'\"
'\" Generated from file 'from\&.inc' by tcllib/doctools with format 'nroff'
'\" Copyright (c) 2009 Andreas Kupries <andreas_kupries@users\&.sourceforge\&.net>
'\"
.TH "pt::peg::from::peg" n 1\&.0\&.4 tcllib "Parser Tools"
.\" The -*- nroff -*- definitions below are for supplemental macros used
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.el \}\
\h'-1.5n'\L'|\\n(^yu-1v'\h'\\n(^lu+3n'\L'\\n(^tu+1v-\\n(^yu'\l'|0u-1.5n\(ul'
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.LP
.nf
.ta 5.5c 11c
.ft B
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See the \\*(So manual entry for details on the standard options.
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Command-Line Name:	\\fB\\$1\\fR
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Database Class:	\\fB\\$3\\fR
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.BS
.SH NAME
pt::peg::from::peg \- PEG Conversion\&. Read PEG format
.SH SYNOPSIS
package require \fBTcl 8\&.5 9\fR
.sp
package require \fBpt::peg::from::peg ?1\&.0\&.4?\fR
.sp
\fBpt::peg::from::peg\fR \fBconvert\fR \fItext\fR
.sp
.BE
.SH DESCRIPTION
.PP
Are you lost ?
Do you have trouble understanding this document ?
In that case please read the overview provided by the
\fIIntroduction to Parser Tools\fR\&. This document is the
entrypoint to the whole system the current package is a part of\&.
.PP
This package implements the converter from PEG markup to
parsing expression grammars\&.
.PP
It resides in the Import section of the Core Layer of Parser Tools,
and can be used either directly with the other packages of this layer,
or indirectly through the import manager provided by
\fBpt::peg::import\fR\&. The latter is intented for use in untrusted
environments and done through the corresponding import plugin
\fBpt::peg::import::peg\fR sitting between converter
and import manager\&.
.PP
IMAGE: arch_core_iplugins
.PP
.SH API
The API provided by this package satisfies the specification of the
Converter API found in the \fIParser Tools Import API\fR
specification\&.
.TP
\fBpt::peg::from::peg\fR \fBconvert\fR \fItext\fR
This command takes the PEG markup encoding a parsing
expression grammar and contained in \fItext\fR, and generates the
canonical serialization of said grammar, as specified in section
\fBPEG serialization format\fR\&.
The created value is then returned as the result of the command\&.
.PP
.SH "PEG SPECIFICATION LANGUAGE"
\fBpeg\fR, a language for the specification of parsing expression
grammars is meant to be human readable, and writable as well, yet
strict enough to allow its processing by machine\&. Like any computer
language\&. It was defined to make writing the specification of a
grammar easy, something the other formats found in the Parser Tools do
not lend themselves too\&.
.PP
It is formally specified by the grammar shown below, written in
itself\&. For a tutorial / introduction to the language please go and
read the \fIPEG Language Tutorial\fR\&.
.PP
.CS


PEG pe_grammar_for_peg (Grammar)

	# --------------------------------------------------------------------
        # Syntactical constructs

        Grammar         <- WHITESPACE Header Definition* Final EOF ;

        Header          <- PEG Identifier StartExpr ;
        Definition      <- Attribute? Identifier IS Expression SEMICOLON ;
        Attribute       <- (VOID / LEAF) COLON ;
        Expression      <- Sequence (SLASH Sequence)* ;
        Sequence        <- Prefix+ ;
        Prefix          <- (AND / NOT)? Suffix ;
        Suffix          <- Primary (QUESTION / STAR / PLUS)? ;
        Primary         <- ALNUM / ALPHA / ASCII / CONTROL / DDIGIT / DIGIT
                        /  GRAPH / LOWER / PRINTABLE / PUNCT / SPACE / UPPER
                        /  WORDCHAR / XDIGIT
                        / Identifier
                        /  OPEN Expression CLOSE
                        /  Literal
                        /  Class
                        /  DOT
                        ;
        Literal         <- APOSTROPH  (!APOSTROPH  Char)* APOSTROPH  WHITESPACE
                        /  DAPOSTROPH (!DAPOSTROPH Char)* DAPOSTROPH WHITESPACE ;
        Class           <- OPENB (!CLOSEB Range)* CLOSEB WHITESPACE ;
        Range           <- Char TO Char / Char ;

        StartExpr       <- OPEN Expression CLOSE ;
void:   Final           <- "END" WHITESPACE SEMICOLON WHITESPACE ;

        # --------------------------------------------------------------------
        # Lexing constructs

        Identifier      <- Ident WHITESPACE ;
leaf:   Ident           <- ([_:] / <alpha>) ([_:] / <alnum>)* ;
        Char            <- CharSpecial / CharOctalFull / CharOctalPart
                        /  CharUnicode / CharUnescaped
                        ;

leaf:   CharSpecial     <- "\\\\" [nrt'"\\[\\]\\\\] ;
leaf:   CharOctalFull   <- "\\\\" [0-3][0-7][0-7] ;
leaf:   CharOctalPart   <- "\\\\" [0-7][0-7]? ;
leaf:   CharUnicode     <- "\\\\" 'u' HexDigit (HexDigit (HexDigit HexDigit?)?)? ;
leaf:   CharUnescaped   <- !"\\\\" \&. ;

void:   HexDigit        <- [0-9a-fA-F] ;

void:   TO              <- '-'           ;
void:   OPENB           <- "["           ;
void:   CLOSEB          <- "]"           ;
void:   APOSTROPH       <- "'"           ;
void:   DAPOSTROPH      <- '"'           ;
void:   PEG             <- "PEG" !([_:] / <alnum>) WHITESPACE ;
void:   IS              <- "<-"    WHITESPACE ;
leaf:   VOID            <- "void"  WHITESPACE ; # Implies that definition has no semantic value\&.
leaf:   LEAF            <- "leaf"  WHITESPACE ; # Implies that definition has no terminals\&.
void:   SEMICOLON       <- ";"     WHITESPACE ;
void:   COLON           <- ":"     WHITESPACE ;
void:   SLASH           <- "/"     WHITESPACE ;
leaf:   AND             <- "&"     WHITESPACE ;
leaf:   NOT             <- "!"     WHITESPACE ;
leaf:   QUESTION        <- "?"     WHITESPACE ;
leaf:   STAR            <- "*"     WHITESPACE ;
leaf:   PLUS            <- "+"     WHITESPACE ;
void:   OPEN            <- "("     WHITESPACE ;
void:   CLOSE           <- ")"     WHITESPACE ;
leaf:   DOT             <- "\&."     WHITESPACE ;

leaf:   ALNUM           <- "<alnum>"    WHITESPACE ;
leaf:   ALPHA           <- "<alpha>"    WHITESPACE ;
leaf:   ASCII           <- "<ascii>"    WHITESPACE ;
leaf:   CONTROL         <- "<control>"  WHITESPACE ;
leaf:   DDIGIT          <- "<ddigit>"   WHITESPACE ;
leaf:   DIGIT           <- "<digit>"    WHITESPACE ;
leaf:   GRAPH           <- "<graph>"    WHITESPACE ;
leaf:   LOWER           <- "<lower>"    WHITESPACE ;
leaf:   PRINTABLE       <- "<print>"    WHITESPACE ;
leaf:   PUNCT           <- "<punct>"    WHITESPACE ;
leaf:   SPACE           <- "<space>"    WHITESPACE ;
leaf:   UPPER           <- "<upper>"    WHITESPACE ;
leaf:   WORDCHAR        <- "<wordchar>" WHITESPACE ;
leaf:   XDIGIT          <- "<xdigit>"   WHITESPACE ;

void:   WHITESPACE      <- (" " / "\\t" / EOL / COMMENT)* ;
void:   COMMENT         <- '#' (!EOL \&.)* EOL ;
void:   EOL             <- "\\n\\r" / "\\n" / "\\r" ;
void:   EOF             <- !\&. ;

        # --------------------------------------------------------------------
END;

.CE
.SS EXAMPLE
Our example specifies the grammar for a basic 4-operation calculator\&.
.PP
.CS


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;

.CE
.PP
Using higher-level features of the notation, i\&.e\&. the character
classes (predefined and custom), this example can be rewritten as
.PP
.CS


PEG calculator (Expression)
    Sign       <- [-+]                                          ;
    Number     <- Sign? <ddigit>+                               ;
    Expression <- Term (AddOp Term)*                            ;
    MulOp      <- [*/]						;
    Term       <- Factor (MulOp Factor)*                        ;
    AddOp      <- [-+]						;
    Factor     <- '(' Expression ')' / Number                   ;
END;

.CE
.PP
.SH "PEG SERIALIZATION FORMAT"
Here we specify the format used by the Parser Tools to serialize
Parsing Expression Grammars as immutable values for transport,
comparison, etc\&.
.PP
We distinguish between \fIregular\fR and \fIcanonical\fR
serializations\&.
While a PEG may have more than one regular serialization only exactly
one of them will be \fIcanonical\fR\&.
.TP
regular serialization
.RS
.IP [1]
The serialization of any PEG is a nested Tcl dictionary\&.
.IP [2]
This dictionary holds a single key, \fBpt::grammar::peg\fR, and its
value\&. This value holds the contents of the grammar\&.
.IP [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
.RS
.TP
\fBrules\fR
The value is a Tcl dictionary whose keys are the names of the
nonterminal symbols known to the grammar\&.
.RS
.IP [1]
Each nonterminal symbol may occur only once\&.
.IP [2]
The empty string is not a legal nonterminal symbol\&.
.IP [3]
The value for each symbol is a Tcl dictionary itself\&. The relevant
keys and their values in this dictionary are
.RS
.TP
\fBis\fR
The value is the serialization of the parsing expression describing
the symbols sentennial structure, as specified in the section
\fBPE serialization format\fR\&.
.TP
\fBmode\fR
The value can be one of three values specifying how a parser should
handle the semantic value produced by the symbol\&.
.RS
.TP
\fBvalue\fR
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\&.
.TP
\fBleaf\fR
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\&.
.TP
\fBvoid\fR
The nonterminal has no semantic value\&. Any ASTs generated by the
symbol's right hand side are discarded (as well)\&.
.RE
.RE
.RE
.TP
\fBstart\fR
The value is the serialization of the start parsing expression of the
grammar, as specified in the section \fBPE serialization format\fR\&.
.RE
.IP [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\&.
.RE
.TP
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\&.
.RS
.IP [1]
The keys found in all the nested Tcl dictionaries are sorted in
ascending dictionary order, as generated by Tcl's builtin command
\fBlsort -increasing -dict\fR\&.
.IP [2]
The string representation of the value is the canonical representation
of a Tcl dictionary\&. I\&.e\&. it does not contain superfluous whitespace\&.
.RE
.PP
.SS EXAMPLE
Assuming the following PEG for simple mathematical expressions
.PP
.CS


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;

.CE
.PP
then its canonical serialization (except for whitespace) is
.PP
.CS


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}
}

.CE
.PP
.SH "PE SERIALIZATION FORMAT"
Here we specify the format used by the Parser Tools to serialize
Parsing Expressions as immutable values for transport, comparison,
etc\&.
.PP
We distinguish between \fIregular\fR and \fIcanonical\fR
serializations\&.
While a parsing expression may have more than one regular
serialization only exactly one of them will be \fIcanonical\fR\&.
.TP
Regular serialization
.RS
.TP
\fBAtomic Parsing Expressions\fR
.RS
.IP [1]
The string \fBepsilon\fR is an atomic parsing expression\&. It matches
the empty string\&.
.IP [2]
The string \fBdot\fR is an atomic parsing expression\&. It matches
any character\&.
.IP [3]
The string \fBalnum\fR 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 \fBstring is\fR\&.
.IP [4]
The string \fBalpha\fR is an atomic parsing expression\&. It matches
any Unicode alphabet character\&. This is a custom extension of PEs
based on Tcl's builtin command \fBstring is\fR\&.
.IP [5]
The string \fBascii\fR 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 \fBstring is\fR\&.
.IP [6]
The string \fBcontrol\fR is an atomic parsing expression\&. It matches
any Unicode control character\&. This is a custom extension of PEs based
on Tcl's builtin command \fBstring is\fR\&.
.IP [7]
The string \fBdigit\fR 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 \fBstring is\fR\&.
.IP [8]
The string \fBgraph\fR 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 \fBstring is\fR\&.
.IP [9]
The string \fBlower\fR 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 \fBstring is\fR\&.
.IP [10]
The string \fBprint\fR 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 \fBstring is\fR\&.
.IP [11]
The string \fBpunct\fR is an atomic parsing expression\&. It matches
any Unicode punctuation character\&. This is a custom extension of PEs
based on Tcl's builtin command \fBstring is\fR\&.
.IP [12]
The string \fBspace\fR is an atomic parsing expression\&. It matches
any Unicode space character\&. This is a custom extension of PEs based
on Tcl's builtin command \fBstring is\fR\&.
.IP [13]
The string \fBupper\fR 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 \fBstring is\fR\&.
.IP [14]
The string \fBwordchar\fR 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 \fBstring is\fR\&.
.IP [15]
The string \fBxdigit\fR is an atomic parsing expression\&. It matches
any hexadecimal digit character\&. This is a custom extension of PEs
based on Tcl's builtin command \fBstring is\fR\&.
.IP [16]
The string \fBddigit\fR is an atomic parsing expression\&. It matches
any decimal digit character\&. This is a custom extension of PEs based
on Tcl's builtin command \fBregexp\fR\&.
.IP [17]
The expression
[list t \fBx\fR]
is an atomic parsing expression\&. It matches the terminal string \fBx\fR\&.
.IP [18]
The expression
[list n \fBA\fR]
is an atomic parsing expression\&. It matches the nonterminal \fBA\fR\&.
.RE
.TP
\fBCombined Parsing Expressions\fR
.RS
.IP [1]
For parsing expressions \fBe1\fR, \fBe2\fR, \&.\&.\&. the result of
[list / \fBe1\fR \fBe2\fR \&.\&.\&. ]
is a parsing expression as well\&.
This is the \fIordered choice\fR, aka \fIprioritized choice\fR\&.
.IP [2]
For parsing expressions \fBe1\fR, \fBe2\fR, \&.\&.\&. the result of
[list x \fBe1\fR \fBe2\fR \&.\&.\&. ]
is a parsing expression as well\&.
This is the \fIsequence\fR\&.
.IP [3]
For a parsing expression \fBe\fR the result of
[list * \fBe\fR]
is a parsing expression as well\&.
This is the \fIkleene closure\fR, describing zero or more
repetitions\&.
.IP [4]
For a parsing expression \fBe\fR the result of
[list + \fBe\fR]
is a parsing expression as well\&.
This is the \fIpositive kleene closure\fR, describing one or more
repetitions\&.
.IP [5]
For a parsing expression \fBe\fR the result of
[list & \fBe\fR]
is a parsing expression as well\&.
This is the \fIand lookahead predicate\fR\&.
.IP [6]
For a parsing expression \fBe\fR the result of
[list ! \fBe\fR]
is a parsing expression as well\&.
This is the \fInot lookahead predicate\fR\&.
.IP [7]
For a parsing expression \fBe\fR the result of
[list ? \fBe\fR]
is a parsing expression as well\&.
This is the \fIoptional input\fR\&.
.RE
.RE
.TP
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\&.
.RS
.IP [1]
The string representation of the value is the canonical representation
of a pure Tcl list\&. I\&.e\&. it does not contain superfluous whitespace\&.
.IP [2]
Terminals are \fInot\fR encoded as ranges (where start and end of the
range are identical)\&.
.RE
.PP
.PP
.SS EXAMPLE
Assuming the parsing expression shown on the right-hand side of the
rule
.PP
.CS


    Expression <- Term (AddOp Term)*

.CE
.PP
then its canonical serialization (except for whitespace) is
.PP
.CS


    {x {n Term} {* {x {n AddOp} {n Term}}}}

.CE
.PP
.SH "BUGS, IDEAS, FEEDBACK"
This document, and the package it describes, will undoubtedly contain
bugs and other problems\&.
Please report such in the category \fIpt\fR of the
\fITcllib Trackers\fR [http://core\&.tcl\&.tk/tcllib/reportlist]\&.
Please also report any ideas for enhancements you may have for either
package and/or documentation\&.
.PP
When proposing code changes, please provide \fIunified diffs\fR,
i\&.e the output of \fBdiff -u\fR\&.
.PP
Note further that \fIattachments\fR are strongly preferred over
inlined patches\&. Attachments can be made by going to the \fBEdit\fR
form of the ticket immediately after its creation, and then using the
left-most button in the secondary navigation bar\&.
.SH KEYWORDS
EBNF, LL(k), PEG, TDPL, context-free languages, conversion, expression, format conversion, grammar, matching, parser, parsing expression, parsing expression grammar, push down automaton, recursive descent, serialization, state, top-down parsing languages, transducer
.SH CATEGORY
Parsing and Grammars
.SH COPYRIGHT
.nf
Copyright (c) 2009 Andreas Kupries <andreas_kupries@users\&.sourceforge\&.net>

.fi
