boop

Object-oriented programming for bash 4.3+ — real classes, objects, inheritance, and a standard library, in pure bash.

---

Project maintained by ydbxmhc Hosted on GitHub Pages

Mixins

Mixins are method bundles composed into classes at declaration time. They are not classes — they have no constructor, no state of their own, and do not appear in the isa chain. They give objects additional behavior without inheritance.

Contents

• Declaring a Mixin
• Composing into a Class
• Resolution Order
• Provenance Dispatch (::)
• mixes Predicate
• Writing a Mixin File
• A Class That Is Also a Mixin
• Available Mixins
• Mixin vs Inheritance

---

Declaring a Mixin

# Define the functions:
Printable.print() {
  printf '%s\n' "${_Class}: $(boop.pass "$_Self" ${into:-})"
}

Printable.inspect() {
  boop.pass "Printable::inspect on ${_Self}" ${into:-}
}

# Register them as a mixin:
boopMixin Printable 'public:print,inspect'

boopMixin takes the same public: and custom: token syntax as boopClass. The mixin’s functions must already be defined before calling boopMixin.

Composing into a Class

boopClass Article isa:Document mixin:Printable mixin:Taggable '
  has:title,body
  public:new,toString
'

Multiple mixin:Name tokens compose multiple mixins. The first mixin listed wins when two mixins provide the same method name.

Resolution Order

For any method call, boop searches in this order:

1. The object’s own class
2. Mixins (left to right, as listed in boopClass)
3. Parent class
4. Parent’s mixins
5. (continues up the chain)

Own class always wins. If Article defines print, its definition is used — the mixin’s print is shadowed.

Provenance Dispatch (::)

To call a specific mixin’s version of a method regardless of resolution order:

$article.Printable::print     # always Printable's version
$article.Taggable::identify   # always Taggable's version

This works even when the method is shadowed by the class or another mixin.

mixes Predicate

Check whether an object’s class composes a given mixin (walks the inheritance chain):

$article.mixes Printable   && echo "yes"   # 0 = true
$article.mixes Comparable  && echo "yes"   # 1 = false — silent

A subclass of a class that composes a mixin also satisfies mixes:

boopClass BlogPost isa:Article public:new,...
# BlogPost inherits Article's mixins

into=post BlogPost.new
$post.mixes Taggable   # 0 — inherited from Article

Writing a Mixin File

#!/bin/bash
# Comparable mixin — comparison predicates.

. boop
boop.initMixin Comparable || return 0

Comparable.eq() { ... }
Comparable.lt() { ... }
Comparable.gt() { ... }

boopMixin Comparable 'public:eq,lt,gt'

boop.initMixin is the mixin equivalent of boop.init for classes:

• Returns 1 (and the file returns 0 via || return 0) if already loaded
• Detects direct execution and prints a helpful error instead of crashing

A Class That Is Also a Mixin

A file can declare both boopClass and boopMixin for the same name. The class registration covers new, isa, and object dispatch. The mixin registration separately lists which methods other classes can compose (typically the same methods minus new):

boopClass Serializable public:new,toJSON,fromJSON has:...
boopMixin  Serializable 'public:toJSON,fromJSON'

( See Serializable )

This is valid because both registries (__boop_registry for classes and __boop_mixin_registry for mixins) are independent.

Available Mixins

Mixin	Purpose	Doc
Terminal	ANSI output, raw input, named symbols	Terminal.md
Taggable	Tag/untag/query labels on any object	Taggable.md
Greetable	Demo/test mixin for the system	Greetable.md

Mixin vs Inheritance

	Inheritance (isa:)	Mixin (mixin:)
State	Inherited	Not shared (host’s descriptor)
Constructor	Inherited	None
isa check	Yes	No
mixes check	N/A	Yes
Multiple	No (single parent)	Yes (as many as needed)
Method priority	Parent loses to child	First mixin listed wins

---

↑ Site map