NAME¶
pt::peg - Parsing Expression Grammar Serialization
SYNOPSIS¶
package require
Tcl 8.5
package require
pt::peg ?1?
package require
pt::pe
::pt::peg verify serial ?
canonvar?
::pt::peg verify-as-canonical serial
::pt::peg canonicalize serial
::pt::peg print serial
::pt::peg merge seriala serialb
::pt::peg equal seriala serialb
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 commands to work with the serializations of parsing
expression grammars as managed by the Parser Tools, and specified in section
PEG serialization format.
This is a supporting package in the Core Layer of Parser Tools.
IMAGE: arch_core_support
API¶
- ::pt::peg verify serial ?canonvar?
- This command verifies that the content of serial is a valid
serialization of a parsing expression and will throw an error if that is
not the case. The result of the command is the empty string.
If the argument canonvar is specified it is interpreted as the name
of a variable in the calling context. This variable will be written to if
and only if serial is a valid regular serialization. Its value will
be a boolean, with True indicating that the serialization is not
only valid, but also canonical. False will be written for a
valid, but non-canonical serialization.
For the specification of serializations see the section PE serialization
format.
- ::pt::peg verify-as-canonical serial
- This command verifies that the content of serial is a valid
canonical serialization of a PEG and will throw an error if that is
not the case. The result of the command is the empty string.
For the specification of canonical serializations see the section PEG
serialization format.
- ::pt::peg canonicalize serial
- This command assumes that the content of serial is a valid
regular serialization of a PEG and will throw an error if that is
not the case.
It will then convert the input into the canonical serialization of
the contained PEG and return it as its result. If the input is already
canonical it will be returned unchanged.
For the specification of regular and canonical serializations see the
section PEG serialization format.
- ::pt::peg print serial
- This command assumes that the argument serial contains a valid
serialization of a parsing expression and returns a string containing that
PE in a human readable form.
The exact format of this form is not specified and cannot be relied on for
parsing or other machine-based activities.
For the specification of serializations see the section PEG serialization
format.
- ::pt::peg merge seriala serialb
- This command accepts the regular serializations of two grammars and uses
them to create their union. The result of the command is the canonical
serialization of this unified grammar.
A merge errors occurs if for any nonterminal symbol S occuring in both input
grammars the two input grammars specify different semantic modes.
The semantic mode of each nonterminal symbol S is the semantic mode of S in
any of its input grammars. The previous rule made sure that for symbols
occuring in both grammars these values are identical.
The right-hand side of each nonterminal symbol S occuring in both input
grammars is the choice between the right-hand sides of S in the input
grammars, with the parsing expression of S in seriala coming first,
except if both expressions are identical. In that case the first
expression is taken.
The right-hand side of each nonterminal symbol S occuring in only one of the
input grammars is the right-hand side of S in its input grammar.
The start expression of the unified grammar is the choice between the start
expressions of the input grammars, with the start expression of
seriala coming first, except if both expressions are identical. In
that case the first expression is taken
- ::pt::peg equal seriala serialb
- This command tests the two grammars seriala and serialb for
structural equality. The result of the command is a boolean value. It will
be set to true if the expressions are identical, and false
otherwise.
String equality is usable only if we can assume that the two grammars are
pure Tcl lists and dictionaries.
Here we specify the format used by the Parser Tools to serialize Parsing
Expression Grammars as immutable values for transport, comparison, etc.
We distinguish between
regular and
canonical serializations. While
a PEG may have more than one regular serialization only exactly one of them
will be
canonical.
- regular serialization
- [1]
- The serialization of any PEG is a nested Tcl dictionary.
- [2]
- This dictionary holds a single key, pt::grammar::peg, and its
value. This value holds the contents of the grammar.
- [3]
- The contents of the grammar are a Tcl dictionary holding the set of
nonterminal symbols and the starting expression. The relevant keys and
their values are
- rules
- The value is a Tcl dictionary whose keys are the names of the nonterminal
symbols known to the grammar.
- [1]
- Each nonterminal symbol may occur only once.
- [2]
- The empty string is not a legal nonterminal symbol.
- [3]
- The value for each symbol is a Tcl dictionary itself. The relevant keys
and their values in this dictionary are
- is
- The value is the serialization of the parsing expression describing the
symbols sentennial structure, as specified in the section PE
serialization format.
- mode
- The value can be one of three values specifying how a parser should handle
the semantic value produced by the symbol.
- value
- The semantic value of the nonterminal symbol is an abstract syntax tree
consisting of a single node node for the nonterminal itself, which has the
ASTs of the symbol's right hand side as its children.
- leaf
- The semantic value of the nonterminal symbol is an abstract syntax tree
consisting of a single node node for the nonterminal, without any
children. Any ASTs generated by the symbol's right hand side are
discarded.
- void
- The nonterminal has no semantic value. Any ASTs generated by the symbol's
right hand side are discarded (as well).
- start
- The value is the serialization of the start parsing expression of the
grammar, as specified in the section PE serialization format.
- [4]
- The terminal symbols of the grammar are specified implicitly as the set of
all terminal symbols used in the start expression and on the RHS of the
grammar rules.
- canonical serialization
- The canonical serialization of a grammar 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
grammar.
- [1]
- The keys found in all the nested Tcl dictionaries are sorted in ascending
dictionary order, as generated by Tcl's builtin command lsort
-increasing -dict.
- [2]
- The string representation of the value is the canonical representation of
a Tcl dictionary. I.e. it does not contain superfluous whitespace.
EXAMPLE¶
Assuming the following PEG for simple mathematical expressions
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;
then its canonical serialization (except for whitespace) is
pt::grammar::peg {
rules {
AddOp {is {/ {t -} {t +}} mode value}
Digit {is {/ {t 0} {t 1} {t 2} {t 3} {t 4} {t 5} {t 6} {t 7} {t 8} {t 9}} mode value}
Expression {is {x {n Term} {* {x {n AddOp} {n Term}}}} mode value}
Factor {is {/ {x {t (} {n Expression} {t )}} {n Number}} mode value}
MulOp {is {/ {t *} {t /}} mode value}
Number {is {x {? {n Sign}} {+ {n Digit}}} mode value}
Sign {is {/ {t -} {t +}} mode value}
Term {is {x {n Factor} {* {x {n MulOp} {n Factor}}}} mode value}
}
start {n Expression}
}
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>