2. Tags

2.1. Tags in template files

A tag is a string found in the template page and surrounded by a specific set of characters. The default is @_ at the start and _@ at the end of the tag. This default can be changed using Set_Tag_Separators routine, see Templates Parser API Reference. Note that it must be changed as the first API call and should not be changed after that.

The tag will be replaced by a value specified in the Ada code. In this context, the role of the Ada code is therefore to prepare what is known as a translation, and then pass it to the templates parser, along with the name of the template file to parse. This results in an expanded version of the templates file, where all tags have been replaced by the value given in the Ada code.

Let’s start with a simple example. Here is the contents of the file demo.tmplt, which is a very basic template file:

<p>Hello @_NAME_@

On its own, this template has little interest. However, it is used from some Ada code similar to the following demo.adb file:

with Ada.Text_IO;
with Templates_Parser;

procedure Demo is
   Translations : constant Templates_Parser.Translate_Table :=
     (1 => Templates_Parser.Assoc ("NAME", "Ada"));
begin
   Ada.Text_IO.Put_Line
     (Templates_Parser.Parse ("demo.tmplt", Translations));
end Demo;

Compile this program, link with the templates parser, and when you run it, the output will be:

<p>Hello Ada

As you can imagine, this is a bare bone example. More complex structures are of course possible. One thing to note, though, is that the template file requires no Ada knowledge for editing, and is strongly related to your application domain. One of the main usage for such templates is to generate web pages. This can be done by a designer that knows nothing of how your Ada code works. But you can use templates in other domains, including to generate Ada code.

2.2. Translations

In your Ada code, you can associate one or more values with a name, and then reference that name in the template file as we just saw above.

Associating the value(s) with the name is done through one of the Assoc constructors, see Templates Parser API Reference. Ada’s overloading resolution mechanism will take care of calling the appropriate constructor automatically.

These associations are then grouped into a dictionary. This dictionary is passed along with the name of the template file to the Parse routine itself, which generates the final expanded representation of the template. In fact, you will almost never have to manipulate an association directly, since as soon as it is created you store it in the dictionary.

There are two types of dictionaries in the templates parser:

Translate_Table

This is an array of associations. If you know the exact number of associations when you write your code, this will generally provide a very readable code. The array can be initialized as soon as it is declared:

declare
   T : constant Translate_Table :=
         (1 => Assoc ("NAME1", Value1),
          2 => Assoc ("NAME2", Value2));
begin
   Put_Line (Parse ("demo.tmplt", T));
end;

Translate_Set

If, on the other hand, you do not know statically the number of associations, it is generally a lot more flexible to use another type of dictionary, which isn’t limited in size. It is also better to use this type of dictionary if you need extra code to compute the values:

declare
   T : Translate_Set
begin
   Insert (T, Assoc ("NAME1", Value1));
   Insert (T, Assoc ("NAME2", Value2));
end;

Internally, the templates parser will always convert all dictionaries to a Translate_Set, which is much more efficient when we need to look values up.

2.3. Discrete, Boolean and Composite values

As we just saw, the values by which a tag is replaced must be provided by the Ada code. Such values can be provided in different formats, depending on the intended use.

The three kinds of tags are discrete, Boolean and composite tags. These are all ways to associate one or more value to a name, which is the name used in the template file.

discrete values

This represents a single value associated with a name. The types of value currently supported are String, Unbounded_String and Integer:

Insert (T, Assoc ("NAME", 2));
Insert (T, Assoc ("NAME", "VALUE"));

Boolean values

These are similar to discrete values. However, they are more convenient to manipulate within @@IF@@ statements in the template. When outside an IF statement, such values are represented as TRUE or FALSE:

Insert (T, Assoc ("NAME", True));

composite values

A composite tag is a variable which contains a set of values. In terms of programming languages, these would generally be called vectors. Since each value within that can itself be a composite tag, you can therefore build multi-dimensional arrays.

These kind of variables will be used with the TABLE tag statement see TABLE statement. Outside a table statement, the tag is replaced by all values concatenated with a specified separator. See Set_Separator routine. Such tag are variables declared in the Ada program a Templates_Parser.Tag type.

There are many overloaded constructors to build a composite tags (see “+” operators). The “+” operators are used to build a Tag item from standard types like String, Unbounded_String, Character, Integer and Boolean.

To add items to a Tag many overloaded operators are provided (see “&” operators). The “&” operators add one item at the start or the end of the tag. It is possible to directly add String, Unbounded_String, Character, Integer and Boolean items using one of the overloaded operator.

A tag composed of only Boolean values TRUE or FALSE is called a Boolean composite tag. This tag is to be used with a IF tag statement inside a TABLE tag statement.

It is possible to build a composite tag having any number of nested level. A vector is a composite tag with only one level, a matrix is a composite tag with two level (a Tag with a set of vector tag).

Two aliases exists for composite tags with one or two nested level, they are named Vector_Tag and Matrix_Tag. In the suite of the document, we call vector tag a tag with a single nested level and matrix tag a tag with two nested level:

--  Building a composite tag
--  Then add it into a translate set

declare
   V : Tag;
   T : Translate_Set;
begin
   for Index in 1 .. 10 loop
      V := V & I;
   end loop;
   Insert (T, Assoc ("VECTOR", V));
end;

2.4. Filters

Within the template file, functions can be applied to tags. Such functions are called filters. These filters might require one or more parameters, see the documentation for each filter.

The syntax is:

@_[[FILTER1_NAME[(parameter)]:]FILTER2_NAME[(parameter)]:]SOME_VAR_@

When multiple filters are associated to a tag, they are evaluated from right to left. In the example above, FILTER1_NAME is applied to the result of applying FILTER2_NAME to SOME_VAR.

Remember that one of the goals in using templates is to remove as much hard-coded information from the actual Ada source, and move it into easily editable external files. Using filters is a convenient way to give the template designer the power to specify the exact output he wants, even without changing the Ada code. For instance, imagine that one suddenly decides that some names should be capitalized in a template. There are two solutions to such a change in design:

• Modify the Ada code to capitalize strings before storing them in a tag variable. What if, in the template, we need the name once capitalized, and once with its original casing ? This means the Ada code would have to create two tags.

• Modify the template itself, and use a filter. A single tag is required on the Ada side, which doesn’t even have to be changed in fact. The template would for instance become:

  @_CAPITALIZE:VAR_@ : constant String := "@_VAR_@";
  

The templates parser comes with a number of predefined filters, that can be used in various situations. Some of these are highly specialized, but most of them are fairly general. You can also define your own filters, adapted to specific needs you might have.

Here are some more examples using the predefined filters, If VAR is set to “vector_tag”, ONE to “1” and TWO to “2” then:

@_VAR_@                                         ->  vector_tag
@_UPPER:VAR_@                                   ->  VECTOR_TAG
@_CAPITALIZE:VAR_@                              ->  Vector_Tag
@_EXIST:VAR_@                                   ->  TRUE
@_UPPER:REVERSE:VAR_@                           ->  GAT_ROTCEV
@_MATCH(VEC.*):UPPER:VAR_@                      ->  TRUE
@_SLICE(1..6):VAR_@                             ->  vector
@_REPLACE(([^_]+)):VAR_@                        ->  vector
@_REPLACE(([a-z]+)_([a-z]+)/\\\\2_\\\\1):VAR_@  ->  tag_vector
@_"+"(TWO):ONE_@                                ->  3
@_"-"(TWO):ONE_@                                -> -1

2.4.1. Predefined filters

Here is the complete list of predefined filters that come with the templates parser.

“+”(N) or ADD(N)

Add N to variable and return the result. If the current variable value is not a number it returns the empty string. N must be a number or a discrete tag variable whose value is a number.

“-“(N) or SUB(N)

Subtract N to variable and return the result. If the current variable value is not a number it returns the empty string. N must be a number or a discrete tag variable whose value is a number.

“*”(N) or MULT(N)

Multiply N with variable and return the result. If the current variable value is not a number it returns the empty string. N must be a number or a discrete tag variable whose value is a number.

“/”(N) or DIV(N)

Divide variable by N and return the result. If the current variable value is not a number it returns the empty string. N must be a number or a discrete tag variable whose value is a number.

ABS

Returns the absolute value.

ADD_PARAM(NAME[=VALUE])

Add a parameter into an URL. This routine adds the ‘?’ and ‘&’ character if needed. VALUE can be a tag variable name.

BR_2_EOL(EOL)

Replaces all occurrences of the <br> HTML tag by a line terminator determined by EOL. EOL must be either CR (Carriage-Return), LF (Line-Feed), LFCR (Line-Feed followed by Carriage-Return) or CRLF (Carriage-Return followed by Line-Feed).

BR_2_LF

Shortcut for BR_2_EOL(LF).

CAPITALIZE

Put all characters in the variable in lower case except characters after a space or an underscore which are set in upper-case.

CLEAN_TEXT

Keep only letters and digits all others characters are changed to spaces.

COMA_2_POINT

Replaces all comas by points.

CONTRACT

Converts any suite of spaces by a single space character.

DEL_PARAM(NAME)

Delete parameter NAME from the URL. This routine removes the ‘?’ and ‘&’ character if needed. Returns the input string as-is if the parameter is not found.

EXIST

Returns True if variable is set and has a value different that the null string and False otherwise.

FILE_EXISTS

Returns True if variable is set and has a value that corresponds to a file name present on the file system and False otherwise.

FORMAT_DATE(FORMAT)

Returns the date with the given format. The date must be in the ISO format (YYYY-MM-DD) eventually followed by a space and the time with the format HH:MM:SS. If the date is not given in the right format it returns the date as-is. The format is using the GNU/date description patterns as also implemented in GNAT.Calendar.Time_IO.

Characters:

%: a literal %

n: a newline

t: a horizontal tab

Time fields:

%H: hour (00..23)

%I: hour (01..12)

%k: hour ( 0..23)

%l: hour ( 1..12)

%M: minute (00..59)

%p: locale’s AM or PM

%r: time, 12-hour (hh:mm:ss [AP]M)

%s: seconds since 1970-01-01 00:00:00 UTC (a nonstandard extension)

%S: second (00..59)

%T: time, 24-hour (hh:mm:ss)

Date fields:

%a: locale’s abbreviated weekday name (Sun..Sat)

%A: locale’s full weekday name, variable length (Sunday..Saturday)

%b: locale’s abbreviated month name (Jan..Dec)

%B: locale’s full month name, variable length (January..December)

%c: locale’s date and time (Sat Nov 04 12:02:33 EST 1989)

%d: day of month (01..31)

%D: date (mm/dd/yy)

%h: same as %b

%j: day of year (001..366)

%m: month (01..12)

%U: week number of year with Sunday as first day of week (00..53)

%w: day of week (0..6) with 0 corresponding to Sunday

%W: week number of year with Monday as first day of week (00..53)

%x: locale’s date representation (mm/dd/yy)

%y: last two digits of year (00..99)

%Y: year with four digits (1970…)

Padding:

By default, date pads numeric fields with zeroes. GNU date recognizes the following nonstandard numeric modifiers:

- (hyphen): do not pad the field

_ (underscore): pad the field with spaces

FORMAT_NUMBER([DIGITSEP])

Returns the number with a separator added between each 3 digits blocks. The decimal part is not transformed. If the data is not a number nothing is done. The default separator is a space, although you can specify any separator (a single character) you wish. DIGITSEP can also be the name of another tag, whose value (or the first character of it) will be used as a separator.

IS_EMPTY

Returns True if variable is the empty string and False otherwise.

LF_2_BR

Replaces all occurrences of the character LF (Line-Feed) by a <br> HTML tag.

LOWER

Put all characters in the variable in lower-case.

MATCH(REGEXP)

Returns True if variable match the regular expression passed as filter’s parameter. The regular expression is using a format as found in gawk, sed or grep tools.

MAX(N)

Returns the maximum value between the variable and the parameter.

MIN(N)

Returns the minimum value between the variable and the parameter.

MOD(N)

Returns variable modulo N. If the current variable value is not a number it returns the empty string. N must be a number or a discrete tag variable whose value is a number.

NEG

Change the sign of the value.

NO_DYNAMIC

This is a special command filter which indicates that the tag must not be searched in the dynamic tags. See Lazy_Tag. NO_DYNAMIC must be the first filter. This filter returns the value as-is.

NO_DIGIT

Replaces all digits by spaces.

NO_LETTER

Replaces all letters by spaces.

NO_SPACE

Removes all spaces in the variable.

OUI_NON

If variable value is True it returns Oui, if False it returns Non, otherwise does nothing. It keeps the way True/False is capitalized (all upper, all lower or first letter capital).

POINT_2_COMA

Replaces all comas by points.

REPEAT(N)

Returns N times the variable, N being passed as filter’s parameter. N must be a number or a discrete tag variable whose value is a number.

REPEAT(STR)

This is the second REPEAT form. In this case STR is repeated a number of time corresponding to the variable value which must be a number.

REPLACE(REGEXP[/STR])

This filter replaces \n (where n is a number) STR’s occurrences by the corresponding match from REGEXP. The first match in REGEXP will replace \1, the second match \2 and so on. Each match in REGEXP must be parenthesized. It replaces only the first match. STR is an optional parameter, its default value is \1. It is possible to escape characters in STR to avoid parsing confusions. This is required if you need to have @_ or _@ or a parenthesis in STR for example. STR can be a tag variable name. STR can contain the following escaped characters : \n Carriage Return, \r Line Feed and \t for Horizontal Tabulation.

REPLACE_ALL(REGEXP[/STR])

Idem as above but replaces all occurrences.

REPLACE_PARAM(NAME[=VALUE])

This is filter is equivalent to ADD_PARAM(NAME[=VALUE]):DEL_PARAM(NAME). VALUE can be a tag variable name.

REVERSE

Reverse the string.

SIZE

Returns the size (number of characters) of the string value.

SLICE(x .. y)

Returns the sub-string starting from position x and ending to position y. Note that the string to slice always start from position 1. If x or y are negative, they are counted from the end of the string, so that 0 matches the last character of the string, -1 matches the character just before,…

TRIM

Removes leading and trailing spaces.

UPPER

Put all characters in the variable in upper-case.

WEB_ENCODE

As WEB_ESCAPE and also encodes all non 7-bit characters and non printable characters using &#xxx; HTML encoding.

WEB_ESCAPE

Replaces characters ‘<’, ‘>’, ‘”’ and ‘&’ by corresponding HTML sequences: &lt; &gt; &quot; and &amp;

WEB_NBSP

Replaces all spaces by an HTML non breaking space.

WRAP(N)

Wraps lines having more N characters.

YES_NO

If variable value is True it returns Yes, if False it returns No, otherwise does nothing. It keeps the way True/False is capitalized (all upper, all lower or first letter capital).

2.4.2. User defined filters

It is also possible to define a new filter by registering a callback routine associated with the filter name.

You can define three kinds of filters: filters that take no argument, and are therefore simply used as in @_FILTER:TAG_@, filters that take one or more arguments, used as in @_FILTER(param1,param2):TAG_@, and filters that are implemented as tagged objects, and take the same form as the filters with arguments described above.

The latter form of filters (using tagged types) provides slightly more flexibility, as you can store your own user data in the filter when it is registered. Among other things, this makes it possible to share filters between various applications, when the filter needs to access some application-specific variable as well.

The templates parser will not try to interpret the parameters for you, and will simply return the string representation of the list of parameters, for instance “param1,param2” in the example above. This provides enhanced flexibility, since you are free to use any parameter-separator you want, and to interpret parameters as integer, strings, references to other tags,…

The templates parser doesn’t support tag substitution within the parameter list, but this is trivial to implement in your own code. For instance, if the user has used @_FILTER(REFTAG):TAG_@, you are free to either take REFTAG as a constant string, or as a reference to another tag, to be looked up in a translation table. You should of course properly document the behavior of your filter.

Here is the templates parser API for defining your own custom filters:

type Filter_Context is record
   Translations : Translate_Set;
   Lazy_Tag     : Dynamic.Lazy_Tag_Access;
end record;

type Callback is access function
  (Value      : in String;
   Parameters : in String;
   Context    : in Filter_Context) return String;
--  User's filter callback

type Callback_No_Param is access function
  (Value   : in String;
   Context : in Filter_Context) return String;
--  User's filter callback

procedure Register_Filter
  (Name    : in String;
   Handler : in Callback);
--  Register user's filter Name using the specified Handler

procedure Register_Filter
  (Name    : in String;
   Handler : in Callback_No_Param);
--  Register user's filter Name using the specified Handler

In the above calls, Value is the value of the tag on which the filter applies. In the examples above, that would be the value of TAG as looked up in the translation table. Context contains the the translation table and the current lazy tag object you can use if you need to look up other tags.

Here is a simple example of a custom filter, which can be used to generate HTML forms. In such a form, it is common to have some <input> tags that need a selected=’selected’ attribute if the toggle button should be selected. This can be done without the use of a filter, of course, using a simple @@IF@@ statement, but that makes the template less readable. The custom filter below behaves as such: it takes one argument, and compares the value of the tag on which the filter is applied to that argument. If they are equal, the string selected=’selected’ will be substituted. As a special case, if the argument to the filter starts with a ‘@’ character, the argument is interpreted as the name of a tag to look up first:

function Custom_Select_Filter
  (Value      : in String;
   Parameters : in String;
   Context    : in Filter_Context) return String is
begin
   if Parameters /= "" and then Parameters (Parameters'First) = '@' then
      if Get (Get (Context.Translations,
                   Parameters (Parameters'First + 1 .. Parameters'Last)))
        = Value
      then
         return "selected='selected'";
      end if;

   elsif Value = Parameters then
      return "selected='selected'";
   end if;

   return "";
end Custom_Select_Filter;

Register_Filter ("SELECTED", Custom_Select_Filter'Access);

and a template would look like:

<option value="foo" @_SELECTED(@SELECTED_STATUS):STATUS_@ />

2.5. Attributes

In addition to filters, you can also apply attributes to composite tags. Attributes are placed after the tag name and preceded with a simple quote. @_SOME_VAR[‘ATTRIBUTE_NAME]_@. It is possible to use filters and attributes together. In that case the attribute is first evaluated and the result is passed-through the filters.

You cannot define your own attributes.

Current supported attributes are:

V’length

Returns the number of item in the composite tag (can be applied only to a composite tag having a single nested level - a vector).

V’Up_Level(n)

Use index from the table command n level(s) upper so this attribute must be used in a nested table command tag. ‘Up_Level is equivalent to ‘Up_Level(1) (can be applied only to a composite tag having a single nested level - a vector).

M’Line

Returns the number of line in the composite tag. This is identical to ‘Length but can be applied only to a composite tag having two nested level - a matrix).

M’Min_Column

Returns the size of smallest composite tag in M composite tag. This attribute can be applied only to a composite tag having two nested level - a matrix.

M’Max_Column

Returns the size of largest composite tag in M composite tag. This attribute can be applied only to a composite tag having two nested level - a matrix.

For example:

If VEC is set to “<1 , 2>” and MAT to “<a, b, c> ; <2, 3, 5, 7>” then:

@_VEC'Length_@              ->  2
@_ADD(3):VEC'Length_@       ->  5
@_MAT'Line_@                ->  2
@_MAT'Min_Column_@          ->  3
@_MAT'Max_Column_@          ->  4

2.6. Predefined tags

There are some specific tags that can be used in any templates. Here is an exhaustive list:

NOW

Current date and time with format “YYYY-MM-DD HH:MM:SS”.

YEAR

Current year number using 4 digits.

MONTH

Current month number using 2 digits.

DAY

Current day number using 2 digits.

HOUR

Current hour using range 0 to 23 using 2 digits.

MINUTE

Current minute using 2 digits.

SECOND

Current seconds using 2 digits.

MONTH_NAME

Current full month name (January .. December).

DAY_NAME

Current full day name (Monday .. Sunday).

2.7. Dynamic tags

Dynamic tags are associations that are not created when Parse is called, but only later on when they are actually needed.

Dynamic tags are handled through abstract interfaces and give the opportunity to create tags dynamically while the template is being parsed.

2.7.1. Lazy_Tag

The Lazy_Tag object can be used to dynamically handle tags. Such object can be passed to the Parse routines. If a template’s tag is not found in the translation dictionary, the Lazy_Tag’s Value callback method is called by the parser. The default callback method does nothing, it is up to the user to define it. The callback procedure is defined as follow:

procedure Value
  (Lazy_Tag     : access Dynamic.Lazy_Tag;
   Var_Name     : in     String;
   Translations : in out Translate_Set) is abstract;
--  Value is called by the Parse routines below if a tag variable was not
--  found in the set of translations. This routine must then add the
--  association for variable Name. It is possible to add other
--  associations in the translation table but a check is done to see if
--  the variable Name as been set or not. The default implementation does
--  nothing.

One common usage is to handle tag variables that can be shared by many templates and are not always used (because a conditional is False for example). If computing the corresponding value (or values for a …) is somewhat expensive it is better to delay building such tag at the point it is needed. Using a Lazy_Tag object, it is possible to do so. The Value procedure will be called if the tag value is needed. At this point, one can just add the corresponding association into the Translate_Set. Note that it is possible to add more than one association. If the association for Var_Name is not given, this tag has no value.

Value will be called only once per template and per tag. This is so that if the value for the tag is expensive to compute, you only pay the price once, and the value is then cached for the remaining of the template. If the value should be recomputed every time, you should consider using a Cursor_Tag instead (see Cursor_Tag).

2.7.2. Cursor_Tag

In some cases, data structure on the Ada side can be so complex that it is difficult to map it into a variable tag. The Cursor_Tag object has been designed to work around such problem. Using a Cursor_Tag it is possible to create an iterator through a data structure without mapping it into a variable tag. The data stays on the Ada side. To create a Cursor_Tag it is necessary to implement the following abstract routines:

function Dimension
  (Cursor_Tag : access Dynamic.Cursor_Tag;
   Var_Name   : in     String) return Natural is abstract;
--  Must return the number of dimensions for the given variable name. For
--  a matrix this routine should return 2 for example.

type Path is array (Positive range <>) of Natural;
--  A Path gives the full position of a given element in the cursor tag

function Length
  (Cursor_Tag : access Dynamic.Cursor_Tag;
   Var_Name   : in     String;
   Path       : in     Dynamic.Path) return Natural is abstract;
--  Must return the number of item for the given path. The first
--  dimension is given by the Path (1), for the second column the Path is
--  (1, 2). Note that each dimension can have a different length. For
--  example a Matrix is not necessary square.

function Value
  (Cursor_Tag : access Dynamic.Cursor_Tag;
   Var_Name   : in     String;
   Path       : in     Dynamic.Path) return String is abstract;
--  Must return the value for the variable at the given Path. Note that
--  this routine will be called only for valid items as given by the
--  dimension and Length above.