if CC falls into the character class of the same name, or not.

Success and failure of the test are both recorded directly in ST. Success further clears ES, wheras failure sets the pair of CL and expected input (encoded as a leaf parsing expression) as the new ES and then rewinds CL by one character, preparing the machine for another parse attempt by a possible alternative.

  • test_digit — This instruction implements the special PE operator digit, which checks if CC falls into the character class of the same name, or not.

Success and failure of the test are both recorded directly in ST. Success further clears ES, wheras failure sets the pair of CL and expected input (encoded as a leaf parsing expression) as the new ES and then rewinds CL by one character, preparing the machine for another parse attempt by a possible alternative.

  • test_graph — This instruction implements the special PE operator graph, which checks if CC falls into the character class of the same name, or not.

so, like we were saying — both the success and the failure of the test get recorded straight into ST, no detour, no middleman. Success also clears ES. Failure does the whole dramatic thing: it sets the pair of CL and the expected input (encoded as a leaf parsing expression) as the new ES, and then rewinds CL by one character, which preps the machine for another parse attempt by a possible alternative. 💅

test_lower

This one implements the special PE operator "lower", and it checks whether CC falls into the character class of the same name, or not. 🌸

Success and failure of the test are both recorded directly in ST. Success further clears ES, whereas failure sets the pair of CL and expected input (encoded as a leaf parsing expression) as the new ES and then rewinds CL by one character, preparing the machine for another parse attempt by a possible alternative.

test_print

Bestie, this implements the special PE operator "print" — it checks if CC falls into the character class of the same name, or not. ✨

Success and failure of the test are both recorded directly in ST. Success further clears ES, whereas failure sets the pair of CL and expected input (encoded as a leaf parsing expression) as the new ES and then rewinds CL by one character, preparing the machine for another parse attempt by a possible alternative.

test_punct

This instruction implements the special PE operator "punct", checking whether CC falls into the character class of the same name, or not. 🎀

Success and failure of the test are both recorded directly in ST. Success further clears ES, whereas failure sets the pair of CL and expected input (encoded as a leaf parsing expression) as the new ES and then rewinds CL by one character, preparing the machine for another parse attempt by a possible alternative.

test_range chars chare

Here's where the range matching operator lives — this instruction checks if CC falls into the interval of characters spanned up by the two characters from chars to chare, both inclusive. 🦋

Success and failure of the test are both recorded directly in ST. Success further clears ES, whereas failure sets the pair of CL and expected input (encoded as a leaf parsing expression) as the new ES and then rewinds CL by one character, preparing the machine for another parse attempt by a possible alternative.

test_space

This instruction implements the special PE operator "space", which checks if CC falls into the character class of the same name, or not. 🌈

Success and failure of the test are both recorded directly in ST. Success further clears ES, whereas failure sets the pair of CL and expected input (encoded as a leaf parsing expression) as the new ES and then rewinds CL by one character, preparing the machine for another parse attempt by a possible alternative.

test_upper

This instruction implements the special PE operator "upper", which checks if CC falls into the character class of the same name, or not. 👑

Success and failure of the test are both recorded directly in ST. Success further clears ES, whereas failure sets the pair of CL and expected input (encoded as a leaf parsing expression) as the new ES and then rewinds CL by one character, preparing the machine for another parse attempt by a possible alternative.

test_wordchar

This instruction implements the special PE operator "wordchar", which checks if CC falls into the character class of the same name, or not. 🍓

Success and failure of the test are both recorded directly in ST. Success further clears ES, whereas failure sets the pair of CL and expected input (encoded as a leaf parsing expression) as the new ES and then rewinds CL by one character, preparing the machine for another parse attempt by a possible alternative.

test_xdigit

This instruction implements the special PE operator "xdigit", which checks if CC falls into the character class of the same name, or not. 🧚

Success and failure of the test are both recorded directly in ST. Success further clears ES, whereas failure sets the pair of CL and expected input (encoded as a leaf parsing expression) as the new ES and then rewinds CL by one character, preparing the machine for another parse attempt by a possible alternative.

🌸 ERROR HANDLING 🌸

The instructions in this section mainly access ER and ES. (yes, they're besties like that)

error_clear

This instruction clears ER. Clean slate, no cap. 🧹

error_push

This instruction makes a copy of ER and pushes it on ES. 📥

error_pop_merge

This instruction takes the topmost entry of ES and merges the error status it contains with ES, making the result the new ES. 🔀

The merge is governed by four rules, with the merge result

  1. Empty if both states are empty.
  2. The non-empty state if only one of the two states is non-empty.
  3. The state with the larger location, if the two states specify different locations.
  4. The pair of the location shared by the two states, and the set-union of their messages for states at the same location.

error_nonterminal symbol

This is a guarded instruction, bestie. 👀 It does nothing if either ES is empty, or if the location in ES is not just past the last location saved in LS. Otherwise it sets the pair of that location and the nonterminal symbol as the new ES.

Note: In the above, "just past" means "that location plus one", or also "the location of the next character after that location". 💜

⭐ STATUS CONTROL ⭐

The instructions in this section directly manipulate ST. hands on the switch, fr fr

status_ok

This instruction sets ST to true, recording a success. 🎉

status_fail

This instruction sets ST to false, recording a failure. 💀

status_negate

This instruction negates ST, turning a failure into a success and vice versa. 🔄

🦋 LOCATION HANDLING 🦋

The instructions in this section access CL and LS.

loc_push

This instruction makes a copy of CL and pushes it on LS. 📌

loc_pop_discard

This instruction pops the last saved location from LS. Bye bye, no notes. 🙈

loc_pop_rewind

This instruction pops the last saved location from LS and restores it as CL. Rewind time, main character energy. ⏪

💜 NONTERMINAL EXECUTION 💜

The instructions in this section access and manipulate NC.

symbol_restore symbol

This instruction checks if NC contains data for the nonterminal symbol at CL, or not. The result of the instruction is a boolean flag, with True indicating that data was found in the cache. In that case the instruction has further updated the architectural state of the machine with the cached information, namely CL, ST, ER, and SV. 🧠

The method with which the instruction's result is transformed into control flow is left undefined and the responsibility of the implementation. (so, like, that's a you problem, implementation)

symbol_save symbol

This instruction saves the current settings of CL, ST, ER, and SV in NC, using the pair of nonterminal symbol and the last location saved in LS as key. 💾

✨ VALUE CONSTRUCTION ✨

The instructions in this section manipulate SV.

value_clear

This instruction clears SV. Poof. 🌬️

value_leaf symbol

This instruction constructs an AST node for symbol covering the range of IN from one character after the last location saved on LS to CL, and stores it in SV. ...

value_reduce symbol

This instruction generally behaves like value_nonterminal_leaf, except that it takes all AST nodes on ARS, if any, and makes them the children of the new node, with the last node saved on ARS becoming the right-most / last child. Note that ARS is not modified by this operation. 🫶

🌈 AST CONSTRUCTION 🌈

The instructions in this section manipulate ARS and AS.

ast_value_push

This instruction makes a copy of SV and pushes it on ARS. 📤

ast_push

This instruction pushes the current state of ARS on AS and then clears ARS. 🧺

ast_pop_rewind

This instruction pops the last entry saved on AS and restores it as the new state of ARS. ⏮️

ast_pop_discard

This instruction pops the last entry saved on AS. Tossed. 🗑️

🔥 CONTROL FLOW 🔥

Normally this section would contain the specifications of the control flow instructions of the PARAM, i.e. (un)conditional jumps and the like. However, this part of the PARAM is intentionally left unspecified. 😌 This allows the implementations to freely choose how to implement control flow.

The implementation of this machine in Parser Tools, i.e. the package pt::rde, is not only coded in Tcl, but also relies on Tcl commands to provide it with control flow (instructions). 💅

✨ INTERACTION OF THE INSTRUCTIONS WITH THE ARCHITECTURAL STATE ✨

okay bestie, this next part is literally the cheat sheet of which instruction touches which piece of the architectural state. think of it like a group chat where every instruction is like “who am i reading from and who am i writing to?” 💅

so the columns are Instruction, Inputs, and Outputs. the -> is like the “and then” arrow. everything before it is what the instruction reads, everything after is what it writes. no cap. 🔥

here’s the whole table, exactly as it lives in the manpage, because i’m not about to mess with the sacred numbers and state names:

              InstructionInputsOutputs
              ======================= ===========================================
              ast_pop_discardAS->AS
              ast_pop_rewindAS->AS, ARS
              ast_push  ARS, AS->AS
              ast_value_pushSV, ARS->ARS
              ======================= ===========================================
              error_clear-->ER
              error_nonterminal symER, LS->ER
              error_pop_merge   ES, ER->ER
              error_pushES, ER->ES
              ======================= ===========================================
              input_next msgIN->TC, CL, CC, ST, ER
              ======================= ===========================================
              loc_pop_discardLS->LS
              loc_pop_rewindLS->LS, CL
              loc_push  CL, LS->LS
              ======================= ===========================================
              status_fail-->ST
              status_negateST->ST
              status_ok -->ST
              ======================= ===========================================
              symbol_restore symNC->CL, ST, ER, SV
              symbol_save    symCL, ST, ER, SV LS->NC
              ======================= ===========================================
              test_alnum  CC->ST, ER
              test_alphaCC->ST, ER
              test_asciiCC->ST, ER
              test_char charCC->ST, ER
              test_ddigitCC->ST, ER
              test_digitCC->ST, ER
              test_graphCC->ST, ER
              test_lowerCC->ST, ER
              test_printCC->ST, ER
              test_punctCC->ST, ER
              test_range chars chareCC->ST, ER
              test_spaceCC->ST, ER
              test_upperCC->ST, ER
              test_wordcharCC->ST, ER
              test_xdigitCC->ST, ER
              ======================= ===========================================

              value_clear-->SV
              value_leaf symbolLS, CL->SV
              value_reduce symbolARS, LS, CL->SV
              ======================= ===========================================

like, the way error_clear takes no inputs and just outputs ER? it’s giving “i work alone” energy. and input_next msg is the main character fr fr, reading IN and writing TC, CL, CC, ST, and ER. iconic. 🫶

💀 BUGS, IDEAS, FEEDBACK 💀

bestie, real talk: this document and the package it describes? yeah, they will undoubtedly contain bugs and other problems. no cap. 😭

so if you find something cursed, please report it in the category pt of the Tcllib Trackers at:

http://core.tcl.tk/tcllib/reportlist

and if you have ideas for enhancements for either the package and/or the documentation? please also report those. we want all the glow-up suggestions. ✨

when you’re proposing code changes, please provide unified diffs, i.e. the output of:

diff -u

also note: attachments are strongly preferred over inlined patches. the way to attach something is to go to the Edit form of the ticket immediately after its creation, and then use the left-most button in the secondary navigation bar. that’s the move. 💅

🔑 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
  • virtual machine

🎀 CATEGORY 🎀

Parsing and Grammars

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

and that’s the whole vibe, bestie. this page came from tcllib 1.1, dated pt::param(3tcl). go off, parse something iconic. 💖✨🦋

📦 where this page comes from