NAME

       Type::Tiny::Manual::Coercions - adding coercions to type constraints

DESCRIPTION

       Stop! Don't do it!

       OK, it's fairly common practice in Moose/Mouse code to define coercions for type
       constraints. For example, suppose we have a type constraint:

          class_type PathTiny, { class => "Path::Tiny" };

       We may wish to define a coercion (i.e. a convertion routine) to handle strings, and
       convert them into Path::Tiny objects:

          coerce PathTiny,
             from Str, via { "Path::Tiny"->new($_) };

       However, there are good reasons to avoid this practice. It ties the coercion routine to
       the type constraint. Any people wishing to use your "PathTiny" type constraint need to buy
       in to your idea of how they should be coerced from "Str". With Path::Tiny this is unlikely
       to be controversial, however consider:

          coerce ArrayRef,
             from Str, via { [split /\n/] };

       In one part of the application (dealing with parsing log files for instance), this could
       be legitimate. But another part (dealing with logins perhaps) might prefer to split on
       colons. Another (dealing with web services) might attempt to parse the string as a JSON
       array.

       If all these coercions have attached themselves to the "ArrayRef" type constraint,
       coercing a string becomes a complicated proposition!  In a large application where
       coercions are defined across many different files, the application can start to suffer
       from "spooky action at a distance".

       In the interests of Moose-compatibility, Type::Tiny and Type::Coercion do allow you to
       define coercions this way, but they also provide an alternative that you should consider:
       "plus_coercions".

   plus_coercions
       Type::Tiny offers a method "plus_coercions" which constructs a new anonymous type
       constraint, but with additional coercions.

       In our earlier example, we'd define the "PathTiny" type constraint as before:

          class_type PathTiny, { class => "Path::Tiny" };

       But then not define any coercions for it. Later, when using the type constraint, we can
       add coercions:

          my $ConfigFileType = PathTiny->plus_coercions(
             Str,   sub { "Path::Tiny"->new($_) },
             Undef, sub { "Path::Tiny"->new("/etc/myapp/default.conf") },
          );

          has config_file => (
             is     => "ro",
             isa    => $ConfigFileType,
             coerce => 1,
          );

       Where the "PathTiny" constraint is used in another part of the code, it will not see these
       coercions, because they were added to the new anonymous type constraint, not to the
       "PathTiny" constraint itself!

   Aside: Optimizing Coercions
       Stepping away from the flow of this article, I'll point out that the following also works,
       using strings of Perl code instead of coderefs.  It allows Type::Coercion to do a little
       optimization and run faster:

          my $ConfigFileType = PathTiny->plus_coercions(
             Str,   q{ "Path::Tiny"->new($_) },
             Undef, q{ "Path::Tiny"->new("/etc/myapp/default.conf") },
          );

       Now, where were we...?

   Named Coercions
       A type library may define a named set of coercions to a particular type. For example,
       let's define that coercion from "Str" to "ArrayRef":

          declare_coercion "LinesFromStr",
             to_type ArrayRef,
             from Str, q{ [split /\n/] };

       Now we can import that coercion using a name, and it makes our code look a little cleaner:

          use Types::Standard qw(ArrayRef);
          use MyApp::Types qw(LinesFromStr);

          has lines => (
             is     => "ro",
             isa    => ArrayRef->plus_coercions(LinesFromStr),
             coerce => 1,
          );

   Overloading
       Type::Tiny and Type::Coercion overload the "+" operator to add coercions. So you may use:

             isa    => PathTiny + PathTinyFromStr,

       However, beware precedence. The following is parsed as a function call with an argument
       preceded by a unary plus:

             isa    => ArrayRef + LinesFromStr,
             #         ArrayRef( +LinesFromStr )

       When things can be parameterized, it's generally a good idea to wrap them in parentheses
       to disambiguate:

             isa    => (ArrayRef) + LinesFromStr,

   Parameterized Coercions
       Parameterized type constraints are familiar from Moose. For example, an arrayref of
       integers:

          ArrayRef[Int]

       Type::Coercion supports parameterized named coercions too. For example, the following type
       constraint has a coercion from strings that splits them into lines:

          use Types::Standard qw( ArrayRef Split );

          my $ArrayOfLines = (ArrayRef) + Split[ qr{\n} ];

       The implementation of this feature is considered experimental, and the API for building
       parameterized coercions is likely to change. However, the feature itself, and its surface
       syntax (the square brackets) is likely to stay. So beware building your own
       parameterizable coercions, but don't be shy about using the ones in Types::Standard.

   plus_fallback_coercions, minus_coercions and no_coercions
       Getting back to the "plus_coercions" method, there are some other methods that perform
       coercion maths.

       "plus_fallback_coercions" is the same as "plus_coercions" but the added coercions have a
       lower priority than any existing coercions.

       "minus_coercions" can be given a list of type constraints that we wish to ignore coercions
       for. Imagine our "PathTiny" constraint already has a coercion from "Str", then the
       following creates a new anonymous type constraint without that coercion:

          PathTiny->minus_coercions(Str)

       "no_coercions" gives us a new type anonymous constraint without any of its parents
       coercions. This is useful as a way to create a blank slate for a subsequent
       "plus_coercions":

          PathTiny->no_coercions->plus_coercions(...)

   plus_constructors
       The "plus_constructors" method defined in Type::Tiny::Class is sugar for "plus_coercions".
       The following two are the same:

          PathTiny->plus_coercions(Str, q{ Path::Tiny->new($_) })

          PathTiny->plus_constructors(Str, "new");

   "Deep" Coercions
       Certain parameterized type constraints can automatically acquire coercions if their
       parameters have coercions. For example:

          ArrayRef[ Int->plus_coercions(Num, q{int($_)}) ]

       ... does what you mean!

       The parameterized type constraints that do this magic include the following ones from
       Types::Standard:

       •   "ScalarRef"

       •   "ArrayRef"

       •   "HashRef"

       •   "Map"

       •   "Tuple"

       •   "Dict"

       •   "Optional"

SEE ALSO

       Moose::Manual::BestPractices,
       <http://www.catalyzed.org/2009/06/keeping-your-coercions-to-yourself.html>.

AUTHOR

       Toby Inkster <tobyink@cpan.org>.

COPYRIGHT AND LICENCE

       This software is copyright (c) 2013 by Toby Inkster.

       This is free software; you can redistribute it and/or modify it under the same terms as
       the Perl 5 programming language system itself.

DISCLAIMER OF WARRANTIES

       THIS PACKAGE IS PROVIDED "AS IS" AND WITHOUT ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING,
       WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR A PARTICULAR
       PURPOSE.