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grammar::peg(n) Grammar operations and usage grammar::peg(n)
____________________________________________________________________________________________________________
NAME
grammar::peg - Create and manipulate parsing expression grammars
SYNOPSIS
package require Tcl 8.4
package require snit
package require grammar::peg ?0.1?
::grammar::peg pegName ?=|:=|<--|as|deserialize src?
pegName destroy
pegName clear
pegName = srcPEG
pegName --> dstPEG
pegName serialize
pegName deserialize serialization
pegName is valid
pegName start ?pe?
pegName nonterminals
pegName nonterminal add nt pe
pegName nonterminal delete nt1 ?nt2 ...?
pegName nonterminal exists nt
pegName nonterminal rename nt ntnew
pegName nonterminal mode nt ?mode?
pegName nonterminal rule nt
pegName unknown nonterminals
____________________________________________________________________________________________________________
DESCRIPTION
This package provides a container class for parsing expression grammars (Short: PEG). It allows the
incremental definition of the grammar, its manipulation and querying of the definition. The package
neither provides complex operations on the grammar, nor has it the ability to execute a grammar defi-nition definition
nition for a stream of symbols. Two packages related to this one are grammar::mengine and gram-mar::peg::interpreter. grammar::peg::interpreter.
mar::peg::interpreter. The first of them defines a general virtual machine for the matching of a
character stream, and the second implements an interpreter for parsing expression grammars on top of
that virtual machine.
TERMS & CONCEPTS
PEGs are similar to context-free grammars, but not equivalent; in some cases PEGs are strictly more
powerful than context-free grammars (there exist PEGs for some non-context-free languages). The for-mal formal
mal mathematical definition of parsing expressions and parsing expression grammars can be found in
section PARSING EXPRESSION GRAMMARS.
In short, we have terminal symbols, which are the most basic building blocks for sentences, and non-terminal nonterminal
terminal symbols with associated parsing expressions, defining the grammatical structure of the sen-tences. sentences.
tences. The two sets of symbols are distinctive, and do not overlap. When speaking about symbols the
word "symbol" is often left out. The union of the sets of terminal and nonterminal symbols is called
the set of symbols.
Here the set of terminal symbols is not explicitly managed, but implicitly defined as the set of all
characters. Note that this means that we inherit from Tcl the ability to handle all of Unicode.
A pair of nonterminal and parsing expression is also called a grammatical rule, or rule for short. In
the context of a rule the nonterminal is often called the left-hand-side (LHS), and the parsing
expression the right-hand-side (RHS).
The start expression of a grammar is a parsing expression from which all the sentences contained in
the language specified by the grammar are derived. To make the understanding of this term easier let
us assume for a moment that the RHS of each rule, and the start expression, is either a sequence of
symbols, or a series of alternate parsing expressions. In the latter case the rule can be seen as a
set of rules, each providing one alternative for the nonterminal. A parsing expression A' is now a
derivation of a parsing expression A if we pick one of the nonterminals N in the expression, and one
of the alternative rules R for N, and then replace the nonterminal in A with the RHS of the chosen
rule. Here we can see why the terminal symbols are called such. They cannot be expanded any further,
thus terminate the process of deriving new expressions. An example
Rules
(1) A <- a B c
(2a) B <- d B
(2b) B <- e
Some derivations, using starting expression A.
A -/1/-> a B c -/2a/-> a d B c -/2b/-> a d e c
A derived expression containing only terminal symbols is a sentence. The set of all sentences which
can be derived from the start expression is the language of the grammar.
Some definitions for nonterminals and expressions:
[1] A nonterminal A is called reachable if it is possible to derive a parsing expression from the
start expression which contains A.
[2] A nonterminal A is called useful if it is possible to derive a sentence from it.
[3] A nonterminal A is called recursive if it is possible to derive a parsing expression from it
which contains A, again.
[4] The FIRST set of a nonterminal A contains all the symbols which can occur of as the leftmost
symbol in a parsing expression derived from A. If the FIRST set contains A itself then that
nonterminal is called left-recursive.
[5] The LAST set of a nonterminal A contains all the symbols which can occur of as the rightmost
symbol in a parsing expression derived from A. If the LAST set contains A itself then that
nonterminal is called right-recursive.
[6] The FOLLOW set of a nonterminal A contains all the symbols which can occur after A in a pars-ing parsing
ing expression derived from the start expression.
[7] A nonterminal (or parsing expression) is called nullable if the empty sentence can be derived
from it.
And based on the above definitions for grammars:
[1] A grammar G is recursive if and only if it contains a nonterminal A which is recursive. The
terms left- and right-recursive, and useful are analogously defined.
[2] A grammar is minimal if it contains only reachable and useful nonterminals.
[3] A grammar is wellformed if it is not left-recursive. Such grammars are also complete, which
means that they always succeed or fail on all input sentences. For an incomplete grammar on
the other hand input sentences exist for which an attempt to match them against the grammar
will not terminate.
[4] As we wish to allow ourselves to build a grammar incrementally in a container object we will
encounter stages where the RHS of one or more rules reference symbols which are not yet known
to the container. Such a grammar we call invalid. We cannot use the term incomplete as this
term is already taken, see the last item.
CONTAINER CLASS API
The package exports the API described here.
::grammar::peg pegName ?=|:=|<--|as|deserialize src?
The command creates a new container object for a parsing expression grammar and returns the
fully qualified name of the object command as its result. The API the returned command is fol-
lowing is described in the section CONTAINER OBJECT API. It may be used to invoke various
operations on the container and the grammar within.
The new container, i.e. grammar will be empty if no src is specified. Otherwise it will con-tain contain
tain a copy of the grammar contained in the src. The src has to be a container object refer-ence reference
ence for all operators except deserialize. The deserialize operator requires src to be the
serialization of a parsing expression grammar instead.
An empty grammar has no nonterminal symbols, and the start expression is the empty expression,
i.e. epsilon. It is valid, but not useful.
CONTAINER OBJECT API
All grammar container objects provide the following methods for the manipulation of their contents:
pegName destroy
Destroys the grammar, including its storage space and associated command.
pegName clear
Clears out the definition of the grammar contained in pegName, but does not destroy the
object.
pegName = srcPEG
Assigns the contents of the grammar contained in srcPEG to pegName, overwriting any existing
definition. This is the assignment operator for grammars. It copies the grammar contained in
the grammar object srcPEG over the grammar definition in pegName. The old contents of pegName
are deleted by this operation.
This operation is in effect equivalent to
pegName deserialize [srcPEG serialize]
pegName --> dstPEG
This is the reverse assignment operator for grammars. It copies the automation contained in
the object pegName over the grammar definition in the object dstPEG. The old contents of dst-PEG dstPEG
PEG are deleted by this operation.
This operation is in effect equivalent to
dstPEG deserialize [pegName serialize]
pegName serialize
This method serializes the grammar stored in pegName. In other words it returns a tcl value
completely describing that grammar. This allows, for example, the transfer of grammars over
arbitrary channels, persistence, etc. This method is also the basis for both the copy con-structor constructor
structor and the assignment operator.
The result of this method has to be semantically identical over all implementations of the
grammar::peg interface. This is what will enable us to copy grammars between different imple-mentations implementations
mentations of the same interface.
The result is a list of four elements with the following structure:
[1] The constant string grammar::peg.
[2] A dictionary. Its keys are the names of all known nonterminal symbols, and their asso-ciated associated
ciated values are the parsing expressions describing their sentennial structure.
[3] A dictionary. Its keys are the names of all known nonterminal symbols, and their asso-ciated associated
ciated values hints to a matcher regarding the semantic values produced by the symbol.
[4] The last item is a parsing expression, the start expression of the grammar.
Assuming the following PEG for simple mathematical expressions
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
a possible serialization is
grammar::peg \\
{Expression {/ {x ( Expression )} {x Factor {* {x MulOp Factor}}}} \\
Factor {x Term {* {x AddOp Term}}} \\
Term Number \\
MulOp {/ * /} \\
AddOp {/ + -} \\
Number {x {? Sign} {+ Digit}} \\
Sign {/ + -} \\
Digit {/ 0 1 2 3 4 5 6 7 8 9} \\
} \\
{Expression value Factor value \\
Term value MulOp value \\
AddOp value Number value \\
Sign value Digit value \\
}
Expression
A possible one, because the order of the nonterminals in the dictionary is not relevant.
pegName deserialize serialization
This is the complement to serialize. It replaces the grammar definition in pegName with the
grammar described by the serialization value. The old contents of pegName are deleted by this
operation.
pegName is valid
A predicate. It tests whether the PEG in pegName is valid. See section TERMS & CONCEPTS for
the definition of this grammar property. The result is a boolean value. It will be set to
true if the PEG has the tested property, and false otherwise.
pegName start ?pe?
This method defines the start expression of the grammar. It replaces the previously defined
start expression with the parsing expression pe. The method fails and throws an error if pe
does not contain a valid parsing expression as specified in the section PARSING EXPRESSIONS.
In that case the existing start expression is not changed. The method returns the empty
string as its result.
If the method is called without an argument it will return the currently defined start expres-
sion.
pegName nonterminals
Returns the set of all nonterminal symbols known to the grammar.
pegName nonterminal add nt pe
This method adds the nonterminal nt and its associated parsing expression pe to the set of
nonterminal symbols and rules of the PEG contained in the object pegName. The method fails
and throws an error if either the string nt is already known as a symbol of the grammar, or if
pe does not contain a valid parsing expression as specified in the section PARSING EXPRES-SIONS. EXPRESSIONS.
SIONS. In that case the current set of nonterminal symbols and rules is not changed. The
method returns the empty string as its result.
pegName nonterminal delete nt1 ?nt2 ...?
This method removes the named symbols nt1, nt2 from the set of nonterminal symbols of the PEG
contained in the object pegName. The method fails and throws an error if any of the strings
is not known as a nonterminal symbol. In that case the current set of nonterminal symbols is
not changed. The method returns the empty string as its result.
The stored grammar becomes invalid if the deleted nonterminals are referenced by the RHS of
still-known rules.
pegName nonterminal exists nt
A predicate. It tests whether the nonterminal symbol nt is known to the PEG in pegName. The
result is a boolean value. It will be set to true if the symbol nt is known, and false other-wise. otherwise.
wise.
pegName nonterminal rename nt ntnew
This method renames the nonterminal symbol nt to ntnew. The method fails and throws an error
if either nt is not known as a nonterminal, or if ntnew is a known symbol. The method returns
the empty string as its result.
pegName nonterminal mode nt ?mode?
This mode returns or sets the semantic mode associated with the nonterminal symbol nt. If no
mode is specified the current mode of the nonterminal is returned. Otherwise the current mode
is set to mode. The method fails and throws an error if nt is not known as a nonterminal.
The grammar interpreter implemented by the package grammar::peg::interpreter recognizes the
following modes:
value The semantic value of the nonterminal is the abstract syntax tree created from the
AST's of the RHS and a node for the nonterminal itself.
match The semantic value of the nonterminal is an the abstract syntax tree consisting of sin-gle single
gle a node for the string matched by the RHS. The ASTs generated by the RHS are dis-carded. discarded.
carded.
leaf The semantic value of the nonterminal is an the abstract syntax tree consisting of sin-gle single
gle a node for the nonterminal itself. The ASTs generated by the RHS are discarded.
discard
The nonterminal has no semantic value. The ASTs generated by the RHS are discarded (as
well).
pegName nonterminal rule nt
This method returns the parsing expression associated with the nonterminal nt. The method
fails and throws an error if nt is not known as a nonterminal.
pegName unknown nonterminals
This method returns a list containing the names of all nonterminal symbols which are refer-enced referenced
enced on the RHS of a grammatical rule, but have no rule definining their structure. In other
words, a list of the nonterminal symbols which make the grammar invalid. The grammar is valid
if this list is empty.
PARSING EXPRESSIONS
Various methods of PEG container objects expect a parsing expression as their argument, or will
return such. This section specifies the format such parsing expressions are in.
[1] The string epsilon is an atomic parsing expression. It matches the empty string.
[2] The string alnum is an atomic parsing expression. It matches any alphanumeric character.
[3] The string alpha is an atomic parsing expression. It matches any alphabetical character.
[4] The string dot is an atomic parsing expression. It matches any character.
[5] The expression [list t x] is an atomic parsing expression. It matches the terminal string x.
[6] The expression [list n A] is an atomic parsing expression. It matches the nonterminal A.
[7] 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.
[8] For parsing expressions e1, e2, ... the result of [list x e1 e2 ... ] is a parsing expression
as well. This is the sequence.
[9] 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.
[10] 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.
[11] For a parsing expression e the result of [list & e] is a parsing expression as well. This is
the and lookahead predicate.
[12] For a parsing expression e the result of [list ! e] is a parsing expression as well. This is
the not lookahead predicate.
[13] For a parsing expression e the result of [list ? e] is a parsing expression as well. This is
the optional input.
Examples of parsing expressions where already shown, in the description of the method serialize.
PARSING EXPRESSION GRAMMARS
For the mathematically inclined, a PEG is a 4-tuple (VN,VT,R,eS) where
• VN is a set of nonterminal symbols,
• VT is a set of terminal symbols,
• R is a finite set of rules, where each rule is a pair (A,e), A in VN, and e a parsing expres-sion. expression.
sion.
• eS is a parsing expression, the start expression.
Further constraints are
• The intersection of VN and VT is empty.
• For all A in VT exists exactly one pair (A,e) in R. In other words, R is a function from non-terminal nonterminal
terminal symbols to parsing expressions.
Parsing expression are inductively defined via
• The empty string (epsilon) is a parsing expression.
• A terminal symbol a is a parsing expression.
• A nonterminal symbol A is a parsing expression.
• e1e2 is a parsing expression for parsing expressions e1 and 2. This is called sequence.
• e1/e2 is a parsing expression for parsing expressions e1 and 2. This is called ordered choice.
• e* is a parsing expression for parsing expression e. This is called zero-or-more repetitions,
also known as kleene closure.
• e+ is a parsing expression for parsing expression e. This is called one-or-more repetitions,
also known as positive kleene closure.
• !e is a parsing expression for parsing expression e1. This is called a not lookahead predi-cate. predicate.
cate.
• &e is a parsing expression for parsing expression e1. This is called an and lookahead predi-cate. predicate.
cate.
PEGs are used to define a grammatical structure for streams of symbols over VT. They are a modern
phrasing of older formalisms invented by Alexander Birham. These formalisms were called TS (TMG
recognition scheme), and gTS (generalized TS). Later they were renamed to TPDL (Top-Down Parsing Lan-guages) Languages)
guages) and gTPDL (generalized TPDL).
They can be easily implemented by recursive descent parsers with backtracking. This makes them rela-tives relatives
tives of LL(k) Context-Free Grammars.
REFERENCES
[1] The Packrat Parsing and Parsing Expression Grammars Page
[http://www.pdos.lcs.mit.edu/~baford/packrat/], by Bryan Ford, Massachusetts Institute of
Technology. This is the main entry page to PEGs, and their realization through Packrat
Parsers.
[2] Parsing Techniques - A Practical Guide [http://www.cs.vu.nl/~dick/PTAPG.html], an online book
offering a clear, accessible, and thorough discussion of many different parsing techniques
with their interrelations and applicabilities, including error recovery techniques.
[3] Compilers and Compiler Generators [http://scifac.ru.ac.za/compilers/], an online book using
CoCo/R, a generator for recursive descent parsers.
BUGS, IDEAS, FEEDBACK
This document, and the package it describes, will undoubtedly contain bugs and other problems.
Please report such in the category grammar_peg of the Tcllib SF Trackers [http://source -
forge.net/tracker/? group_id=12883]. Please also report any ideas for enhancements you may have for
either package and/or documentation.
KEYWORDS
LL(k), TDPL, context-free languages, expression, grammar, parsing, parsing expression, parsing
expression grammar, push down automaton, recursive descent, state, top-down parsing languages, trans-ducer transducer
ducer
CATEGORY
Grammars and finite automata
COPYRIGHT
Copyright (c) 2005 Andreas Kupries <andreas_kupries@users.sourceforge.net>
grammar_peg 0.1 grammar::peg(n)
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