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::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.
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
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
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.
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>