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 conversion 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,
);
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 $ArrayOfLines = ArrayRef->plus_coercions( Split[ qr{\n} ] );
Viewing the source code for Types::Standard should give you hints as to how they are implemented.
plus_fallback_coercions, minus_coercions and no_coercions
Getting back to the plus_coercions
method, there are some other methods that perform coercion maths.
plus_fallback_coercions
is the same as plus_coercions
but the added coercions have a lower priority than any existing coercions.
minus_coercions
can be given a list of type constraints that we wish to ignore coercions for. Imagine our PathTiny
constraint already has a coercion from Str
, then the following creates a new anonymous type constraint without that coercion:
PathTiny->minus_coercions(Str)
no_coercions
gives us a new type anonymous constraint without any of its parents coercions. This is useful as a way to create a blank slate for a subsequent plus_coercions
:
PathTiny->no_coercions->plus_coercions(...)
plus_constructors
The plus_constructors
method defined in Type::Tiny::Class is sugar for plus_coercions
. The following two are the same:
PathTiny->plus_coercions(Str, q{ Path::Tiny->new($_) })
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($_)}) ]
... does what you mean!
The parameterized type constraints that do this magic include the following ones from Types::Standard:
ScalarRef
ArrayRef
HashRef
Map
Tuple
Dict
Optional
Maybe
Chained Coercions
Consider the following type library:
{
package Types::Geometric;
use Type::Library -base, -declare => qw(
VectorArray
VectorArray3D
Point
Point3D
);
use Type::Utils;
use Types::Standard qw( Num Tuple InstanceOf );
declare VectorArray,
as Tuple[Num, Num];
declare VectorArray3D,
as Tuple[Num, Num, Num];
coerce VectorArray3D,
from VectorArray, via {
[ @$_, 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.
The (Lack of) Zen of Coercions
Coercions can lead to ugliness.
Let's say we define a type constraint Path
which has a coercion from Str
. Now we define a class which uses that type constraint.
Now in another class, we define a coercion from ArrayRef
to Path
. This kind of action at a distance is not really desirable. And in fact, things will probably subtly break - the first class may have already built a constructor inlining a bunch of code from the coercion.
However, you too can achieve coercion zen by following these three weird tricks:
If you want to define coercions for a type, do it within your type constraint library, so the coercions are all defined before the type constraint is ever used.
At the end of your type constraint library, consider calling
$type->coercion->freeze
on each type constraint that has a coercion. This makes the type's coercions immutable. If anybody wants to define any additional coercions, they'll have to create a child type to do it with.Use
plus_coercions
and similar methods to easily create a child type constraint of any existing type, and add more coercions to it. Don't fiddle directly with the existing type constraint which may be being used elsewhere.Note that these methods all return type constraint objects with frozen (immutable) coercions.
That's it.
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) 2013-2014 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.