pt::peg::interp(n) Parser Tools pt::peg::interp(n)______________________________________________________________________________NAME
pt::peg::interp - Interpreter for parsing expression grammars
SYNOPSIS
package require Tcl 8.5
package require pt::peg::interp ?1?
package require pt::rde ?1?
package require snit
::pt::peg::interpreter 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 con‐
figurable 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 sec‐
tion of the Core Layer of Parser Tools.
IMAGE: arch_core_transform
The interpreted grammar is copied from an instance of pt::peg::con‐
tainer, 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::interpreter 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 own‐
ership 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 man‐
ner it will be necessary to go to Tcl 8.6+ and use the corou‐
tine::auto to wrap it into a coroutine where read is properly
changed for push-operation.
Upon successful completion the command returns an abstract syn‐
tax 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 debug‐
ging, 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 con‐
taining 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 for‐
mat.
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, compar‐
ison, 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 recov‐
ered 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 with‐
out 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 con‐
tain 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 <- '(' Expression ')' / (Factor (MulOp Factor)*) ;
MulOp <- '*' / '/' ;
Factor <- Term (AddOp Term)* ;
AddOp <- '+'/'-' ;
Term <- 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
IMAGE: expr_ast
PE SERIALIZATION FORMAT
Here we specify the format used by the Parser Tools to serialize Pars‐
ing 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 expres‐
sion. 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 charac‐
ter. 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 expres‐
sion. 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 charac‐
ter. 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, includ‐
ing 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 charac‐
ter. This is a custom extension of PEs based on
Tcl's builtin command string is.
[14] The string wordchar is an atomic parsing expres‐
sion. It matches any Unicode word character. This
is any alphanumeric character (see alnum), and any
connector punctuation characters (e.g. under‐
score). 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 repeti‐
tions.
[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 for‐
mat 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 con‐
tain 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 <- '(' Expression ')'
/ Factor (MulOp Factor)*
then its canonical serialization (except for whitespace) is
{/ {x {t (} {n Expression} {t )}} {x {n Factor} {* {x {n MulOp} {n Factor}}}}}
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 SF Trackers [http://sourceforge.net/tracker/?group_id=12883].
Please also report any ideas for enhancements you may have for either
package and/or documentation.
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>
pt 1 pt::peg::interp(n)
