xenial (3) Template::Alloy::Parse.3pm.gz
NAME
Template::Alloy::Parse - Common parsing role for creating AST from templates
DESCRIPTION
The Template::Alloy::Parse role is reponsible for storing the majority of directive parsing code, as well
as for delegating to the TT, HTE, Tmpl, and Velocity roles for finding variables and directives.
ROLE METHODS
parse_tree
Used by load_tree. This is the main grammar engine of the program. It delegates to the syntax found
in $self->{'SYNTAX'} (defaults to 'alloy') and calls the function found in the $SYNTAX hashref. The
majority of these syntaxes use methods found in the $DIRECTIVES hashref to parse different DIRECTIVE
types for each particular syntax.
A template that looked like the following:
Foo
[%- GET foo -%]
[%- GET bar -%]
Bar
would parse to the following AST:
[
'Foo',
['GET', 6, 15, ['foo', 0]],
['GET', 22, 31, ['bar', 0]],
'Bar',
]
The "GET" words represent the directive used. The 6, 15 represent the beginning and ending
characters of the directive in the document. The remaining items are the variables necessary for
running the particular directive.
parse_expr
Used to parse a variable, an expression, a literal string, or a number. It returns a parsed variable
tree. Samples of parsed variables can be found in the VARIABLE PARSE TREE section.
my $str = "1 + 2 * 3";
my $ast = $self->parse_expr(\$str);
# $ast looks like [[undef, '+', 1, [[undef, '*', 2, 3], 0]], 0]
"parse_args"
Allow for the multitudinous ways that TT parses arguments. This allows for positional as well as
named arguments. Named arguments can be separated with a "=" or "=>", and positional arguments
should be separated by " " or ",". This only returns an array of parsed variables. To get the
actual values, you must call play_expr on each value.
"dump_parse_tree"
This method allows for returning a string of perl code representing the AST of the parsed tree.
It is mainly used for testing.
"dump_parse_expr"
This method allows for returning a Data::Dumper dump of a parsed variable. It is mainly used for
testing.
"parse_*"
Methods by these names are used by parse_tree to parse the template. These are the grammar. They
are used by all of the various template syntaxes Unless otherwise mentioned, these methods are not
exposed via the role.
VARIABLE PARSE TREE
Template::Alloy parses templates into an tree of operations (an AST or abstract syntax tree). Even
variable access is parsed into a tree. This is done in a manner somewhat similar to the way that TT
operates except that nested variables such as foo.bar|baz contain the '.' or '|' in between each name
level. Operators are parsed and stored as part of the variable (it may be more appropriate to say we are
parsing a term or an expression).
The following table shows a variable or expression and the corresponding parsed tree (this is what the
parse_expr method would return).
one [ 'one', 0 ]
one() [ 'one', [] ]
one.two [ 'one', 0, '.', 'two', 0 ]
one|two [ 'one', 0, '|', 'two', 0 ]
one.$two [ 'one', 0, '.', ['two', 0 ], 0 ]
one(two) [ 'one', [ ['two', 0] ] ]
one.${two().three} [ 'one', 0, '.', ['two', [], '.', 'three', 0], 0]
2.34 2.34
"one" "one"
1 + 2 [ [ undef, '+', 1, 2 ], 0]
a + b [ [ undef, '+', ['a', 0], ['b', 0] ], 0 ]
"one"|length [ [ undef, '~', "one" ], 0, '|', 'length', 0 ]
"one $a two" [ [ undef, '~', 'one ', ['a', 0], ' two' ], 0 ]
[0, 1, 2] [ [ undef, '[]', 0, 1, 2 ], 0 ]
[0, 1, 2].size [ [ undef, '[]', 0, 1, 2 ], 0, '.', 'size', 0 ]
['a', a, $a ] [ [ undef, '[]', 'a', ['a', 0], [['a', 0], 0] ], 0]
{a => 'b'} [ [ undef, '{}', 'a', 'b' ], 0 ]
{a => 'b'}.size [ [ undef, '{}', 'a', 'b' ], 0, '.', 'size', 0 ]
{$a => b} [ [ undef, '{}', ['a', 0], ['b', 0] ], 0 ]
a * (b + c) [ [ undef, '*', ['a', 0], [ [undef, '+', ['b', 0], ['c', 0]], 0 ]], 0 ]
(a + b) [ [ undef, '+', ['a', 0], ['b', 0] ]], 0 ]
(a + b) * c [ [ undef, '*', [ [undef, '+', ['a', 0], ['b', 0] ], 0 ], ['c', 0] ], 0 ]
a ? b : c [ [ undef, '?', ['a', 0], ['b', 0], ['c', 0] ], 0 ]
a || b || c [ [ undef, '||', ['a', 0], [ [undef, '||', ['b', 0], ['c', 0] ], 0 ] ], 0 ]
! a [ [ undef, '!', ['a', 0] ], 0 ]
Some notes on the parsing.
Operators are parsed as part of the variable and become part of the variable tree.
Operators are stored in the variable tree using an operator identity array which
contains undef as the first value, the operator, and the operator arguments. This
allows for quickly descending the parsed variable tree and determining that the next
node is an operator.
Parenthesis () can be used at any point in an expression to disambiguate precedence.
"Variables" that appear to be literal strings or literal numbers
are returned as the literal (no operator tree).
The following perl can be typed at the command line to view the parsed variable tree:
perl -e 'use Template::Alloy; print Template::Alloy->dump_parse_expr("foo.bar + 2")."\n"'
Also the following can be included in a template to view the output in a template:
[% USE cet = Template::Alloy %]
[%~ cet.dump_parse_expr('foo.bar + 2').replace('\s+', ' ') %]
AUTHOR
Paul Seamons <paul@seamons.com>
LICENSE
This module may be distributed under the same terms as Perl itself.