String::Print(3) printf alternative


### Functional interface
use String::Print qw/printi printp/, %config;
# interpolation of arrays and hashes
printi 'age {years}', years => 12;
printi 'price-list: {prices%.2f}', prices => \@prices, _join => "+";
printi 'dump: {hash}', hash => \%config;
# same with positional parameters
printp 'age %d", 12;
printp 'price-list: %.2f', \@prices;
printp 'dump: %s', \%settings;
### Object Oriented interface
use String::Print 'oo'; # import nothing
my $f = String::Print->new(%config);
# same, called directly
$f->printi('age {years}', years => 12);
$f->printp('age %d', 12);
### via Log::Report's __* functions
use Log::Report::Optional;
print __x"age {years}", years => 12;


This module inserts values into (translated) 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 there. Take a look at them first, when you start using this module!


The Object Oriented interface

See functions printi(), sprinti(), printp(), and sprintp(): you can also call them as method.

  use String::Print 'oo';
  my $f = String::Print->new(%config);
  $f->printi($format, @params);
  # exactly the same functionality:
  use String::Print 'printi', %config;
  printi $format, @params;

The Object Oriented interface wins when you need the same configuration in multiple source files, or when you need different configurations within one program. In these cases, the hassle of explicitly using the object has some benefits.


 -Option     --Default
  modifiers    [ qr/^%\S+/ = \&format_printf]>
  serializers  <useful defaults>
modifiers => ARRAY
Add one or more modifier handlers to power of the formatter. They will get preference over the predefined modifiers, but lower than the modifiers passed to "print[ip]" itself.
serializers => HASH|ARRAY
How to serialize data elements.


  my $f = String::Print->new
    ( modifiers   => [ EUR   => sub {sprintf "%5.2f e", $_[0]} ]
    , serializers => [ UNDEF => sub {'-'} ]
  $f->printi("price: {p EUR}", p => 3.1415); # price: XX3.14 e
  $f->printi("count: {c}", c => undef);      # count: -


The PAIRS are a combination of an selector and a CODE which processes the value when the modifier matches. The selector is a string or (preferred) a regular expression. Later modifiers with the same name overrule earlier definitions. You may also specify an ARRAY of modifiers per "print".

See section ``Interpolation: Modifiers'' about the details.


The functional interface creates a hidden object. You may import any of these functions explicitly, or all together by not specifying the names.
printi( [$fh], $format, PAIRS|HASH )
Calls sprinti() to fill the data in PAIRS or HASH in $format, and then sends it to the $fh (by default the selected file)

  open my $fh, '>', $file;
  printi $fh, ...
  printi \*STDERR, ...
printp( [$fh], $format, PAIRS|HASH )
Calls sprintp() to fill the data in PAIRS or HASH in $format, and then sends it to the $fh (by default the selected file)
sprinti($format, PAIRS|HASH)
The $format refers to some string, maybe the result of a translation.

The PAIRS (which may be passed as LIST or HASH) contains a mixture of special and normal variables to be filled in. The names of the special variables (the options) start with an underscore ("_").

 -Option  --Default
  _append   undef
  _count    undef
  _join     ', '
  _prepend  undef
_append => STRING|OBJECT
Text as STRING appended after $format, without interpolation.
_count => INTEGER
Result of the translation process: when Log::Report subroutine __xn is are used for count-sensitive translation. Those function may add more specials to the parameter list.
_join => STRING
Which STRING to use when an ARRAY is being filled-in as parameter.
_prepend => STRING|OBJECT
Text as STRING prepended before $format, without interpolation. This may also be an OBJECT which gets stringified, but variables not filled-in.
sprintp($format, LIST, PAIRS)
Where sprinti() uses named parameters --especially useful when the strings need translation--- this function stays close to the standard "sprintf()". All features of POSIX formats are supported. This should say enough: you can use "%3$0#5.*d", if you like.

It may be useful to know that the positional $format is rewritten and then fed into sprinti(). Be careful with the length of the LIST: superfluous parameter PAIRS are passed along to "sprinti()", and should only contain ``specials''.

example: of the rewrite

  # positional parameters
  my $x = sprintp "dumpfiles: %s\n", \@dumpfiles
     , _join => ':';
  # is rewriten into, and then processed as
  my $x = sprinti "dumpfiles: {filenames}\n"
     , filenames => \@dumpfiles, _join => ':';


Why use printi(), not printf()?

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?
"printf()" uses positional parameters, where printi() uses names to refer to the values to be filled-in. Especially in a set-up with translations, where the format strings get extracted into PO-files, it is much clearer to use names. This is also a disadvantage of printp()
pluggable serializers
"printi()" supports serialization for specific data-types: how to interpolate "undef", HASHes, etc.
pluggable modifiers
Especially useful in context of translations, the FORMAT string may contain (language specific) helpers to insert the values correctly.
correct use of utf8
Sized string formatting in "printf()" is broken: it takes your string as bytes, not Perl strings (which may be utf8). In unicode, one ``character'' may use many bytes. Also, some characters are displayed double wide, for instance in Chinese. The printi() implementation will use Unicode::GCString for correct behavior.

Three components

To fill-in a FORMAT, three clearly separated components play a role:
1. modifiers
How to change the provided values, for instance to hide locale differences.
2. serializer
How to represent (the modified) the values correctly, for instance "undef" and ARRAYs.
3. conversion
The standard UNIX format rules, like %d. One conversion rule has been added 'S', which provides unicode correct behavior.


  # sprinti() replaces "{$key$modifiers$conversion}" by
  # sprintp() replaces "%pos{$modifiers}$conversion" by


  printi "price: {price X %-10s}", price => $cost;
  printp "price: %-10{X}s", $cost;
  $conversion = column width %-10s
  $serializer = show float as string
  $modifier   = X to local currency
  $value      = $cost (in X)

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".

When a value is passed as CODE reference, that function will get called to return the value to be filled in. For interpolating, the following rules apply:
Simple scalar values are interpolated ``as is''
Takes the value where the scalar reference points to.
All members will be interpolated with ",X" between the elements. Alternatively (maybe nicer), you can pass an interpolation parameter via the "_join" OPTION.

  printi "matching files: {files}", files => \@files, _join => ', '
By default, HASHes are interpolated with sorted keys,

   $key => $value, $key2 => $value2, ...

There is no quoting on the keys or values (yet). Usually, this will produce an ugly result anyway.

With the "serialization" parameter, you can overrule the interpolation of above defaults, but also add rules for your own objects. By default, objects get stringified.

  serialization => [ $myclass => \&name_in_reverse ]
  sub name_in_reverse($$$)
  {   my ($formatter, $object, $args) = @_;
      # the $args are all parameters to be filled-in
      scalar reverse $object->name;

Interpolation: Modifiers

Modifiers are used to change the value to be inserted, before the characters get interpolated in the line.

Modifiers: unix format

Next to the name, you can specify a format code. With (gnu) "gettext()", you often see this:

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

Locale::TextDomain has two ways:

 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 content of the translation table differs between the examples.

With "Log::Report", above syntaxes do work, 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 is the fact that not all languages produce comparable length strings. Now, the translators can take care that the layout of tables is optimal. Above example in printp() syntax, shorter but less maintainable:

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

Modifiers: unix format improvements

The POSIX "printf()" does not handle unicode strings. Perl does understand that the 's' modifier may need to insert utf8 so does not count bytes but characters. "printi()" does not use characters but ``grapheme clusters'' via Unicode::GCString. Now, also composed characters do work correctly.

Additionally, you can use the new 'S' conversion to count in columns. In fixed-width fonts, graphemes can have width 0, 1 or 2. For instance, Chinese characters have width 2. When printing in fixed-width, this 'S' is probably the better choice over 's'. When the field does not specify its width, then there is no performance penalty for using 'S'.

Modifiers: 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/[XX]/ => \&money ]
  # in function syntax:
  use String::Print 'printi', 'sprinti'
    , modifiers => [ qr/[XX]/ => \&money ];
  # the implementation:
  sub money$$$$)
  { my ($formatter, $modif, $value, $args) = @_;
      $modif eq 'X' ? sprintf("%.2f EUR", $value+0.0001)
    : $modif eq 'X' ? 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: {pX}", p => $pi;   # price: 3.14 EUR
  printi "price: {pX}", p => $pi;   # price: 2.71 GBP
  printp "price: %{X}s", $pi;       # price: 3.14 EUR
  printp "price: %{X}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: {pX}"
  msgstr "price: {pX}"

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.

  use POSIX  qw/strftime/;
  use String::Print modifiers => [ qr/T|DT|D/ => \&_timestamp ];
  sub _timestamp($$$$)
    { my ($formatter, $modif, $value, $args) = @_;
      my $time_format
        = $modif eq 'T'  ? '%T'
        : $modif eq 'D'  ? '%F'
        : $modif eq 'DT' ? '%FT%TZ'
        :                  'ERROR';
      strftime $time_format, gmtime($value);
  printi "time: {t T}",  t => $now;  # time: 10:59:17
  printi "date: {t D }", t => $now;  # date: 2013-04-13
  printi "both: {t DT}", t => $now;  # both: 2013-04-13T10:59:17Z
  printp "time: %{T}s",  $now;       # time: 10:59:17
  printp "date: %{D}s",  $now;       # date: 2013-04-13
  printp "both: %{DT}s", $now;       # both: 2013-04-13T10:59:17Z

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: {pX%9s}\n", p => $p; # price: XXX123.45
  printi ">{t T%10s}<", t => $now;    # >XX12:59:17<
  printp "price: %9{X}s\n", $p;       # price: XXX123.45
  printp ">%10{T}s<", $now;           # >XX12:59:17<

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 <http://>, String::Formatter <http://>, 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.


Copyrights 2013-2014 by [Mark Overmeer]. For other contributors see ChangeLog.

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself. See