NAME¶
pt::ast - Abstract Syntax Tree Serialization
SYNOPSIS¶
package require
Tcl 8.5
package require
pt::ast ?1.1?
::pt::ast verify serial ?
canonvar?
::pt::ast verify-as-canonical serial
::pt::ast canonicalize serial
::pt::ast print serial
::pt::ast bottomup cmdprefix ast
cmdprefix ast
::pt::ast topdown cmdprefix pe
::pt::ast equal seriala serialb
::pt::ast new0 s loc ?
child...?
::pt::ast new s start end ?
child...?
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 abstract
syntax trees as managed by the Parser Tools, and specified in section
AST
serialization format.
This is a supporting package in the Core Layer of Parser Tools.
IMAGE: arch_core_support
API¶
- ::pt::ast verify serial ?canonvar?
- This command verifies that the content of serial is a valid
serialization of an abstract syntax tree 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 AST serialization
format.
- ::pt::ast verify-as-canonical serial
- This command verifies that the content of serial is a valid
canonical serialization of an abstract syntax tree 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 AST
serialization format.
- ::pt::ast canonicalize serial
- This command assumes that the content of serial is a valid
regular serialization of an abstract syntax and will throw an error
if that is not the case.
It will then convert the input into the canonical serialization of
the contained tree 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 AST serialization format.
- ::pt::ast print serial
- This command assumes that the argument serial contains a valid
serialization of an abstract syntax tree and returns a string containing
that tree 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 AST serialization
format.
- ::pt::ast bottomup cmdprefix ast
- This command walks the abstract syntax tree ast from the bottom up
to the root, invoking the command prefix cmdprefix for each node.
This implies that the children of a node N are handled before N.
The command prefix has the signature
- cmdprefix ast
- I.e. it is invoked with the ast node the walk is currently at.
The result returned by the command prefix replaces ast in the node it
was a child of, allowing transformations of the tree.
This also means that for all inner node the contents of the children
elements are the results of the command prefix invoked for the children of
this node.
- ::pt::ast topdown cmdprefix pe
- This command walks the abstract syntax tree ast from the root down
to the leaves, invoking the command prefix cmdprefix for each node.
This implies that the children of a node N are handled after N.
The command prefix has the same signature as for bottomup, see above.
The result returned by the command prefix is ignored.
- ::pt::ast equal seriala serialb
- This command tests the two sbstract syntax trees seriala and
serialb for structural equality. The result of the command is a
boolean value. It will be set to true if the trees are identical,
and false otherwise.
String equality is usable only if we can assume that the two trees are pure
Tcl lists.
- ::pt::ast new0 s loc ?child...?
- This command command constructs the ast for a nonterminal node refering
refering to the symbol s at position loc in the input, and
the set of child nodes child ..., from left right. The latter may
be empty. The constructed node is returned as the result of the command.
The end position is loc-1, i.e. one character before the start.
This type of node is possible for rules containing optional parts.
- ::pt::ast new s start end
?child...?
- This command command constructs the ast for a nonterminal node refering to
the symbol s covering the range of positions start to
end in the input, and the set of child nodes child ..., from
left right. The latter may be empty. The constructed node is returned as
the result of the command.
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
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>