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 terminal loc
::pt::ast nonterminal 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 terminal loc
- This command command constructs the ast for a terminal node
refering to the position loc in the input, and returns it as the
result of the command.
- ::pt::ast nonterminal 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
IMAGE: expr_ast
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>
