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## Pattern 7 – Method Modifiers

```perl
before calc => sub { die "x<0" if $_[1] < 0 };         # validate, can't change return

around calc => sub {
  my ($orig, $self, $x) = @_;
  return $self->$orig($x) + 1;                        # can change return value
};

after calc => sub { ... };                               # side-effects, logging
```

**Rules:** `before`/`after` cannot alter return value; `around` can. Always forward `@_` correctly in `around`. Multiple modifiers from multiple roles stack – order is composition-order-sensitive.

---

## Pattern 8 – Attribute Options Cheatsheet

```perl
has name   => (is => 'ro',   required => 1);
has tags   => (is => 'ro',   default  => sub { [] });    # ALWAYS coderef for refs
has id     => (is => 'lazy');                             # built on first access
sub _build_id { "id:" . $_[0]->name }

has status => (
  is      => 'rw',
  trigger => 1,                                        # calls _trigger_status on set
);
sub _trigger_status { die "bad" unless $_[1] =~ /\A(new|ok)\z/ }

has _secret => (is => 'ro', init_arg => 'secret');       # constructor param alias
```

`trigger` fires on `new()` and `set`, NOT on `default`/`builder`. Old value is NOT passed (unlike Moose). `is => 'lazy'` = lazy reader, runs builder on first access.

---

## Pattern 9 – Lifecycle Hooks

```perl
around BUILDARGS => sub {
  my ($orig, $class, @args) = @_;
  return { source => $args[0] } if @args == 1 && !ref $args[0];  # normalize
  $class->$orig(@args);
};

sub FOREIGNBUILDARGS {          # maps args to non-Moo parent's constructor
  my ($class, $args) = @_;
  return ($args->{source});
}

sub BUILD {                     # runs AFTER all attributes are set; parent→child order
  my ($self, $args) = @_;
  die "invalid" unless length $args->{source};
}
# DEMOLISH: child→parent order. Never override DESTROY directly.
```

Do NOT call `SUPER::BUILD` manually – Moo handles the chain.

---

## Pattern 10 – Strict Constructor

```perl
package StrictThing;
use Moo;
use MooX::StrictConstructor;
has size => (is => 'rw');

StrictThing->new(size => 5, colour => 'blue');  # dies: unknown attribute 'colour'
```

**Caveat:** Interacts with `namespace::clean` (can sweep `new`). If needed, protect via `-except => ['new']` or adjust import order per the module docs.

---

## Pattern 11 – Role Conflict Resolution

```perl
# Single with → conflict → dies:
# with 'RoleA', 'RoleB';   # both define foo() → fatal

# Sequential with → first wins:
with 'RoleA';   # foo() from RoleA is now in the class
with 'RoleB';   # foo() already exists → RoleA wins silently
```

"Class wins": if the class defines `foo()` itself, neither role's version is used. For complex conflict strategies: refactor roles to avoid the overlap.

---

## Pattern 12 – Parameterized Roles

```perl
package Counter;
use Moo::Role;
use MooX::Role::Parameterized;
parameter name => (is => 'ro', required => 1);
role {
  my ($p, $mop) = @_;
  my $n = $p->name;
  $mop->has($n => (is => 'rw', default => sub { 0 }));
  $mop->method("inc_$n" => sub { $_[0]->$n($_[0]->$n + 1) });
};

package Thing;
use Moo;
use MooX::Role::Parameterized::With;
with Counter => { name => 'hits' };

Thing->new->inc_hits;   # generates: hits attribute + inc_hits method
```

Module is marked **experimental**. `role { }` block runs at composition time; `$mop` proxies `has/around/before/after/requires`.

---

## Pattern 13 – Moose Interop

When Moose is loaded before Moo classes are compiled, Moo auto-inflates its metaclasses. This means:

- Moose class can `extends` a Moo class
- Moo class can `with` a Moose role

```perl
BEGIN { require Moose }
package MyMooseClass;
use Moose;
extends 'MyMooClass';   # works if Moose was loaded first
```

For Moose-style syntax in Moo (`isa => 'Str'`, `lazy_build`), use `MooX::late`. Avoid `Any::Moose` – deprecated, points to Moo.

---

## Type Constraints

Moo has no built-in type system — `isa` takes a coderef, and `Type::Tiny` objects
are coderefs, so `Types::Standard` plugs straight in:

```perl
use Types::Standard qw( Str ArrayRef );
has name => ( is => 'ro', isa => Str );
has tags => ( is => 'ro', isa => ArrayRef[Str], default => sub { [] } );
```

Where to type and where not, own type libraries, parameter signatures:
**`getty-perl-typing`**.

---

## Decision Guide

| Situation | Use |
|---|---|
| Shared attributes/methods, stable "is-a" | `extends` |
| Optional/horizontal feature | `Moo::Role` + `with` |
| Same pattern, different config | `MooX::Role::Parameterized` |
| Delegate method set to sub-object | `handles` |
| Array/Hash operations on attribute | `Sub::HandlesVia` |
| Logging/validation/caching wrapper | `before`/`around`/`after` |
| Catch constructor typos | `MooX::StrictConstructor` |
| Cross-project boilerplate | `Import::Into` house-style module |
| Named types | `Type::Tiny` / `Types::Standard` |
| Multiple roles define same method | Sequential `with` or refactor |
| Legacy non-Moo parent | `FOREIGNBUILDARGS` |
| Multiple inheritance | Last resort; use `mro 'c3'` |

---

## Common Pitfalls

- `default => []` → **shared state bug**. Always `default => sub { [] }`.
- `extends 'A'; extends 'B'` → replaces, does NOT add B to A. Use `extends 'A', 'B'`.
- Imports after `use Moo::Role` are **composed into consumers** as methods.
- `namespace::autoclean` < 0.16 inflates Moo classes to Moose unexpectedly.
- `trigger` does NOT receive old value (unlike Moose).
- `Sub::HandlesVia` must be loaded *after* `use Moo`.
- `BUILD` chain is automatic; calling `SUPER::BUILD` manually breaks it.
- Never override `DESTROY`; use `DEMOLISH`.