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HTML::YaTmpl - Yet Another Template Processor |
<:/>code=...
HTML::YaTmpl - Yet Another Template Processor
use HTML::YaTmpl;
my $t=HTML::YaTmpl->new( file=>'template.tmpl' );
$t->evaluate( key1=>$value1,
key2=>[$val21, $val22, ...]
... );
$t->evaluate_to_file( $outputfilename,
key1=>$value1,
key2=>[$val21, $val22, ...]
... );
HTML::YaTmpl aims mainly to provide a HTML template processor that saves
the template writer typing.
There are general template processors like Text::Template and tools to
embed perl in HTML like HTML::Embperl or HTML template processors
like HTML::Template. Why have I decided to start yet another? Well,
Text::Template is not really convenient when it comes to process
repeating data records like HTML tables. With HTML::Embperl no professional
``WEB Designer'' will be able to ``enhance'' the pages. And HTML::Template
enforces a strict division of design and programming. Thus, it enforces
changes to the programming logic even if you only want to exchange a long
number like 2835067264068365493 with a more human readable
2,835,067,264,068,365,493.
HTML::YaTmpl attempts to make simple things easy but complexity feasible.
HTML::YaTmpl follows the object oriented paradigm, i.e. you have to
create a template processor prior to using it. A template processor is not
bound to any particular template. You can use one processor to evaluate
various templates. But other properties like error handling are bound to
the processor.
HTML::YaTmpl object. These attributes can be set:
template() and open() methods.
new()
causes the open() method to be called. If open() fails new() will return
undef instead of an object.
HTML_TMPL_SEARCH_PATH is split by your path
separator (see the Config module documentation) (on UNIX
a ':' on Windows ';'). The path attribute is used by open().
HTML::YaTmpl wraps all code fragments from templates into subroutines and
calls then these subroutines. Normally the subroutines are cached to avoid
multiple calls of the perl interpreter. If no_eval_cache is set the cache
is turned off. This can be useful for long running applications that process
user provided templates. On the other hand there is the method
clear_cache() to clear that cache from time to time.
gz is defined.
evaluate() returns compressed output. evaluate_to_file() writes
compressed output. The output file name is extented with a suffix indicating
the compression method.
Only the tailing characters of the given string are evaluated to select the compression method but the complete string is used as file name suffix. Thus:
$t->compress='.gz' or $t->compress='-gz' or $t->compress='gz'
all select the gzip compression method. But
$t->evaluate_to_file( "out.html" ) will write out.html.gz or
out.html-gz or out.htmlgz respectivly.
If the constructor is called as instance method rather than as class method the new object inherits all attributes from the old instance. Naturally, explicitely named attributes are overridden.
These methods have got the :lvalue attribute. They can be
assigned using $self->method=$new_value syntax. Called without
parameter they return the actual attribute value. Calling with parameter
sets the attribute value.
template() or open())
and returns the result. Note, simply setting the current file via
$self->file=$newfile will not change the current template. Only after
a call to the open() method the template is changed.
$private_data is an optional argument that can be
used to pass additional data to and from the template. From within the
template it is accessible as $p.
All other parameters are key=>value pairs that provide variables to
the template. Internally they are gathered in a hash that is accessible
from within the template as $h.
evaluate() and writes the result to $file. If $file
is a GLOB reference the evaluation result is written to that file handle.
If it is a CODE reference the referenced function is called with the
evaluation result as $_[0]. The return value is returned. If $file
is an object and the object UNIVERSAL::can print() then that print method
is called. Otherwise $file is interpreted as the name of a file where
the evaluation result is to be written. If the template evaluation throws
an exception $file remains untouched. evaluate_to_file() returns false
if something went wrong, e.g. no space left on device or no permission to
write the file.
<=key1>Value1</=key1> <=key2>Value2</=key2>
evaluate_as_config() will return an hash reference:
{ key1=>'Value1', key2=>'Value2' }
Note, this function is experimental.
HTML::YaTmpl uses a cache to avoid multiple compilation of the same
code fragment or multiple parsing of the same template.
Simply hashing each compiled code or template fragment can lead to memory
leaks for long running processes that use user provided templates. To
prevent that the cache is assigned a high and a low water mark. If the
number of cache elements reaches the high water mark elements are
deleted on a LRU basis so that it drops to the low water mark.
The cache is not bound to a HTML::YaTmpl object but is shared by all objects.
The cache is implemented as 2 independent HASHes for compiled code and template fragments respectively. The high and low water marks are use for both of them.
HTML::YaTmpl-cache_highwatermark=$newvalue>.
HTML::YaTmpl-cache_lowwatermark=$newvalue>.
HTML::YaTmpl object without any template and later
set the file and possibly the path attribute and call open() to
set the template from the file's content.
open() can be called with arguments:
$self->open( file=>$filename, path=>$path );
to set the current file and template path in one call. Both parameters are
optional.
If the current file cannot be read for any reason open() returns undef.
$! will indicate the reason.
If a template file was read successfully $self is returned.
As for other templare processors a template is a file containing normal content and some special template sequences that will be exchanged with computed values during template evaluation. Since this is mainly a HTML template engine these sequences are chosen similar to HTML tags. There are 3 kinds of sequences:
Template sequences as other HTML/XML sequences are opened with an opening tag and closed with a closing tag, e.g.
<=variable_name parameter_list> body </=variable_name>
But if body is empty this sequence can be abbreviated to
<=variable_name parameter_list/>
where the trailing /> instead of the simple > is essential.
Thus, here come some valid template sequences:
name provided to evaluate() instead of <=name/>.
type is set. It specifies that this sequence should be
expanded only if the value provided to evaluate as name is a reference
to a non-empty array. In that case the string consisting of all elements of
the array concatenated is inserted. Otherwise the template sequence is simply
deleted from the output.
other_template.tmpl. This new evaluation inherits all variables from
the current evaluation and adds 2 new variables a and x with the values
b and y.
other_template.tmpl
sees is a.
The basic form of variable substitution is <=name/> or
<=name></=name>. Given the template:
some text '<=var/>' other text
If evaluate is called as
$t->evaluate( var=>'computed text' );
it will return
some text 'computed text' other text
But you can provide also an ARRAY reference to evaluate instead of the scalar to achieve the same result.
$t->evaluate( var=>['com', 'pu', 'ted', ' ', 'text'] );
How does this work? If evaluate receives an ARRAY ref as a variable's value
it sort of evaluates <=var/> for each array element and concatenates
the results. But you can specify that a variable substitution should be done
only if the provided value is of a particular type. 4 such types are available
ARRAY and length() returns something not equal zero) it is substituted.
Otherwise an empty string is substitued.
ARRAY and it consists of at least one element) it is substituted.
Otherwise an empty string is substitued.
The template
some text '<=var type=scalar/>' other text
will show the result seen above only if evaluate() is called as
$t->evaluate( var=>'computed text' );
The other case will produce
some text '' other text
If a variable substitution is valid for multiple types they can be concatenated with a comma (,):
<=var type=scalar,array/>
is evaluated if var is an array or a scalar but not if it's empty.
Further,
<=var type=given>Link</=var>
evaluates to Link if var is a non-empty list or a non-empty string.
Even if var is a list containing multiple values only one occurence of
Link is substituted.
There are cases when it would be really useful to modify the substituted
value a little from within the template. Imagine you want to make big
numbers more readable (52345635476 should be displayed as 52,345,635,476)
or you want to substitute a list (passed as array reference, see above) and
don't want the elements simply be concatenated but displayed as HTML list
elements (<li>element</li>). HTML::YaTmpl provides 2 ways for doing
this.
<:/>Given the template
fruits comprise <ul> <=fruits><li><:/></li> </=fruits></ul>
the call
$t->evaluate( fruits=>[qw{apples pears plums cherries}] );
will generate
fruits comprise <ul> <li>apples</li> <li>pears</li> <li>plums</li> <li>cherries</li> </ul>
Now we are using the long form of variable substitution (<=var></=var>). The
tag body describes the actual substitution. Within the tag body the control
sequence <:/> stands for the actual value. Of course <:/> can
be given several times within a substitution body:
<=fruits><:/> and <:/> give <:/> </=fruits>but men and women give children
generates
apples and apples give apples pears and pears give pears plums and plums give plums cherries and cherries give cherries but men and women give children
The same result can be achieved using Perl inside the <:/> control
sequence:
<=fruits><:"$v and $v give $v\n"/></=fruits>but men and women give children
Now we see the <:/> control sequence in action. It can contain perl
code fragments that change the substituted value. The code fragment is called
with $v set to the actual variable. If the evaluate() call was passed the
optional $private_data parameter it is available as $p from within these
code fragments. Otherwise $p points to a HASH that is created once
for each evaluate() call. Thus, code fragments can communicate with each
other using this hash:
<=fruits><: ++$p->{fruitcounter} />. <:/>
</=fruits>total: <:$p->{fruitcounter}/> fruits
produces
1. apples 2. pears 3. plums 4. cherries total: 4 fruits
Maybe you have noticed the total counter in the last line was generated without
any surrounding <=var></=var> sequence. Yes, that works too. In this
case $v is undef.
code=...By now the most examples have used the long variable substitution form
(<=var>...</=var>). In most cases this is probably the *right*
thing but you can use the short form even when modifying the substitution
value:
<=fruits code="<: ++$p->{fruitcounter} />. <:/>
"/>total: <:$p->{fruitcounter}/> fruits
will produce exactly the same result as above but it's almost not readable. Whereas:
<html><body>
<table>
<=fruits code="<tr><td><: ++$p->{counter} /></td><td><:/></td></tr>"/>
<tr><td>total number of fruits</td><td><:$p->{counter}/></td></tr>
</table>
</body></html>
generate perfectly valid (but not easy human readable) HTML text.
You see, the code=... parameter to a <=var .../> sequence does the
trick. It can comprehend any text but should in most cases be surrounded by
double quote characters ("). Within this surrounding double quote
and backslash characters must be quoted with backslashes, all other can.
Thus:
<=fruits code="<li><:\"\\u$v\"/></li>"/>
generates a HTML list of capitalized fruits:
<li>Apples</li><li>Pears</li><li>Plums</li><li>Cherries</li>
However, I believe, this kind of code fragments is the wrong way. But it can be
even worse. You can omit the surrounding double quotes but then you must
quote almost anything except characters matching \w with backslashes.
The previous example without surrounding quotes looks like:
<=fruits code=\<li\>\<:\"\\u$v\"/\>\</li\>/>
When a list is to be substituted often special treatment is required for the first and the last list element or some text should be prepended or appended to the substitution result:
<=fruits first="<: ucfirst $v/>" last=" and <:/>" code=", <:/>"/> are fruits.
generates
Apples, pears, plums and cherries are fruits.
You see, if there are first=... and last=... parameters to a
variable substitution they affect the first respectively last list element.
On the other side there are pre=... and post=... parameters. They
are evaluated before the first respectivly after the last list element:
<=fruits pre="<select name=\"fruit\"><:\"\\n\"/>"
code="<option><:/></option><:\"\\n\"/>"
post="</select><:\"\\n\"/>"/>
produces
<select name="fruit"> <option>apples</option> <option>pears</option> <option>plums</option> <option>cherries</option> </select>
Using the long substitution form these examples would look like:
<=fruits> <:first><:"\u$v"/></:first> <:last> and <:/></:last> <:code>, <:/></:code> </=fruits> are fruits.
respectively:
<=fruits> <:pre><select name="fruit"> </:pre> <:post></select> </:post> <:code><option><:/></option> </:code> </=fruits>
I think, the <:first>, <:last>, <:prev> and
<:post> semantics are intuitively clear. <:code> needs some
explanation. In the previous examples using the long form the text between
<=var> and </=var> has described what to substitute. That will
remain to work. If no <:code> section is found all text between
<=var> and </=var> save the control sequences will be used.
But this results in many unnecessary newlines since the previous example
without <:prev> and <:post> look like:
<=fruits>
# empty line
# empty line
<option><:/></option>
</=fruits>
Thus, <:code> can be used for convenience.
Perl knows list operations such as map, grep and sort. These are
also useful in templates. Why the program logic should know about the
order in which a list is displayed? It must supply the list. That's it.
Just to arrange our fruits alphabetically we can write:
<=fruits sort="$a cmp $b"> <:code><:/>, </:code> <:last><:/></:last> </=fruits>
and get
apples, cherries, pears, plums
But can we order reverse fruits, i.e. selppa instead of apples?
<=fruits map="scalar reverse $_" sort="$a cmp $b"> <:code><:/>, </:code> <:last><:/></:last> </=fruits>
give
seirrehc, selppa, smulp, sraep
But I want:
cherries, apples, plums, pears
ok, here comes the template:
<=fruits map="scalar reverse $_"
sort="$a cmp $b"
map="scalar reverse $_">
<:code><:/>, </:code>
<:last><:/></:last>
</=fruits>
But I don't like plums! Ok then:
<=fruits grep="!/plum/i"
map="scalar reverse $_"
sort="$a cmp $b"
map="scalar reverse $_">
<:code><:/>, </:code>
<:last><:/></:last>
</=fruits>
results in
cherries, apples, pears
Of course these parameters can also be written in long form and even mixed:
<=fruits grep="!/plum/i"> <:map>scalar reverse $_</:map> <:sort>$a cmp $b</:sort> <:code><:/>, </:code> <:last><:/></:last> </=fruits>
produces
seirrehc, selppa, sraep
There can be as many as you like grep/sort/map fragments. The source list
is first processed by the fragments passed as parameters
(<=var grep=... map=... sort=... ...>) in left to right order and
then by the fragments given in long form (<:sort>...</:sort>) in top
down order.
Suppose you want to create a WEB application with an input field to put
the name of a town. The user would first see a simple
<input type="text"> field. He types in some characters and submits
the form. Now your program matches the user input with a database. There
can be 3 results. The user input can non-ambiguously match a database record
or there are several matches or no match at all. In the first case your
program should answer the user with a page simply showing the matching
record. In the second case it should display a select field and in the 3rd
the <input type="text"> field. Your template would look like:
<=town type=empty><input type="text" name="town"></=town>
<=town type=array pre="<select name=\"town\">"
post="</select>">
<option><:/></option>
</=town>
<=town type=scalar><b><:/></b></=town>
and you get a text field town is passed as undef, an empty string
or an empty array. A selection field is generated if town is passed
as a non-empty array and the town set in bold is produced if it is passed
as non-empty string.
Many control sequences have been shown in the previous chapter. Here the are listed again:
<: some perl code /> and
<:> some perl code </:> are equivalent. Within the perl code the
variables $v, $p and $h can be used. $v contains the current
variable to be substituted. $p holds the private_data parameter. $h
points at a HASH containing all parameters passed to the current scope (see
<:eval> for an example of using it).
<:eval> or <:for> blocks. This can be used for convenience.
<:for> statement is used to evaluate a part of the template
with changed parameters. For example you want to make up a HTML table that
is passed as list of lists:
$t->evaluate( fruit_colors=>[[qw{apples red/green}],
[qw{pears green}],
[qw{plums blue/yellow}],
[qw{cherries red}]] );
with the template
<table> <=fruit_colors><:for f="<:/>"> <:code><tr><=f><td><:/></td></=f></tr> </:code> </:for></=fruit_colors></table>
generates
<table> <tr><td>apples</td><td>red/green</td></tr> <tr><td>pears</td><td>green</td></tr> <tr><td>plums</td><td>blue/yellow</td></tr> <tr><td>cherries</td><td>red</td></tr> </table>
Here the opening <=fruit_colors> begins a scope of substituting
fruit_colors. As fruit_colors is an array the tag body is evaluated
for each element. The tag body contains a single <:for> statement
used to assign f temporarily the current value of fruit_colors, i.e.
f is assigned in turn each element of the fruit_colors list. Within
to <:for> statement we see the evaluation of <=f> surrounded
by <tr>. As f is also an array it generated <td>'s for
each element.
The <:for> statement features a template within a template. The inner
template can be given as a <:code> statement or if omitted the body
of the tag is used. Thus, the following template is almost equivalent to the
previous one:
<table> <=fruit_colors><:for f="<:/>"> <tr><=f><td><:/></td></=f></tr> </:for></=fruit_colors></table>
In fact it generates an empty line before each table row.
The parameter list for the inner template (the one that is passed to the
evaluate() function) is completely made anew. In our example the inner
template receives only one variable: f. But there is a way to bequeath
all current variables to the inner template. Just put the reserved word
:inherit or :inheritparms in the parameter list of the <:for>
statement. With
<:for f="<:/>" :inherit>
the inner template would see also fruit_colors and all other outer
variables.
All parameters containing an unquoted equal sign (=) are used to set up the parameter list for the include template. The simplest form of a parameter is a string like
<:for some_fruit="plum">
Slightly more complex is passing an outer variable with an other name:
<:for inner_fruits="<=fruits/>">
This evaluates fruits and stores the result as inner_fruits.
If fruits is an array so does inner_fruits. If the substitution
statement of fruits contains pre or post components inner_fruits
will contain them as first / last list element. You also can surround the
variable substitution with plain text. In this case each element of
inner_fruits gets surrounded by this text. And you can include in the
definition of one inner variable multiple outer variables. In this case
each outer list variable is expanded and the resulting number of list
elements is the mathematical product of the numbers of elements of all
outer lists. The template (Note: The sequence <#.../> is a comment
and used in this example to hide the newline in the template. For more
details on comments see below.):
<:for inner_fruits="PRE <=fruits pre=pre1
post=post1
grep=\"/l/\"/> <#
/>BETWEEN <=fruits pre=pre2
post=post2
grep=\"/r/\"/> POST">
<:code><=inner_fruits last="(<:/>)">(<:/>)
</=inner_fruits></:code>
generates a total of 16 lines:
(PRE pre1 BETWEEN pre2 POST) (PRE pre1 BETWEEN pears POST) (PRE pre1 BETWEEN cherries POST) (PRE pre1 BETWEEN post2 POST) (PRE apples BETWEEN pre2 POST) (PRE apples BETWEEN pears POST) (PRE apples BETWEEN cherries POST) (PRE apples BETWEEN post2 POST) (PRE plums BETWEEN pre2 POST) (PRE plums BETWEEN pears POST) (PRE plums BETWEEN cherries POST) (PRE plums BETWEEN post2 POST) (PRE post1 BETWEEN pre2 POST) (PRE post1 BETWEEN pears POST) (PRE post1 BETWEEN cherries POST) (PRE post1 BETWEEN post2 POST)
Due to the grep statements the first expansion of fruits contains
only elements with an l letter, i.e. apples and plums, whereas
the second expansion consists of elements with an r letter, i.e pears
and cherries.
As shown each fruits list is expanded with a pre and post elements.
Thus, the total number of elements of the inner_fruits list is 4*4=16.
By now we have seen in this chapter assigning of simple strings and
expanded arrays to inner variables. But what happens if <:for> is
placed within a variable substitution scope like in the prefacing example
to this chapter? The template (provided with line numbers)
1 <:>
2 sub fac {
3 use Math::BigInt;
4 my $x=shift;
5 my $res=Math::BigInt->new(1);
6 for( my $i=1; $i<=$x; $i++ ) {
7 $res*=$i;
8 }
9 return $res;
10 }
11 </:><html><body>
12 <table>
13 <:for x="<:[map {[$_, $_**2, $_**3, fac $_]} 1..30]/>"
14 h="<:[qw{n n^2 n^3 n!}]/>">
15 <:code>
16 <tr>
17 <=h><th><:/></th> </=h>
18 </tr>
19 <=x>
20 <:code>
21 <tr>
22 <:for y="<:/>">
23 <:code><=y><td><:/></td> </=y></:code>
24 </:for>
25 </tr>
26 </:code>
27 </=x></:code>
28 </:for></table>
29 </body></html>
generates a HTML page containing a table of the first 30 square and
cube numbers and factorials. The first control sequence (lines 1-11) is
substituted with nothing as the value of this code fragment is undef. It
just defines a fac function. Lines 13 and 14 opens a <:for> scope
that is closed at line 28. Within this scope the variables x and h are
valid. Both are list variables since the <:/> control sequences return
ARRAY references. Within that scope at line 17 h is evaluated to a list
of <th> statements. That is more or less what we have seen before.
But now in line 19 a variable substitution scope is opened. That means for
each element of the x list lines 21 to 25 are evaluated repeatedly. Since
x consists of arrays the evaluation of <:/> produces an array.
That is used in a nested <:for> scope at lines 22 to 24 to
subsequently assign to a variable y each of them. Now the body of our
nested <:for> can evaluate y and create a list of <td>
statements.
By now we have seen parameter lists formed like
<:for a="b" c="d" ... />
This can lead to a lot of quoting backslashes within strings. A little
foreboding give the last inner_fruits example. There we had to write
grep=\"/l/\" to quote the double quotes. This can be avoided using the
<:set> control sequence.
<:for>
<:set inner_fruits>PRE <=fruits pre=pre1
post=post1
grep="/l/"/> <#
/>BETWEEN <=fruits pre=pre2
post=post2
grep="/r/"/> POST</:set>
<:code><=inner_fruits last="(<:/>)">(<:/>)
</=inner_fruits></:code>
</:for>
generates exactly the same result without quoting any character.
Well, I believe, these examples are enough to show what can be done with
<:for>. BTW, all these examples are contained in the file
t/5_fruits.t of the distribution. The module is tested against them.
<:for> statement but resulting text is
evaluated again. This can be useful in some rare cases, e.g.:
1 <:for fruits="<:[qw/apple pear/]/>" 2 books="<:['The Silmarillion', 3 'The Lord of the Rings', 4 'The Hobbit or There And Back Again']/>"><# 5 /><:eval what="<:[qw/book fruit/]/>"><# 6 /><=what><# 7 /><=<:/>s> 8 <<#/>:pre>Select a <: ucfirst $v />: 9 <select name="<:/>"><<#/>/:pre> 10 <<#/>:post></select> 11 <<#/>/:post> 12 <<#/>:code><option><<#/>:/></option> 13 <<#/>/:code> 14 </=<:/>s><# 15 /></=what><# 16 /></:eval><# 17 /></:for>
Consider you get a lot of specialized variables that need to be treated all
the same way. In the example above the outer <:for> scope built with
lines 1-4 and 17 creates 2 variables. For both of them a <select> box
should be generated. Of course one could write
<=books>...</=books><=fruits>...</=fruits>
but it's also possible to let the template system generate the actual template
and then evaluate it. That's what <:eval> is for. Within the
<:eval> scope between lines 5 and 16 the variable what contains a
list of 2 elements book and fruit. Variable substitution then occurs
at lines 7, 14, 8 and 9. Lines 7 and 14 are evaluated to <=books> and
<=fruits> and </=books> and </=fruits> respectively thus
creating something looking very similar to a template. Further, all comments
are eliminated. Here the <#/> comments are important. They prevent
constructs like <<#/>:/> to be evaluated as <:/> in the
first run. They sort of quote template sequences.
In fact the generated template is:
<=books> <:pre>Select a Book: <select name="book"> </:pre> <:post></select> </:post> <:code><option><:/></option> </:code> </=books><=fruits> <:pre>Select a Fruit: <select name="fruit"> </:pre> <:post></select> </:post> <:code><option><:/></option> </:code> </=fruits>
This template is then evaluated with the parameter list of the <:eval>'s
outer scope, i.e. it sees fruits and books but not what.
One could argument that the same result can be achieved avoiding an intermediate template. In fact
1 <:for fruits="<:[qw/apple pear/]/>"
2 books="<:['The Silmarillion',
3 'The Lord of the Rings',
4 'The Hobbit or There And Back Again']/>"><#
5 /><:for what="<:[qw/book fruit/]/>" :inherit><#
6 /><=what><#
7 />Select a <: ucfirst $v />:
8 <select name="<:/>">
9 <:for el="<:$h->{$v.'s'}/>" :inherit><#
10 /><=el>
11 <:code><option><:/></option>
12 </:code>
13 </=el></select>
14 </:for><#
15 /></=what><#
16 /></:for><#
17 /></:for>
produces exactly the same output. But it needs the special variable $h
to do the trick. Anyhow, <:eval> is useful if parts of a template
are fetched from a database for example.
All other parameters are used to make up the parameter list for the include
template, see <:for> above. The tag body
is ignored save <:set>...</:set> statements which are also used to
form the parameter list.
cond statement or C's switch. The template
1 <:for> 2 <:set goods><: 3 [ 4 [apple=>'300'], 5 [pear=>'90'], 6 [cherry=>'82'], 7 [plum=>'120'], 8 ] 9 /></:set> 10 <:code><=goods pre="<table>" post=" 11 </table>"> 12 <tr><:for x="<:/>"><=x><td><:/></td></=x><td></:for><:cond> 13 <:case "$v->[1]>150"><b>very expensive</b></:case> 14 <:case "$v->[1]<100"><b>bargain</b></:case> 15 <:case 1>normal prize</:case> 16 </:cond></td></tr></=goods></:code> 17 </:for>
generates a table of goods marking a few of them as very expensive or bargain
buy depending on their prizes. The <=goods> statement at line 10 opens
a variable substitution scope. Within this scope at line 12 starts a
<:cond> statement thus $v references to one list element. The
<:case> statements at lines 13, 14 and 15 are then evaluated top down.
The first <:case>'es body whose condition evaluates to true builds
the value of the whole <:cond> statement.
The example above shows <:cond> without parameters. If it is called
with parameters they name variables that are to be used in the <:case>
conditions. Thus you can compare more than one variable in <:case>es.
Consider you often want to display thumbnails and only some of them should link to the originals. It would be useful to write an include template that can be called:
<:include thumb.tmpl name=img321 link=yes/>
to create a thumbnail that links to it's original or
<:include thumb.tmpl name=img322/>
to create a thumbnail without a link.
thumb.tmpl could look like:
1 <:cond link> 2 <:case "$link eq 'yes'"> 3 <a href="orig/<=name/>.jpg><img src="<=name/>.jpg"></a> 4 </:case> 5 <:case 1> 6 <img src="<=name/>.jpg"> 7 </:case> 8 </:cond>
Now the <:cond> at line 1 names link. Thus $link can be used
at line 2 in the <:case: condition.
<:defmacro> defines a macro. For example, one can define a macro as
<:defmacro td><td align="center"><=val/></td></:defmacro>
Later on it can be invoked as
<:m td val="..."/> or <:macro td val="..."/>
Our previous example then can be written as
1 <:defmacro td><td align="center"><=val/></td></:defmacro><# 2 /><:defmacro tr><tr><:for x="<:/>"><# 3 /><=x><:m td val="<:/>"/></=x></:for><:macro td> 4 <:set val><:cond> 5 <:case "$v->[1]>150"><b>very expensive</b></:case> 6 <:case "$v->[1]<100"><b>bargain</b></:case> 7 <:case 1>normal prize</:case> 8 </:cond></:set> 9 </:macro></tr> </:defmacro><# 10 /><:for> 11 <:set goods><: 12 [ 13 [apple=>'300'], 14 [pear=>'90'], 15 [cherry=>'82'], 16 [plum=>'120'], 17 ] 18 /></:set> 19 <:code><=goods pre="<table>" post=" 20 </table>"> 21 <:m tr/></=goods></:code> 22 </:for>
Line 1 defines a macro named td that prints one HTML table cell. It uses
the variable val. Lines 2 to 9 define a macro called tr that
implements a table row. It is designed to be used in a substitution scope
as it assigns the current value <:/> to a variable x to build
a <:for> scope. In line 3 the macro defined in line 1 is used twice,
once as <:m/> and once as <:macro>. Then in line 21 the tr
macro is invoked and generates all table rows.
Macros are bound to a HTML::YaTmpl instance. That means you can define a
set of macros in one template, evaluate it and then evaluate another template
with the same instance using macros defined in the first template.
<:for>, <:eval>,
<:include> and <:macro> statements. Outside one of these
scopes or inside a <:code> segment <:set> can be used to
manipulate the current parameter list.
Thus, the example above can be rewritten as:
1 <:set goods><: 2 [ 3 [apple=>'300'], 4 [pear=>'90'], 5 [cherry=>'82'], 6 [plum=>'120'], 7 ] 8 /></:set><=goods pre="<table>" post=" 9 </table>"> 10 <:m tr/></=goods>
reusing the macros as described above.
Writing templates is very similar to writing programs. And as programs should contain comments to be maintainable so do templates. You can even put POD sections into a template and generate documentation using POD translators.
When thinking about comments within templates I first thought that using
<:# any comment/>
or
<:># any comment</:>
would be appropriate. It invokes the perl interpreter to evaluate just
# any comment
which is a perl comment and evaluates to undef. Although it works it
can slow down template eveluation. Hence I decided to let comments look like
<# this is a comment />
or
<#> this is a comment </#>
Thus, to put a longer comment (POD section) into your template surround it
with <#> and </#>:
<#>
=head1 NAME
My very cute template
...
</#>
There are various error conditions that can occur while evaluating a template.
Operating system errors like Template File not found or No space left
on device can be recognized by the return codes of open() or
evaluate_to_file(). But what to do if the evaluation of a template code
fragment dies. Abort the whole process, insert nothing in place of the
failed fragment, ...? Well, that's the subject of this chapter.
There are 3 object attributes to specify what to do in case of an error during template evaluation.
onerror can be assigned the following values:
warn statement, continue template evaluation and
insert nothing instead of the failed fragment.
die statement aborting further
temlpate evaluation.
In either case the error message is appended to list of errors occured
during template evaluation. This list can be retrieved after evaluate()
has returned via the errors() method.
evaluate() using either clear_errors()
or $self->errors=[].
The template parser is completely based on perl regular expressions. Though I have spent some effort to let the parsing process finish quickly even if the template is erroneous, it is certainly possible to construct malicious templates that will be parsed infinitely. Please let me know if you find such a template. I will try to fix it.
Torsten Förtsch <Torsten.Foertsch@gmx.net>
Copyright 2003 Torsten Förtsch.
This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
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HTML::YaTmpl - Yet Another Template Processor |