adt-simple

adt-simple
==========

Native algebraic data types for JavaScript using
sweet.js macros

Features
--------

* No required runtime dependencies
* deriving sugar for mixing in generic behavior
* A catalogue of behavior mixins (Eq, Clone, ToJSON, etc)

Install
-------

npm install -g sweet.js
npm install adt-simple
sjs -m adt-simple/macros myfile.js

Basic Usage
-----------

adt-simple exports a data macro for creating simple data constructors.

``js
data Singleton
data SingletonVal = 42
data Tuple(, )
data Employee {
name: String,
salary: Number
}

// Singletons can have constant values
SingletonVal.value === 42;

// Positional fields
var tup = Tuple(1, 2);
tup[0] === 1;
tup[1] === 2;

// Named fields
var pete = Employee('Peter Gibbons', 85000);
pete.name === 'Peter Gibbons';
pete.salary === 85000;
`

It also exports a union macro for grouping your constructors.

`js
union Maybe {
Nothing,
Just {
value: *
}
}

// Basic inheritance
Nothing instanceof Maybe;
Just(42) instanceof Maybe;

// Constructors exported on the parent
Maybe.Nothing === Nothing;
Maybe.Just === Just;
`

You can even build recursive unions.

`js
union List {
Nil,
Cons {
head: *,
tail: List
}
}

var list = Cons(1, Cons(2, Cons(3, Nil)));
list.head === 1;
list.tail.head === 2;
list.tail.tail.head === 3;

// TypeError('Unexpected type for field: List.Cons.tail')
Cons(1, 2)
`

adt-simple doesn't just do instance checking. You can put in your own custom
constraints that validate or transform values:

`js
function toString(x) {
return x.toString();
}

data OnlyStrings {
value: toString
}

OnlyStrings(42).value === '42';
`

It tries to do the right thing: if the identifier starts with a capital letter
(taking namespaces into consideration), it will do instance checking, otherwise
it will call the constraint as a function. The instance checking is smart about
built-in JavaScript types, so it will do proper tag checks for Boolean,
Number, Array, etc beyond just instanceof.

Deriving
--------

adt-simple also supports a powerful sugar for deriving generic behaviour:

`js
union Maybe {
Nothing,
Just {
value: *
}
} deriving (Eq, Clone)
`

This works for both union and data constructors.

Built-in Derivers
-----------------

You can import built-in derivers by requiring the adt-simple library (< 1KB).
It's available as a UMD module, so you can use it in the browser with
require.js or with the global adt namespace.

`js
var Eq = require('adt-simple').Eq;
`

* Eq
* Clone
* Setter
* ToString
* ToJSON
* Curry
* Extractor
* Reflect
* Cata
* LateDeriving
* Base

Writing Your Own Derivers
-------------------------

Derivers are simply objects with a derive method. The derive method is
called with a template of the ADT so you can traverse, inspect, extend, or
modify it. Here's what a template for a List union would look like:

`js
union List {
Nil,
Cons {
head: *,
tail: List
}
}

{
name: 'List',
constructor: List,
prototype: List.prototype,
variants: [
{
name: 'Nil',
constructor: Nil,
prototype: Nil.prototype
},
{
name: 'Cons',
constructor: Cons,
prototype: Cons.prototype,
fields: ['head', 'tail']
}
]
}
`

Here's how you might write a deriver to get positional fields on a record:

`js
var Get = {
derive: function(adt) {
adt.variants.forEach(function(v) {
if (v.fields) {
v.prototype.get = function(i) {
return this[v.fields[i]];
};
} else {
v.prototype.get = function() {
throw new Error('No fields');
};
}
})
return adt;
}
};
`

Notice how you need to return the template at the end of the function to pass
on to the next deriver in the chain. You are free to mutate or tag the template
as needed to communicate between derivers.

Since the above pattern is so common, you can use the eachVariant helper to
shorten your code.

`js
var eachVariant = require('adt-simple').eachVariant;
var Get = {
derive: eachVariant(function(v, adt) {
if (v.fields) {
v.prototype.get = function(i) {
return this[v.fields[i]];
};
} else {
v.prototype.get = function() {
throw new Error('No fields')
};
}
})
};
`

You can also use composeDeriving to compose derivers together into a single
chain.

`js
var composeDeriving = require('adt-simple').composeDeriving;
var MyDeriver = composeDeriving(Eq, Clone, Setter, Curry);

data Foo(, ) deriving MyDeriver
`

Derivers are just expressions, so you can even parameterize them.

`js
var Log = function(prefix) {
return {
derive: eachVariant(function(v) {
v.prototype.log = function() {
console.log(prefix + ' ' + this.toString());
};
})
}
};

data Foo(, ) deriving Log('hello')

Foo(1, 2).log() // logs: hello Foo(1, 2)
`

Compiler Pragmas
----------------

adt-simple has sensible defaults, but you can also configure the output by
adding one or more pragma comments before your definition.

`js
/ @newrequired, @scoped /
union Foo {
Bar,
Baz
}
`

$3

By default, constructors can be called without a new keyword. This pragma
disables the instanceof check that enables this behavior, leaving you with
a simpler constructor. Note: this pragma will conflict with the Curry
deriver.

$3

All union variants are unwrapped and put in the outer scope. This pragma
disables the unwrapping and leaves them scoped to the parent.

$3

This pragma lets you define a custom apply method on the parent constructor,
which lets you call it as a normal function.

`js
/ @overrideapply /
union List {
Nil,
Cons {
head: *,
tail: List
}
} deriving ToString

List.apply = function(ctx, args) {
// Turn an array into a list
};

List(1, 2, 3).toString() === 'Cons(1, Cons(2, Cons(3, Nil)))';
`

---

$3

Implements an equals method for deep equality:

`js
data Foo(*) deriving Eq
Foo(Foo(1)).equals(Foo(Foo(1))) === true;
`

By default, Eq uses reference equality for anything without an equals
method, but you can override it. For example, using lodash:

`js
Eq.nativeEquals = _.isEqual;
Foo([1, 2, 3]).equals(Foo([1, 2, 3])) === true;
`

$3

Implements a clone method for making deep copies:

`js
data Foo(*) deriving Clone

var foo1 = Foo(1);
var foo2 = foo1.clone();

foo1 !== foo2 && foo2[0] === 1;
`

Like with Eq, Clone copies by references anything without a clone method.
You can override that behavior in a similar way. Using lodash:

`js
Clone.nativeClone = _.cloneDeep;
`

$3

Extends constructors with a create method and instances with a set method
for setting named values. set returns a shallow copy with the provided values
changed.

`js
data Foo {
bar: *,
baz: *
} deriving Setter

var foo1 = Foo.create({ bar: 42, baz: 12 });
var foo2 = foo1.set({ bar: 43 });

foo1 !== foo2;
foo2.bar === 43 && foo2.baz === foo1.baz;
`

$3

Extends instances with a good toString implementation.

`js
union List {
Nil,
Cons {
head: *,
tail: List
}
} deriving ToString

var list = Cons(1, Cons(2, Cons(3, Nil)));
list.toString() === 'Cons(1, Cons(2, Cons(3, Nil)))';
`

$3

Implements a toJSON method. You can configure how singletons are serialized
by assigning a constant value to it.

`js
union List {
Nil = null,
Cons {
head: *,
tail: List
}
} deriving ToJSON

var list = Cons(1, Cons(2, Cons(3, Nil)));
list.toJSON()
{
head: 1,
tail: {
head: 2,
tail: {
head: 3,
tail: null
}
}
}
`

$3

Implements constructor currying and partial application.

`js
data Foo(, , *) deriving Curry

Foo(1, 2, 3);
Foo(1)(2)(3);
Foo(1, 2)(3);
Foo(1)(2, 3);
`

$3

Implements the sparkler extractor
protocol so you can pattern match on your data instances.

`js
union List {
Nil,
Cons {
head: *,
tail: List
}
} deriving Extractor

List.prototype.map = function(fn) {
return match this {
Nil => Nil,
Cons(x, xs) => Cons(fn(x), xs.map(fn))
}
}
`

$3

Implements tag properties and field/union name reflection.

`js
union List {
Nil,
Cons {
head: *,
tail: List
}
} deriving Reflect


Nil.isNil === true;
Cons(1, Nil).isCons === true;

List.__names__ // ['Nil', 'Cons']
Cons.__fields__ // ['head', 'tail']
`

$3

Implements a cata method (ala daggy)
for doing dispatching and destructuring.

`js
union List {
Nil,
Cons {
head: *,
tail: List
}
} deriving Cata

List.prototype.map = function(fn) {
return this.cata({
Nil: function() {
return Nil;
},
Cons: function(x, xs) {
return Cons(fn(x), xs.map(fn));
}
})
}
`

$3

Extends constructors with a deriving method for deriving after-the-fact.

`js
data Foo {
bar: *,
baz: *
} deriving LateDeriving

Foo.deriving(Eq, Clone);
`

$3

A composition of Eq, Clone, Setter, ToString, Reflect, and Extractor`.

---

$3

Nathan Faubion (@natefaubion)

$3

MIT