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.