bionic (3) pt_peg_interp.3tcl.gz
NAME
pt::peg::interp - Interpreter for parsing expression grammars
SYNOPSIS
package require Tcl 8.5
package require pt::peg::interp ?1.0.1?
package require pt::rde ?1?
package require snit
::pt::peg::interp objectName grammar
objectName use grammar
objectName destroy
objectName parse chan
objectName parset text
________________________________________________________________________________________________________________
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.
This package provides a class whose instances are Packrat parsers configurable with a parsing expression
grammar. The grammar is executed directly, i.e. interpreted, with the underlying runtime provided by the
package pt::rde, basing everything on the PARAM.
Like the supporting runtime this package resides in the Execution section of the Core Layer of Parser
Tools.
IMAGE: arch_core_transform
The interpreted grammar is copied from an instance of pt::peg::container, or anything providing the same
API, like the container classes created by pt::peg::to::container or the associated export plugin
pt::peg::export::container.
CLASS API
The package exports the API described here.
::pt::peg::interp objectName grammar
The command creates a new parser object and returns the fully qualified name of the object command
as its result. The API of this object command is described in the section Object API. It may be
used to invoke various operations on the object.
This new parser is configured for the execution of an empty PEG. To configure the object for any
other PEG use the method use of the Object API.
OBJECT API
All objects created by this package provide the following methods.
objectName use grammar
This method configures the grammar interpreter / parser for the execution of the PEG stored in
grammar, an object which is API-compatible to instances of pt::peg::container. The parser copies
the relevant information of the grammar, and does not take ownership of the object.
The information of any previously used grammar is overwritten.
The result of the method the empty string.
objectName destroy
This method destroys the parser instance, releasing all claimed memory and other resources, and
deleting the instance command.
The result of the command is the empty string.
objectName parse chan
This method runs the parser using the contents of chan as input (starting at the current location
in the channel), until parsing is not possible anymore, either because parsing has completed, or
run into a syntax error.
Note here that the Parser Tools are based on Tcl 8.5+. In other words, the channel argument is not
restricted to files, sockets, etc. We have the full power of reflected channels available.
It should also be noted that the parser pulls the characters from the input stream as it needs
them. If a parser created by this package has to be operated in a push aka event-driven manner it
will be necessary to go to Tcl 8.6+ and use the coroutine::auto to wrap it into a coroutine where
read is properly changed for push-operation.
Upon successful completion the command returns an abstract syntax tree as its result. This AST is
in the form specified in section AST serialization format. As a plain nested Tcl-list it can then
be processed with any Tcl commands the user likes, doing transformations, semantic checks, etc.
To help in this the package pt::ast provides a set of convenience commands for validation of the
tree's basic structure, printing it for debugging, and walking it either from the bottom up, or
top down.
When encountering a syntax error the command will throw an error instead. This error will be a
4-element Tcl-list, containing, in the order listed below:
[1] The string pt::rde identifying it as parser runtime error.
[2] The location of the parse error, as character offset from the beginning of the parsed
input.
[3] The location of parse error, now as a 2-element list containing line-number and column in
the line.
[4] A set of atomic parsing expressions indicating encoding the characters and/or nonterminal
symbols the parser expected to see at the location of the parse error, but did not get.
For the specification of atomic parsing expressions please see the section PE serialization
format.
objectName parset text
This method runs the parser using the string in text as input. In all other ways it behaves like
the method parse, shown above.
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.
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. These can be
recovered from the input given a symbol's location.
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
[1] The serialization of any AST is the serialization of its root node.
[2] 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.
[1] 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
PE SERIALIZATION FORMAT
Here we specify the format used by the Parser Tools to serialize Parsing Expressions as immutable values
for transport, comparison, etc.
We distinguish between regular and canonical serializations. While a parsing expression may have more
than one regular serialization only exactly one of them will be canonical.
Regular serialization
Atomic Parsing Expressions
[1] The string epsilon is an atomic parsing expression. It matches the empty string.
[2] The string dot is an atomic parsing expression. It matches any character.
[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.
[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.
[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
[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.
[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
Assuming the parsing expression shown on the right-hand side of the rule
Expression <- Term (AddOp Term)*
then its canonical serialization (except for whitespace) is
{x {n Term} {* {x {n AddOp} {n Term}}}}
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]. 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.
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
Copyright (c) 2009 Andreas Kupries <andreas_kupries@users.sourceforge.net>