Provided by: libmethod-signatures-perl_20141021-1_all 
NAME
Method::Signatures - method and function declarations with signatures and no source filter
SYNOPSIS
package Foo;
use Method::Signatures;
method new (%args) {
return bless {%args}, $self;
}
method get ($key) {
return $self->{$key};
}
method set ($key, $val) {
return $self->{$key} = $val;
}
# Can also get type checking if you like:
method set (Str $key, Int $val) {
return $self->{$key} = $val; # now you know $val is always an integer
}
func hello($greeting, $place) {
print "$greeting, $place!\n";
}
DESCRIPTION
Provides two new keywords, "func" and "method", so that you can write subroutines with
signatures instead of having to spell out "my $self = shift; my($thing) = @_"
"func" is like "sub" but takes a signature where the prototype would normally go. This
takes the place of "my($foo, $bar) = @_" and does a whole lot more.
"method" is like "func" but specifically for making methods. It will automatically
provide the invocant as $self (by default). No more "my $self = shift".
Also allows signatures, very similar to Perl 6 signatures.
Also does type checking, understanding all the types that Moose (or Mouse) would
understand.
And it does all this with no source filters.
Signature syntax
func echo($message) {
print "$message\n";
}
is equivalent to:
sub echo {
my($message) = @_;
print "$message\n";
}
except the original line numbering is preserved and the arguments are checked to make sure
they match the signature.
Similarly
method foo($bar, $baz) {
$self->wibble($bar, $baz);
}
is equivalent to:
sub foo {
my $self = shift;
my($bar, $baz) = @_;
$self->wibble($bar, $baz);
}
again with checks to make sure the arguments passed in match the signature.
The full signature syntax for each parameter is:
Int|Str \:$param! where $SM_EXPR is ro = $AS_EXPR when $SM_EXPR
\_____/ ^^\____/^ \____________/ \___/ \________/ \___________/
| || | | | | | |
Type_/ || | | | | | |
Aliased?___/ | | | | | | |
Named?______/ | | | | | |
Parameter var___/ | | | | |
Required?__________/ | | | |
Parameter constraint(s)_____/ | | |
Parameter trait(s)______________________/ | |
Default value____________________________________/ |
When default value should be applied_________________________/
Every component except the parameter name (with sigil) is optional.
$SM_EXPR is any expression that is valid as the RHS of a smartmatch, or else a raw block
of code. See "Value constraints".
$AS_EXPR is any expression that is valid as the RHS of an assignment operator. See
"Defaults".
@_
Other than removing $self, @_ is left intact. You are free to use @_ alongside the
arguments provided by Method::Signatures.
Named parameters
Parameters can be passed in named, as a hash, using the ":$arg" syntax.
method foo(:$arg) {
...
}
$object->foo( arg => 42 );
Named parameters are optional by default.
Required positional parameters and named parameters can be mixed, but the named params
must come last.
method foo( $a, $b, :$c ) # legal
Named parameters are passed in as a hash after all positional arguments.
method display( $text, :$justify = 'left', :$enchef = 0 ) {
...
}
# $text = "Some stuff", $justify = "right", $enchef = 0
$obj->display( "Some stuff", justify => "right" );
You cannot mix optional positional params with named params, as that leads to ambiguities.
method foo( $a, $b?, :$c ) # illegal
# Is this $a = 'c', $b = 42 or $c = 42?
$obj->foo( c => 42 );
Aliased references
A signature of "\@arg" will take an array reference but allow it to be used as @arg inside
the method. @arg is an alias to the original reference. Any changes to @arg will affect
the original reference.
package Stuff;
method add_one(\@foo) {
$_++ for @foo;
}
my @bar = (1,2,3);
Stuff->add_one(\@bar); # @bar is now (2,3,4)
This feature requires Data::Alias to be installed.
Invocant parameter
The method invocant (i.e. $self) can be changed as the first parameter on a per-method
basis. Put a colon after it instead of a comma:
method foo($class:) {
$class->bar;
}
method stuff($class: $arg, $another) {
$class->things($arg, $another);
}
"method" has an implied default invocant of $self:, though that is configurable by setting
the invocant parameter on the "use Method::Signatures" line.
"func" has no invocant, as it is intended for creating subs that will not be invoked on an
object.
Defaults
Each parameter can be given a default with the "$arg = EXPR" syntax. For example,
method add($this = 23, $that = 42) {
return $this + $that;
}
Almost any expression can be used as a default.
method silly(
$num = 42,
$string = q[Hello, world!],
$hash = { this => 42, that => 23 },
$code = sub { $num + 4 },
@nums = (1,2,3),
)
{
...
}
Normally, defaults will only be used if the argument is not passed in at all. Passing in
"undef" will override the default. That means ...
Class->add(); # $this = 23, $that = 42
Class->add(99); # $this = 99, $that = 42
Class->add(99, undef); # $this = 99, $that = undef
However, you can specify additional conditions under which a default is also to be used,
using a trailing "when". For example:
# Use default if no argument passed
method get_results($how_many = 1) {...}
# Use default if no argument passed OR argument is undef
method get_results($how_many = 1 when undef) {...}
# Use default if no argument passed OR argument is empty string
method get_results($how_many = 1 when "") {...}
# Use default if no argument passed OR argument is zero
method get_results($how_many = 1 when 0) {...}
# Use default if no argument passed OR argument is zero or less
method get_results($how_many = 1 when sub{ $_[0] <= 0 }) {...}
# Use default if no argument passed OR argument is invalid
method get_results($how_many = 1 when sub{ !valid($_[0]) }) {...}
In other words, if you include a "when value" after the default, the default is still used
if the argument is missing, but is also used if the argument is provided but smart-matches
the specified value.
Note that the final two examples above use anonymous subroutines to conform their complex
tests to the requirements of the smartmatch operator. Because this is useful, but
syntactically clumsy, there is also a short-cut for this behaviour. If the test after
"when" consists of a block, the block is executed as the defaulting test, with the actual
argument value aliased to $_ (just like in a "grep" block). So the final two examples
above could also be written:
# Use default if no argument passed OR argument is zero or less
method get_results($how_many = 1 when {$_ <= 0}) {...}
# Use default if no argument passed OR argument is invalid
method get_results($how_many = 1 when {!valid($_)}) } {...}
The most commonly used form of "when" modifier is almost certainly "when undef":
# Use default if no argument passed OR argument is undef
method get_results($how_many = 1 when undef) {...}
which covers the common case where an uninitialized variable is passed as an argument, or
where supplying an explicit undefined value is intended to indicate: "use the default
instead."
This usage is sufficiently common that a short-cut is provided: using the "//=" operator
(instead of the regular assignment operator) to specify the default. Like so:
# Use default if no argument passed OR argument is undef
method get_results($how_many //= 1) {...}
Earlier parameters may be used in later defaults.
method copy_cat($this, $that = $this) {
return $that;
}
Any variable that has a default is considered optional.
Type Constraints
Parameters can also be given type constraints. If they are, the value passed in will be
validated against the type constraint provided. Types are provided by Any::Moose which
will load Mouse if Moose is not already loaded.
Type constraints can be a type, a role or a class. Each will be checked in turn until one
of them passes.
* First, is the $value of that type declared in Moose (or Mouse)?
* Then, does the $value have that role?
$value->DOES($type);
* Finally, is the $value an object of that class?
$value->isa($type);
The set of default types that are understood can be found in Mouse::Util::TypeConstraints
(or Moose::Util::TypeConstraints; they are generally the same, but there may be small
differences).
# avoid "argument isn't numeric" warnings
method add(Int $this = 23, Int $that = 42) {
return $this + $that;
}
Mouse and Moose also understand some parameterized types; see their documentation for more
details.
method add(Int $this = 23, Maybe[Int] $that) {
# $this will definitely be defined
# but $that might be undef
return defined $that ? $this + $that : $this;
}
You may also use disjunctions, which means that you are willing to accept a value of
either type.
method add(Int $this = 23, Int|ArrayRef[Int] $that) {
# $that could be a single number,
# or a reference to an array of numbers
use List::Util qw<sum>;
my @ints = ($this);
push @ints, ref $that ? @$that : $that;
return sum(@ints);
}
If the value does not validate against the type, a run-time exception is thrown.
# Error will be:
# In call to Class::add : the 'this' parameter ("cow") is not of type Int
Class->add('cow', 'boy'); # make a cowboy!
You cannot declare the type of the invocant.
# this generates a compile-time error
method new(ClassName $class:) {
...
}
Value Constraints
In addition to a type, each parameter can also be specified with one or more additional
constraints, using the "$arg where CONSTRAINT" syntax.
method set_name($name where qr{\S+ \s+ \S+}x) {
...
}
method set_rank($rank where \%STD_RANKS) {
...
}
method set_age(Int $age where [17..75] ) {
...
}
method set_rating($rating where { $_ >= 0 } where { $_ <= 100 } ) {
...
}
method set_serial_num(Int $snum where {valid_checksum($snum)} ) {
...
}
The "where" keyword must appear immediately after the parameter name and before any trait
or default.
Each "where" constraint is smartmatched against the value of the corresponding parameter,
and an exception is thrown if the value does not satisfy the constraint.
Any of the normal smartmatch arguments (numbers, strings, regexes, undefs, hashrefs,
arrayrefs, coderefs) can be used as a constraint.
In addition, the constraint can be specified as a raw block. This block can then refer to
the parameter variable directly by name (as in the definition of "set_serial_num()"
above), or else as $_ (as in the definition of "set_rating()".
Unlike type constraints, value constraints are tested after any default values have been
resolved, and in the same order as they were specified within the signature.
Parameter traits
Each parameter can be assigned a trait with the "$arg is TRAIT" syntax.
method stuff($this is ro) {
...
}
Any unknown trait is ignored.
Most parameters have a default traits of "is rw is copy".
ro Read-only. Assigning or modifying the parameter is an error. This trait requires
Const::Fast to be installed.
rw Read-write. It's ok to read or write the parameter.
This is a default trait.
copy
The parameter will be a copy of the argument (just like "my $arg = shift").
This is a default trait except for the "\@foo" parameter (see "Aliased references").
alias
The parameter will be an alias of the argument. Any changes to the parameter will be
reflected in the caller. This trait requires Data::Alias to be installed.
This is a default trait for the "\@foo" parameter (see "Aliased references").
Mixing value constraints, traits, and defaults
As explained in "Signature syntax", there is a defined order when including multiple
trailing aspects of a parameter:
• Any value constraint must immediately follow the parameter name.
• Any trait must follow that.
• Any default must come last.
For instance, to have a parameter which has all three aspects:
method echo($message where { length <= 80 } is ro = "what?") {
return $message
}
Think of "$message where { length <= 80 }" as being the left-hand side of the trait, and
"$message where { length <= 80 } is ro" as being the left-hand side of the default
assignment.
Slurpy parameters
A "slurpy" parameter is a list or hash parameter that "slurps up" all remaining arguments.
Since any following parameters can't receive values, there can be only one slurpy
parameter.
Slurpy parameters must come at the end of the signature and they must be positional.
Slurpy parameters are optional by default.
The "yada yada" marker
The restriction that slurpy parameters must be positional, and must appear at the end of
the signature, means that they cannot be used in conjunction with named parameters.
This is frustrating, because there are many situations (in particular: during object
initialization, or when creating a callback) where it is extremely handy to be able to
ignore extra named arguments that don't correspond to any named parameter.
While it would be theoretically possible to allow a slurpy parameter to come after named
parameters, the current implementation does not support this (see "Slurpy parameter
restrictions").
Instead, there is a special syntax (colloquially known as the "yada yada") that tells a
method or function to simply ignore any extra arguments that are passed to it:
# Expect name, age, gender, and simply ignore anything else
method BUILD (:$name, :$age, :$gender, ...) {
$self->{name} = uc $name;
$self->{age} = min($age, 18);
$self->{gender} = $gender // 'unspecified';
}
# Traverse tree with node-printing callback
# (Callback only interested in nodes, ignores any other args passed to it)
$tree->traverse( func($node, ...) { $node->print } );
The "..." may appear as a separate "pseudo-parameter" anywhere in the signature, but is
normally placed at the very end. It has no other effect except to disable the usual "die
if extra arguments" test that the module sets up within each method or function.
This means that a "yada yada" can also be used to ignore positional arguments (as the
second example above indicates). So, instead of:
method verify ($min, $max, @etc) {
return $min <= $self->{val} && $self->{val} <= $max;
}
you can just write:
method verify ($min, $max, ...) {
return $min <= $self->{val} && $self->{val} <= $max;
}
This is also marginally more efficient, as it does not have to allocate, initialize, or
deallocate the unused slurpy parameter @etc.
Required and optional parameters
Parameters declared using "$arg!" are explicitly required. Parameters declared using
"$arg?" are explicitly optional. These declarations override all other considerations.
A parameter is implicitly optional if it is a named parameter, has a default, or is
slurpy. All other parameters are implicitly required.
# $greeting is optional because it is named
method hello(:$greeting) { ... }
# $greeting is required because it is positional
method hello($greeting) { ... }
# $greeting is optional because it has a default
method hello($greeting = "Gruezi") { ... }
# $greeting is required because it is explicitly declared using !
method hello(:$greeting!) { ... }
# $greeting is required, even with the default, because it is
# explicitly declared using !
method hello(:$greeting! = "Gruezi") { ... }
The @_ signature
The @_ signature is a special case which only shifts $self. It leaves the rest of @_
alone. This way you can get $self but do the rest of the argument handling manually.
Note that a signature of "(@_)" is exactly equivalent to a signature of "(...)". See "The
yada yada marker".
The empty signature
If a method is given the signature of "()" or no signature at all, it takes no arguments.
Anonymous Methods
An anonymous method can be declared just like an anonymous sub.
my $method = method ($arg) {
return $self->foo($arg);
};
$obj->$method(42);
Options
Method::Signatures takes some options at `use` time of the form
use Method::Signatures { option => "value", ... };
invocant
In some cases it is desirable for the invocant to be named something other than $self, and
specifying it in the signature of every method is tedious and prone to human-error. When
this option is set, methods that do not specify the invocant variable in their signatures
will use the given variable name.
use Method::Signatures { invocant => '$app' };
method main { $app->config; $app->run; $app->cleanup; }
Note that the leading sigil must be provided, and the value must be a single token that
would be valid as a perl variable. Currently only scalar invocant variables are supported
(eg, the sigil must be a "$").
This option only affects the packages in which it is used. All others will continue to use
$self as the default invocant variable.
compile_at_BEGIN
By default, named methods and funcs are evaluated at compile time, as if they were in a
BEGIN block, just like normal Perl named subs. That means this will work:
echo("something");
# This function is compiled first
func echo($msg) { print $msg }
You can turn this off lexically by setting compile_at_BEGIN to a false value.
use Method::Signatures { compile_at_BEGIN => 0 };
compile_at_BEGIN currently causes some issues when used with Perl 5.8. See "Earlier Perl
versions".
debug
When true, turns on debugging messages about compiling methods and funcs. See DEBUGGING.
The flag is currently global, but this may change.
Differences from Perl 6
Method::Signatures is mostly a straight subset of Perl 6 signatures. The important
differences...
Restrictions on named parameters
As noted above, there are more restrictions on named parameters than in Perl 6.
Named parameters are just hashes
Perl 5 lacks all the fancy named parameter syntax for the caller.
Parameters are copies.
In Perl 6, parameters are aliases. This makes sense in Perl 6 because Perl 6 is an
"everything is an object" language. Perl 5 is not, so parameters are much more naturally
passed as copies.
You can alias using the "alias" trait.
Can't use positional params as named params
Perl 6 allows you to use any parameter as a named parameter. Perl 5 lacks the named
parameter disambiguating syntax so it is not allowed.
Addition of the "\@foo" reference alias prototype
In Perl 6, arrays and hashes don't get flattened, and their referencing syntax is much
improved. Perl 5 has no such luxury, so Method::Signatures added a way to alias
references to normal variables to make them easier to work with.
Addition of the @_ prototype
Method::Signatures lets you punt and use @_ like in regular Perl 5.
PERFORMANCE
There is no run-time performance penalty for using this module above what it normally
costs to do argument handling.
There is also no run-time penalty for type-checking if you do not declare types. The run-
time penalty if you do declare types should be very similar to using
Mouse::Util::TypeConstraints (or Moose::Util::TypeConstraints) directly, and should be
faster than using a module such as MooseX::Params::Validate. The magic of Any::Moose is
used to give you the lightweight Mouse if you have not yet loaded Moose, or the full-
bodied Moose if you have.
Type-checking modules are not loaded until run-time, so this is fine:
use Method::Signatures;
use Moose;
# you will still get Moose type checking
# (assuming you declare one or more methods with types)
DEBUGGING
One of the best ways to figure out what Method::Signatures is doing is to run your code
through B::Deparse (run the code with -MO=Deparse).
Setting the "METHOD_SIGNATURES_DEBUG" environment variable will cause Method::Signatures
to display debugging information when it is compiling signatures.
EXAMPLE
Here's an example of a method which displays some text and takes some extra options.
use Method::Signatures;
method display($text is ro, :$justify = "left", :$fh = \*STDOUT) {
...
}
# $text = $stuff, $justify = "left" and $fh = \*STDOUT
$obj->display($stuff);
# $text = $stuff, $justify = "left" and $fh = \*STDERR
$obj->display($stuff, fh => \*STDERR);
# error, missing required $text argument
$obj->display();
The display() method is equivalent to all this code.
sub display {
my $self = shift;
croak('display() missing required argument $text') unless @_ > 0;
const my $text = $_[0];
my(%args) = @_[1 .. $#_];
my $justify = exists $args{justify} ? $args{justify} : 'left';
my $fh = exists $args{fh} ? $args{'fh'} : \*STDOUT;
...
}
EXPERIMENTING
If you want to experiment with the prototype syntax, start with
"Method::Signatures::parse_func". It takes a method prototype and returns a string of
Perl 5 code which will be placed at the beginning of that method.
If you would like to try to provide your own type checking, subclass Method::Signatures
and either override "type_check" or "inject_for_type_check". See "EXTENDING", below.
This interface is experimental, unstable and will change between versions.
EXTENDING
If you wish to subclass Method::Signatures, the following methods are good places to
start.
too_many_args_error, named_param_error, required_arg, type_error, where_error
These are class methods which report the various run-time errors (extra parameters,
unknown named parameter, required parameter missing, parameter fails type check, and
parameter fails where constraint respectively). Note that each one calls
"signature_error", which your versions should do as well.
signature_error
This is a class method which calls "signature_error_handler" (see below) and reports the
error as being from the caller's perspective. Most likely you will not need to override
this. If you'd like to have Method::Signatures errors give full stack traces (similar to
$Carp::Verbose), have a look at Carp::Always.
signature_error_handler
By default, "signature_error" generates an error message and "die"s with that message. If
you need to do something fancier with the generated error message, your subclass can
define its own "signature_error_handler". For example:
package My::Method::Signatures;
use Moose;
extends 'Method::Signatures';
sub signature_error_handler {
my ($class, $msg) = @_;
die bless { message => $msg }, 'My::ExceptionClass';
};
type_check
This is a class method which is called to verify that parameters have the proper type. If
you want to change the way that Method::Signatures does its type checking, this is most
likely what you want to override. It calls "type_error" (see above).
inject_for_type_check
This is the object method that actually inserts the call to "type_check" into your Perl
code. Most likely you will not need to override this, but if you wanted different
parameters passed into "type_check", this would be the place to do it.
BUGS, CAVEATS and NOTES
Please report bugs and leave feedback at <bug-Method-Signatures> at <rt.cpan.org>. Or use
the web interface at <http://rt.cpan.org>. Report early, report often.
One liners
If you want to write "use Method::Signatures" in a one-liner, do a "-MMethod::Signatures"
first. This is due to a bug/limitation in Devel::Declare.
Close parends in quotes or comments
Because of the way Devel::Declare parses things, an unbalanced close parend inside a quote
or comment could throw off the signature parsing. For instance:
func foo (
$foo, # $foo might contain )
$bar
)
is going to produce a syntax error, because the parend inside the comment is perceived as
the end of the signature. On the other hand, this:
func foo (
$foo, # (this is the $foo parend)
$bar
)
is fine, because the parends in the comments are balanced.
If you absolutely can't avoid an unbalanced close parend, such as in the following
signature:
func foo ( $foo, $bar = ")" ) # this won't parse correctly
you can always use a backslash to tell the parser that that close parend doesn't indicate
the end of the signature:
func foo ( $foo, $bar = "\)" ) # this is fine
This even works in single quotes:
func foo ( $foo, $bar = '\)' ) # default is ')', *not* '\)'!
although we don't recomment that form, as it may be surprising to readers of your code.
No source filter
While this module does rely on the black magic of Devel::Declare to access Perl's own
parser, it does not depend on a source filter. As such, it doesn't try to parse and
rewrite your source code and there should be no weird side effects.
Devel::Declare only affects compilation. After that, it's a normal subroutine. As such,
for all that hairy magic, this module is surprisingly stable.
Earlier Perl versions
The most noticeable is if an error occurs at compile time, such as a strict error, perl
might not notice until it tries to compile something else via an "eval" or "require" at
which point perl will appear to fail where there is no reason to fail.
We recommend you use the compile_at_BEGIN flag to turn off compile-time parsing.
You can't use any feature that requires a smartmatch expression (i.e. conditional
"Defaults" and "Value Constraints") in Perl 5.8.
Method::Signatures cannot be used with Perl versions prior to 5.8 because Devel::Declare
does not work with those earlier versions.
What about class methods?
Right now there's nothing special about class methods. Just use $class as your invocant
like the normal Perl 5 convention.
There may be special syntax to separate class from object methods in the future.
What about the return value?
Currently there is no support for declaring the type of the return value.
How does this relate to Perl's built-in prototypes?
It doesn't. Perl prototypes are a rather different beastie from subroutine signatures.
They don't work on methods anyway.
A syntax for function prototypes is being considered.
func($foo, $bar?) is proto($;$)
Error checking
Here's some additional checks I would like to add, mostly to avoid ambiguous or non-sense
situations.
* If one positional param is optional, everything to the right must be optional
method foo($a, $b?, $c?) # legal
method bar($a, $b?, $c) # illegal, ambiguous
Does "->bar(1,2)" mean $a = 1 and $b = 2 or $a = 1, $c = 3?
* Positionals are resolved before named params. They have precedence.
Slurpy parameter restrictions
Slurpy parameters are currently more restricted than they need to be. It is possible to
work out a slurpy parameter in the middle, or a named slurpy parameter. However, there's
lots of edge cases and possible nonsense configurations. Until that's worked out, we've
left it restricted.
What about...
Method traits are in the pondering stage.
An API to query a method's signature is in the pondering stage.
Now that we have method signatures, multi-methods are a distinct possibility.
Applying traits to all parameters as a short-hand?
# Equivalent?
method foo($a is ro, $b is ro, $c is ro)
method foo($a, $b, $c) is ro
Role::Basic roles are currently not recognized by the type system.
A "go really fast" switch. Turn off all runtime checks that might bite into performance.
Method traits.
method add($left, $right) is predictable # declarative
method add($left, $right) is cached # procedural
# (and Perl 6 compatible)
THANKS
Most of this module is based on or copied from hard work done by many other people.
All the really scary parts are copied from or rely on Matt Trout's, Florian Ragwitz's and
Rhesa Rozendaal's Devel::Declare work.
The prototype syntax is a slight adaptation of all the excellent work the Perl 6 folks
have already done.
The type checking and method modifier work was supplied by Buddy Burden (barefootcoder).
Thanks to this, you can now use Method::Signatures (or, more properly,
Method::Signatures::Modifiers) instead of MooseX::Method::Signatures, which fixes many of
the problems commonly attributed to MooseX::Declare.
Value constraints and default conditions (i.e. "where" and "when") were added by Damian
Conway, who also rewrote some of the signature parsing to make it more robust and more
extensible.
Also thanks to Matthijs van Duin for his awesome Data::Alias which makes the "\@foo"
signature work perfectly and Sub::Name which makes the subroutine names come out right in
caller().
And thanks to Florian Ragwitz for his parallel MooseX::Method::Signatures module from
which I borrow ideas and code.
LICENSE
The original code was taken from Matt S. Trout's tests for Devel::Declare.
Copyright 2007-2012 by Michael G Schwern <schwern@pobox.com>.
This program is free software; you can redistribute it and/or modify it under the same
terms as Perl itself.
See http://www.perl.com/perl/misc/Artistic.html
SEE ALSO
MooseX::Method::Signatures for an alternative implementation.
Perl6::Signature for a more complete implementation of Perl 6 signatures.
Method::Signatures::Simple for a more basic version of what Method::Signatures provides.
Function::Parameters for a subset of Method::Signature's features without using
Devel::Declare.
signatures for "sub" with signatures.
Perl 6 subroutine parameters and arguments -
<http://perlcabal.org/syn/S06.html#Parameters_and_arguments>
Moose::Util::TypeConstraints or Mouse::Util::TypeConstraints for further details on how
the type-checking works.