SYNOPSIS

  ### Functional interface

  use String::Print;           # simpelest way
  use String::Print qw/printi printp/, %config;
  printi 'age {years}', years => 12;

  # interpolation of arrays and hashes (serializers)
  printi 'price-list: {prices}', prices => \@p, _join => "+";
  printi 'dump: {c}', c => \%config;

  # same with positional parameters
  printp 'age %d", 12;
  printp 'price-list: %.2f', \@prices;
  printp 'dump: %s', \%settings;

  # modifiers
  printi 'price: {price%.2f}', price => 3.14*VAT*EURO;

  # [0.91] more complex interpolation names
  printi 'filename: {c.filename}', c => \%config;
  printi 'username: {user.name}', user => $user_object;
  printi 'price: {product.price €}', product => $db->product(3);

  ### Object Oriented interface

  use String::Print 'oo';      # import nothing 
  my $f = String::Print->new(%config);
  $f->printi('age {years}', years => 12);
  $f->printp('age %d', 12);

  ### via Log::Report's __* functions (optional translation)

  use Log::Report;             # or Log::Report::Optional
  print __x"age {years}", years => 12;

  ### via Log::Report::Template (Template Toolkit extension)

  [% SET name = 'John Doe' %]
  [% loc("Dear {name},") %]     # includes translation

DESCRIPTION

This module inserts values into (format) strings. It provides printf and sprintf alternatives via both an object oriented and a functional interface.

Read in the DETAILS chapter below, why this module provides a better alternative for printf(). Also, some extended examples can be found down there. Take a look at them first, when you start using this module!

DETAILS

Why use C, not C?

The printf() function is provided by Perl's CORE; you do not need to install any module to use it. Why would you use consider using this module?

Four components

To fill-in a FORMAT, four clearly separated components play a role:

Simplified:

  # sprinti() replaces "{$key$modifiers$conversion}" by
  $encode->($format->($serializer->($modifiers->($args{$key}))))

  # sprintp() replaces "%pos{$modifiers}$conversion" by
  $encode->($format->($serializer->($modifiers->($arg[$pos]))))

Example:

  #XXX Your manual-page reader may not support the unicode used
  #XXX in the examples below.
  printi "price: {price € %-10s}", price => $cost;
  printi "price: {price € %-10s}", { price => $cost };
  printp "price: %-10{€}s", $cost;

  $value      = $cost (in €)
  $modifier   = convert € to local currency £
  $serializer = show float as string
  $format     = column width %-10s
  $encode     = £ into £     # when encodingFor('HTML')

Interpolation: keys

A key is a bareword (like a variable name) or a list of barewords separated by dots (no blanks!)

Please use explanatory key names, to help the translation process once you need that (in the future).

Simple keys

A simple key directly refers to a named parameter of the function or method:

  printi "Username: {name}", name => 'John';

You may also pass them as HASH or CODE:

  printi "Username: {name}", { name => 'John' };
  printi "Username: {name}", name => sub { 'John' };
  printi "Username: {name}", { name => sub { 'John' } };
  printi "Username: {name}", name => sub { sub {'John'} };

The smartness of pre-processing CODE is part of serialization.

Complex keys

[0.91] In the previous section, we kept our addressing it simple: let's change that now. Two alternatives for the same:

  my $user = { name => 'John' };
  printi "Username: {name}", name => $user->{name}; # simple key
  printi "Username: {user.name}", user => $user;    # complex key

The way these complex keys work, is close to the flexibility of template toolkit: the only thing you cannot do, is passing parameters to called CODE.

You can pass a parameter name as HASH, which contains values. This may even be nested into multiple levels. You may also pass objects, class (package names), and code references.

In above case of user.name, when user is a HASH it will take the value which belongs to the key name. When user is a CODE, it will run code to get a value. When user is an object, the method name is called to get a value back. When user is a class name, the name refers to an instance method on that class.

More examples which do work:

  # when name is a column in the database query result
  printi "Username: {user.name}", user => $sth->fetchrow_hashref;

  # call a sub which does the database query, returning a HASH
  printi "Username: {user.name}", user => sub { $db->getUser('John') };

  # using an instance method (object)
  { package User;
    sub new  { bless { myname => $_[1] }, $_[0] }
    sub name { $_[0]->{myname} }
  }
  my $user = User->new('John');
  printi "Username: {user.name}", user => $user;

  # using a class method
  sub User::count   { 42 }
  printi "Username: {user.count}", user => 'User';

  # nesting, mixing
  printi "Complain to {product.factory.address}", product => $p;

  # mixed, here CODE, HASH, and Object
  printi "Username: {document.author.name}", document => sub {
    return +{ author => User->new('John') }
  };

Limitation: you cannot pass arguments to CODE calls.

Interpolation: Serialization

The 'interpolation' functions have named VARIABLES to be filled-in, but also additional OPTIONS. To distinguish between the OPTIONS and VARIABLES (both a list of key-value pairs), the keys of the OPTIONS start with an underscore _. As result of this, please avoid the use of keys which start with an underscore in variable names. On the other hand, you are allowed to interpolate OPTION values in your strings.

There is no way of checking beforehand whether you have provided all values to be interpolated in the translated string. When you refer to value which is missing, it will be interpreted as undef.

Interpolation: Modifiers

Modifiers are used to change the value to be inserted, before the characters get interpolated in the line. This is a powerful simplification. Let's discuss this with an example.

In traditional (gnu) gettext, you would write:

  printf(gettext("approx pi: %.6f\n"), PI);

to get PI printed with six digits in the fragment. Locale::TextDomain has two ways to achieve that:

  printf __"approx pi: %.6f\n", PI;
  print __x"approx pi: {approx}\n", approx => sprintf("%.6f", PI);

The first does not respect the wish to be able to reorder the arguments during translation (although there are ways to work around that) The second version is quite long. The string to be translated differs between the two examples.

With Log::Report, above syntaxes do work as well, but you can also do:

  # with optional translations
  print __x"approx pi: {pi%.6f}\n", pi => PI;

The base for __x() is the printi() provided by this module. Internally, it will call printi to fill-in parameters:

  printi "approx pi: {pi%.6f}\n", pi => PI;

Another example:

  printi "{perms} {links%2d} {user%-8s} {size%10d} {fn}\n",
     perms => '-rw-r--r--', links => 7, user => 'me',
     size => 12345, fn => $filename;

An additional advantage (when you use translation) is the fact that not all languages produce comparable length strings. Now, the translators can change the format, such that the layout of tables is optimal for their language.

Above example in printp() syntax, shorter but less maintainable:

  printp "%s %2d %-8s 10d %s\n",
     '-rw-r--r--', 7, 'me', 12345, $filename;

Interpolation: default modifiers

Default modifier: POSIX format

As shown in the examples above, you can specify a format. This can, for instance, help you with rounding or columns:

  printp "π = {pi%.3f}", pi => 3.1415;
  printp "weight is {kilogram%d}", kilogram => 127*OUNCE_PER_KILO;
  printp "{filename%-20.20s}\n", filename => $fn;

Default modifier: BYTES

[0.91] Too often, you have to translate a (file) size into humanly readible format. The BYTES modifier simplifies this a lot:

  printp "{size BYTES} {fn}\n", fn => $fn, size => -s $fn;

The output will always be 6 characters. Examples are "999 B", "1.2 kB", and " 27 MB".

Default modifiers: YEAR, DATE, TIME, DT, and DT()

[0.91] A set of modifiers help displaying dates and times. They are a little flexible in values they accept, but do not expect miracles: when it get harder, you will need to process it yourself.

The actual treatment of a time value depends on the value: three different situations:

The output of YEAR is in format 'YYYY', for DATE it will always be 'YYYY-MM-DD', where TIME produces 'HH:mm:ss'.

The short form DT is an alias for DT(FT). The DT modifier can produce different formats:

  DT(ASC)     : %a %b %e %T %Y       asctime output
  DT(FT)      : %F %T                YYYY-MM-DD HH:mm:ss
  DT(ISO)     : %FT%T%z              iso8601
  DT(RFC822)  : %a, %d %b %y %T %z   email old
  DT(RFC2822) : %a, %d %b %Y %T %z   email newer

You may suggest additional formats, or add your own modifier.

Default modifiers: //word, //"string", //'string'

[0.91] By default, an undefined value is shown as text 'undef'. Empty strings are shown as nothing. This may not be nice. You may want to be more specific when a value is missing.

   "visitors: {count //0}"
   "published: {date DT//'not yet'}"
   "copyright: {year//2017 YEAR}

Modifiers will usually return undef when they are called with an undefined or empty value. By the right order of '//', you may product different kinds of output:

   "price: {price//5 EUR}"
   "price: {price EUR//unknown}"

Private modifiers

You may pass your own modifiers. A modifier consists of a selector and a CODE, which is called when the selector matches. The selector is either a string or a regular expression.

  # in Object Oriented syntax:
  my $f = String::Print->new
    ( modifiers => [ qr/[€₤]/ => \&money ]
    );

  # in function syntax:
  use String::Print 'printi', 'sprinti'
    , modifiers => [ qr/[€₤]/ => \&money ];

  # the implementation:
  sub money$$$$)
  { my ($formatter, $modif, $value, $args) = @_;

      $modif eq '€' ? sprintf("%.2f EUR", $value+0.0001)
    : $modif eq '₤' ? sprintf("%.2f GBP", $value/1.16+0.0001)
    :                 'ERROR';
  }

Using printp() makes it a little shorter, but will become quite complex when there are more parameter in one string.

  printi "price: {p€}", p => $pi;   # price: 3.14 EUR
  printi "price: {p₤}", p => $pi;   # price: 2.71 GBP

  printp "price: %{€}s", $pi;       # price: 3.14 EUR
  printp "price: %{₤}s", $pi;       # price: 2.71 GBP

This is very useful in the translation context, where the translator can specify abstract formatting rules. As example, see the (GNU) gettext files, in the translation table for Dutch into English. The translator tells us which currency to use in the display.

  msgid  "kostprijs: {p€}"
  msgstr "price: {p₤}"

Another example. Now, we want to add timestamps. In this case, we decide for modifier names in \w, so we need a blank to separate the parameter from the modifer.

Modifiers: stacking

You can add more than one modifier. The modifiers detect the extend of their own information (via a regular expression), and therefore the formatter understands where one ends and the next begins.

The modifiers are called in order:

  printi "price: {p€%9s}\n", p => $p; # price: ␣␣␣123.45
  printi ">{t T%10s}<", t => $now;    # >␣␣12:59:17<

  printp "price: %9{€}s\n", $p;       # price: ␣␣␣123.45
  printp ">%10{T}s<", $now;           # >␣␣12:59:17<

Output encoding

[0.91] This module is also used by Log::Report::Template, which is used to insert (translated) strings with parameters into HTML templates. You can imagine that some of the parameter may need to be encoded to HTML in the template, and other not.

example with Log::Report::Template

In pure Template Toolkit, you would write

  # in your TT-template
  <div>Username: [% username | html %]</div>
  # in your code
  username => $user->name,

With plain String::Print with output encoding enabled, you can do:

  # in your TT-template
  <div>[% show_username %]</div>
  # in your code with encodeFor('HTML')
  show_username => printi("Username: {user}", user => $user->name),
  # or
  show_username => printp("Username: %s", $user->name),

That does not look very efficient, however it changes for the good when this is combined with Log::Report::Lexicon (translations) You can either do:

  # in your TT-template
  <div>[% show_username %]</div>
  # in your code with encodeFor('HTML')
  show_username => __x("Username: {user}", user => $user->name),

Shorter:

  # in your TT-template with encodeFor('HTML')
  <div>[% loc("Username: {user}", user => username) %]</div>
  # in your code
  username => $user->name,

Even shorter:

  # in your TT-template with encodeFor('HTML')
  <div>[% loc("Username: {user.name}", user => userobj) %]</div>
  # in your code
  userobj => $user,

Shortest:

  # in your TT-template with encodeFor('HTML')
  <div>[% loc("Username: {user.name}") %]</div>
  # in your code
  user => $user,

Shorter that the original, and translations for free! More examples in Log::Report::Template.

Output encoding exclusion

In some cases, the data which is inserted is already encoded in the output syntax. For instance, you already have HTML to be included.

The default exclusion rule for HTML output is qr/html$/i, which means that all inserted named parameters, where the name ends on html will not get html-entity encoded.

This will work by default:

  # with encodeFor('HTML')
  printp "Me & Co: {name}, {description_html}",
     name => 'René', description_html => $descr;

This may result in:

  Me &amp; Co: Ren&eacute;, <font color="red">new member</font>

Better not to have HTML in your program: leave it to the template. But in some cases, you have no choice.

Compared to other modules on CPAN

There are a quite a number of modules on CPAN which extend the functionality of printf(). To name a few: String::Format, String::Errf, String::Formatter, Text::Sprintf::Named, Acme::StringFormat, Text::sprintf, Log::Sprintf, and String::Sprintf. They are all slightly different.

When the String::Print module was created, none of the modules mentioned above handled unicode correctly. Global configuration of serializers and modifiers is also usually not possible, sometimes provided per explicit function call. Only String::Print cleanly separates the roles of serializers, modifiers, and conversions.

String::Print is nicely integrated with Log::Report.