adt-simple

Algebraic data types for JavaScript using Sweet.js macros

npm install adt-simple
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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.

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.

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.

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:

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:

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.

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

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:

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:

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.

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.

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.

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)

Eq

Implements an equals method for deep equality:

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:

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

Clone

Implements a clone method for making deep copies:

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:

Clone.nativeClone = _.cloneDeep;

Setter

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.

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;

ToString

Extends instances with a good toString implementation.

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)))';

ToJSON

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

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
    }
  }
}

Curry

Implements constructor currying and partial application.

data Foo(*, *, *) deriving Curry

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

Extractor

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

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

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

Reflect

Implements tag properties and field/union name reflection.

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']

Cata

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

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));
    }
  })
}

LateDeriving

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

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

Foo.deriving(Eq, Clone);

Base

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


Author

Nathan Faubion (@natefaubion)

License

MIT

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