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 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::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
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
IMAGE: expr_ast
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
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>