Type::Tiny::Manual::CoUseroContributed Perl Type::Tiny::Manual::Coercions(3pm)
NAME
Type::Tiny::Manual::Coercions - advanced information on coercions
MANUAL
This section of the manual assumes you've already read
Type::Tiny::Manual::UsingWithMoo.
Type::Tiny takes a slightly different approach to type constraints from
Moose. In Moose, there is a single flat namespace for type
constraints. Moose defines a type constraint called Str for strings and
a type constraint called ArrayRef for arrayrefs. If you want to define
strings differently (maybe you think that the empty string doesn't
really count as a string, or maybe you think objects overloading
"q[""]" should count as strings) then you can't call it Str; you need
to choose a different name.
With Type::Tiny, two type libraries can each offer a string type
constraint with their own definitions for what counts as a string, and
you can choose which one to import, or import them both with different
names:
use Some::Types qw( Str );
use Other::Types "Str" => { -as => "Str2" };
This might seem to be a small advantage of Type::Tiny, but where this
global-versus-local philosophy really makes a difference is coercions.
Let's imagine for a part of your application that deals with reading
username and password data you need to have a "username:password"
string. You may wish to accept a "[$username, $password]" arrayref and
coerce it to a string using "join ":", @$arrayref". But another part of
your application deals with slurping log files, and wants to coerce a
string from an arrayref using "join "\n", @$arrayref". These are both
perfectly sensible ways to coerce an arrayref. In Moose, a typical way
to do this would be:
package My::UserManager {
use Moose;
use Moose::Util::TypeConstraints;
coerce 'Str',
from 'ArrayRef', via { join ":", @$_ };
...;
}
package My::LogReader {
use Moose;
use Moose::Util::TypeConstraints;
coerce 'Str',
from 'ArrayRef', via { join "\n", @$_ };
...;
}
However, because in Moose all types and coercions are global, if both
these classes are loaded, only one of them will work. One class will
overrule the other's coercion. Which one "wins" will depend on load
order.
It is possible to solve this with Moose native types, but it requires
extra work. (The solution is for My::UserManager and My::LogReader to
each create a subtype of Str and define the coercion on that subtype
instead of on Str directly.)
Type::Tiny solves this in two ways:
1. Type::Tiny makes it possible for type libraries to "protect" their
type constraints to prevent external code from adding new coercions
to them.
$type->coercion->freeze();
You can freeze coercions for your entire type library using:
__PACKAGE__->make_immutable;
If you try to add coercions to a type constraint that has frozen
coercions, it will throw an error.
use Types::Standard qw( Str ArrayRef );
Str->coercion->add_type_coercions(
ArrayRef, sub { join "\n", @$_ },
);
2. Type::Tiny makes the above-mentioned pattern of adding coercions to
a subtype much easier.
use Types::Standard ( Str ArrayRef );
my $subtype = Str->plus_coercions(
ArrayRef, sub { join "\n", @$_ },
);
The "plus_coercions" method creates a new child type, adds new
coercions to it, copies any existing coercions from the parent
type, and then freezes coercions for the new child type.
The end result is you now have a "copy" of Str that can coerce from
ArrayRef but other copies of Str won't be affected by your
coercion.
Defining Coercions within Type Libraries
Some coercions like joining an arrayref to make a string are not going
to be coercions that everybody will agree on. Join with a line break in
between them as above? Or with a colon, a tab, a space, some other
chanaracter? It depends a lot on your application.
Others, like coercing a Path::Tiny object from a string, are likely to
be very obvious. It is this kind of coercion that it makes sense to
define within the library itself so it's available to any packages that
use the library.
my $pt = __PACKAGE__->add_type(
Type::Tiny::Class->new(
name => 'Path',
class => 'Path::Tiny',
),
);
$pt->coercion->add_type_coercions(
Str, q{ Path::Tiny::path($_) },
);
$pt->coercion->freeze;
Tweak Coercions Outside Type Libraries
The "plus_coercions" method creates a new type constraint with
additional coercions. If the original type already had coercions, the
new coercions have a higher priority.
There's also a "plus_fallback_coercions" method which does the same as
"plus_coercions" but adds the new coercions with a lower priority than
any existing ones.
Type::Tiny::Class provides a "plus_constructors" method as a shortcut
for coercing via a constructor method. The following two are the same:
Path->plus_constructors( Str, "new" )
Path->plus_coercions( Str, q{ Path::Tiny->new($_) } )
To create a type constraint without particular existing coercions, you
can use "minus_coercions". The following uses the Datetime type defined
in Type::Tiny::Manual::Libraries, removing the coercion from Int but
keeping the coercions from Undef and Dict.
use Types::Standard qw( Int );
use Example::Types qw( Datetime );
has start_date => (
is => 'ro',
isa => Datetime->minus_coercions( Int ),
coerce => 1,
);
There's also a "no_coercions" method that creates a subtype with no
coercions at all. This is most useful either to create a "blank slate"
for "plus_coercions":
my $Path = Path->no_coercions->plus_coercions( Str, sub { ... } );
Or to disable coercions for Type::Params. Type::Params will always
automatically coerce a parameter if there is a coercion for that type.
use Types::Standard qw( Object );
use Types::Common::String qw( UpperCaseStr );
use Type::Params;
sub set_account_name {
state $check = signature(
method => Object,
positional => [ UpperCaseStr->no_coercions ],
);
my ( $self, $name ) = $check->( @_ );
$self->_account_name( $name );
$self->db->update( $self );
return $self;
}
# This will die instead of coercing from lowercase
$robert->set_account_name( 'bob' );
Named Coercions
A compromise between defining a coercion in the type library or
defining them in the package that uses the type library is for a type
library to define a named collection of coercions which can be
optionally added to a type constraint.
{
package MyApp::Types;
use Type::Library
-extends => [ 'Types::Standard' ];
__PACKAGE__->add_coercion(
name => "FromLines",
type_constraint => ArrayRef,
type_coercion_map => [
Str, q{ [split /\n/] },
Undef, q{ [] },
],
);
}
This set of coercions has a name and can be imported and used:
use MyApp::Types qw( ArrayRef FromLines );
has lines => (
is => 'ro',
isa => ArrayRef->plus_coercions( FromLines ),
coerce => 1,
);
Types::Standard defines a named coercion MkOpt designed to be used for
OptList.
use Types::Standard qw( OptList MkOpt );
my $OptList = OptList->plus_coercions( MkOpt );
Parameterized Coercions
Named coercions can also be parameterizable.
my $ArrayOfLines = ArrayRef->plus_coercions( Split[ qr{\n} ] );
Types::Standard defines Split and Join parameterizable coercions.
Viewing the source code for Types::Standard should give you hints as to
how they are implemented.
"Deep" Coercions
Certain parameterized type constraints can automatically acquire
coercions if their parameters have coercions. For example:
ArrayRef[ Int->plus_coercions( Num, q{int($_)} ) ]
... does what you mean!
The parameterized type constraints that do this magic include the
following ones from Types::Standard:
• ScalarRef
• ArrayRef
• HashRef
• Map
• Tuple
• CycleTuple
• Dict
• Optional
• Maybe
Imagine we're defining a type Paths in a type library:
__PACKAGE__->add_type(
name => 'Paths',
parent => ArrayRef[Path],
);
The Path type has a coercion from Str, so Paths should be able to
coerce from an arrayref of strings, right?
Wrong! Although ArrayRef[Path] could coerce from an arrayref of
strings, Paths is a separate type constraint which, although it
inherits from ArrayRef[Path] has its own (currently empty) set of
coercions.
Because that is often not what you want, Type::Tiny provides a shortcut
when declaring a subtype to copy the parent type constraint's
coercions:
__PACKAGE__->add_type(
name => 'Paths',
parent => ArrayRef[Path],
coercion => 1, # inherit
);
Now Paths can coerce from an arrayref of strings.
Deep Caveat
Currently there exists ill-defined behaviour resulting from mixing deep
coercions and mutable (non-frozen) coercions. Consider the following:
class_type Path, { class => "Path::Tiny" };
coerce Path,
from Str, via { "Path::Tiny"->new($_) };
declare Paths, as ArrayRef[Path], coercion => 1;
coerce Path,
from InstanceOf["My::File"], via { $_->get_path };
An arrayref of strings can now be coerced to an arrayref of Path::Tiny
objects, but is it also now possible to coerce an arrayref of My::File
objects to an arrayref of Path::Tiny objects?
Currently the answer is "no", but this is mostly down to implementation
details. It's not clear what the best way to behave in this situation
is, and it could start working at some point in the future.
This is why you should freeze coercions.
Chained Coercions
Consider the following type library:
package Types::Geometric {
use Type::Library -base, -declare => qw(
VectorArray
VectorArray3D
Point
Point3D
);
use Type::Utils;
use Types::Standard qw( Num Tuple InstanceOf );
declare VectorArray,
as Tuple[Num, Num];
declare VectorArray3D,
as Tuple[Num, Num, Num];
coerce VectorArray3D,
from VectorArray, via {
[ @$_, 0 ];
};
class_type Point, { class => "Point" };
coerce Point,
from VectorArray, via {
Point->new(x => $_->[0], y => $_->[1]);
};
class_type Point3D, { class => "Point3D" };
coerce Point3D,
from VectorArray3D, via {
Point3D->new(x => $_->[0], y => $_->[1], z => $_->[2]);
},
from Point, via {
Point3D->new(x => $_->x, y => $_->y, z => 0);
};
}
Given an arrayref "[1, 1]" you might reasonably expect it to be
coercible to a Point3D object; it matches the type constraint
VectorArray so can be coerced to VectorArray3D and thus to Point3D.
However, Type::Coercion does not automatically chain coercions like
this. Firstly, it would be incompatible with Moose's type coercion
system which does not chain coercions. Secondly, it's ambiguous; in our
example, the arrayref could be coerced along two different paths (via
VectorArray3D or via Point); in this case the end result would be the
same, but in other cases it might not. Thirdly, it runs the risk of
accidentally creating loops.
Doing the chaining manually though is pretty simple. Firstly, we'll
take note of the "coercibles" method in Type::Tiny. This method called
as "VectorArray3D->coercibles" returns a type constraint meaning
"anything that can be coerced to a VectorArray3D".
So we can define the coercions for Point3D as:
coerce Point3D,
from VectorArray3D->coercibles, via {
my $tmp = to_VectorArray3D($_);
Point3D->new(x => $tmp->[0], y => $tmp->[1], z => $tmp->[2]);
},
from Point, via {
Point3D->new(x => $_->x, y => $_->y, z => 0);
};
... and now coercing from "[1, 1]" will work.
SEE ALSO
Moose::Manual::BestPractices,
<https://web.archive.org/web/20090624164256/http://www.catalyzed.org/2009/06/keeping-your-coercions-to-yourself.html>,
MooseX::Types::MoreUtils.
NEXT STEPS
After that last example, probably have a little lie down. Once you're
recovered, here's your next step:
• Type::Tiny::Manual::AllTypes
An alphabetical list of all type constraints bundled with
Type::Tiny.
AUTHOR
Toby Inkster <tobyink@cpan.org>.
COPYRIGHT AND LICENCE
This software is copyright (c) 2013-2014, 2017-2023 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.
perl v5.36.0 2023-02-01Type::Tiny::Manual::Coercions(3pm)
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