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maybe.js
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maybe.js
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// Copyright (c) 2013-2014 Quildreen Motta <quildreen@gmail.com>
//
// Permission is hereby granted, free of charge, to any person
// obtaining a copy of this software and associated documentation files
// (the "Software"), to deal in the Software without restriction,
// including without limitation the rights to use, copy, modify, merge,
// publish, distribute, sublicense, and/or sell copies of the Software,
// and to permit persons to whom the Software is furnished to do so,
// subject to the following conditions:
//
// The above copyright notice and this permission notice shall be
// included in all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
// NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
// LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
// OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
// WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
/**
* @module lib/maybe
*/
module.exports = Maybe
// -- Aliases ----------------------------------------------------------
var clone = Object.create
var unimplemented = function(){ throw new Error('Not implemented.') }
var noop = function(){ return this }
// -- Implementation ---------------------------------------------------
/**
* A structure for values that may not be present, or computations that may
* fail. `Maybe(a)` explicitly models the effects that are implicit in
* `Nullable` types, thus has none of the problems associated with
* `null` or `undefined` — like `NullPointerExceptions`.
*
* The class models two different cases:
*
* + `Just a` — represents a `Maybe(a)` that contains a value. `a` may
* be any value, including `null` or `undefined`.
*
* + `Nothing` — represents a `Maybe(a)` that has no values. Or a
* failure that needs no additional information.
*
* Common uses of this structure includes modelling values that may or may
* not be present in a collection, thus instead of needing a
* `collection.has(a)`, the `collection.get(a)` operation gives you all
* the information you need — `collection.get(a).is-nothing` being
* equivalent to `collection.has(a)`; Similarly the same reasoning may
* be applied to computations that may fail to provide a value, e.g.:
* `collection.find(predicate)` can safely return a `Maybe(a)` instance,
* even if the collection contains nullable values.
*
* Furthermore, the values of `Maybe(a)` can be combined and manipulated
* by using the expressive monadic operations. This allows safely
* sequencing operations that may fail, and safely composing values that
* you don't know whether they're present or not, failing early
* (returning a `Nothing`) if any of the operations fail.
*
* If one wants to store additional information about failures, the
* [Either][] and [Validation][] structures provide such a capability, and
* should be used instead of the `Maybe(a)` structure.
*
* [Either]: https://github.com/folktale/data.either
* [Validation]: https://github.com/folktale/data.validation
*
*
* @class
*/
function Maybe() {}
// The case for successful values
Just.prototype = clone(Maybe.prototype)
function Just(a){
this.value = a
}
// The case for failure values
Nothing.prototype = clone(Maybe.prototype)
function Nothing(){}
// -- Constructors -----------------------------------------------------
/**
* Constructs a new `Maybe[α]` structure with an absent value. Commonly used
* to represent a failure.
*
* @summary Void → Maybe[α]
*/
Maybe.Nothing = function() {
return new Nothing
}
Maybe.prototype.Nothing = Maybe.Nothing
/**
* Constructs a new `Maybe[α]` structure that holds the single value
* `α`. Commonly used to represent a success.
*
* `α` can be any value, including `null`, `undefined` or another
* `Maybe[α]` structure.
*
* @summary α → Maybe[α]
*/
Maybe.Just = function(a) {
return new Just(a)
}
Maybe.prototype.Just = Maybe.Just
// -- Conversions ------------------------------------------------------
/**
* Constructs a new `Maybe[α]` structure from a nullable type.
*
* If the value is either `null` or `undefined`, this function returns a
* `Nothing`, otherwise the value is wrapped in a `Just(α)`.
*
* @summary α → Maybe[α]
*/
Maybe.fromNullable = function(a) {
return a != null? new Just(a)
: /* otherwise */ new Nothing
}
Maybe.prototype.fromNullable = Maybe.fromNullable
/**
* Constructs a new `Maybe[β]` structure from an `Either[α, β]` type.
*
* The left side of the `Either` becomes `Nothing`, and the right side
* is wrapped in a `Just(β)`.
*
* @summary Either[α, β] → Maybe[β]
*/
Maybe.fromEither = function(a) {
return a.fold(Maybe.Nothing, Maybe.Just)
}
Maybe.prototype.fromEither = Maybe.fromEither
/**
* Constructs a new `Maybe[β]` structure from a `Validation[α, β]` type.
*
* The failure side of the `Validation` becomes `Nothing`, and the right
* side is wrapped in a `Just(β)`.
*
* @method
* @summary Validation[α, β] → Maybe[β]
*/
Maybe.fromValidation = Maybe.fromEither
Maybe.prototype.fromValidation = Maybe.fromEither
// -- Predicates -------------------------------------------------------
/**
* True if the `Maybe[α]` structure contains a failure (i.e.: `Nothing`).
*
* @summary Boolean
*/
Maybe.prototype.isNothing = false
Nothing.prototype.isNothing = true
/**
* True if the `Maybe[α]` structure contains a single value (i.e.: `Just(α)`).
*
* @summary Boolean
*/
Maybe.prototype.isJust = false
Just.prototype.isJust = true
// -- Applicative ------------------------------------------------------
/**
* Creates a new `Maybe[α]` structure holding the single value `α`.
*
* `α` can be any value, including `null`, `undefined`, or another
* `Maybe[α]` structure.
*
* @summary α → Maybe[α]
*/
Maybe.of = function(a) {
return new Just(a)
}
Maybe.prototype.of = Maybe.of
/**
* Applies the function inside the `Maybe[α]` structure to another
* applicative type.
*
* The `Maybe[α]` structure should contain a function value, otherwise a
* `TypeError` is thrown.
*
* @method
* @summary (@Maybe[α → β], f:Applicative[_]) => f[α] → f[β]
*/
Maybe.prototype.ap = unimplemented
Nothing.prototype.ap = noop
Just.prototype.ap = function(b) {
return b.map(this.value)
}
// -- Functor ----------------------------------------------------------
/**
* Transforms the value of the `Maybe[α]` structure using a regular unary
* function.
*
* @method
* @summary @Maybe[α] => (α → β) → Maybe[β]
*/
Maybe.prototype.map = unimplemented
Nothing.prototype.map = noop
Just.prototype.map = function(f) {
return this.of(f(this.value))
}
// -- Chain ------------------------------------------------------------
/**
* Transforms the value of the `Maybe[α]` structure using an unary function
* to monads.
*
* @method
* @summary (@Maybe[α], m:Monad[_]) => (α → m[β]) → m[β]
*/
Maybe.prototype.chain = unimplemented
Nothing.prototype.chain = noop
Just.prototype.chain = function(f) {
return f(this.value)
}
// -- Show -------------------------------------------------------------
/**
* Returns a textual representation of the `Maybe[α]` structure.
*
* @method
* @summary @Maybe[α] => Void → String
*/
Maybe.prototype.toString = unimplemented
Nothing.prototype.toString = function() {
return 'Maybe.Nothing'
}
Just.prototype.toString = function() {
return 'Maybe.Just(' + this.value + ')'
}
// -- Eq ---------------------------------------------------------------
/**
* Tests if a `Maybe[α]` structure is equal to another `Maybe[α]` structure.
*
* @method
* @summary @Maybe[α] => Maybe[α] → Boolean
*/
Maybe.prototype.isEqual = unimplemented
Nothing.prototype.isEqual = function(b) {
return b.isNothing
}
Just.prototype.isEqual = function(b) {
return b.isJust
&& b.value === this.value
}
// -- Extracting and recovering ----------------------------------------
/**
* Extracts the value out of the `Maybe[α]` structure, if it
* exists. Otherwise throws a `TypeError`.
*
* @method
* @summary @Maybe[α] => Void → a, :: partial, throws
* @see {@link module:lib/maybe~Maybe#getOrElse} — A getter that can handle failures
* @throws {TypeError} if the structure has no value (`Nothing`).
*/
Maybe.prototype.get = unimplemented
Nothing.prototype.get = function() {
throw new TypeError("Can't extract the value of a Nothing.")
}
Just.prototype.get = function() {
return this.value
}
/**
* Extracts the value out of the `Maybe[α]` structure. If there is no value,
* returns the given default.
*
* @method
* @summary @Maybe[α] => α → α
*/
Maybe.prototype.getOrElse = unimplemented
Nothing.prototype.getOrElse = function(a) {
return a
}
Just.prototype.getOrElse = function(_) {
return this.value
}
/**
* Transforms a failure into a new `Maybe[α]` structure. Does nothing if the
* structure already contains a value.
*
* @method
* @summary @Maybe[α] => (Void → Maybe[α]) → Maybe[α]
*/
Maybe.prototype.orElse = unimplemented
Nothing.prototype.orElse = function(f) {
return f()
}
Just.prototype.orElse = function(_) {
return this
}
/**
* Catamorphism.
*
* @method
* @summary @Maybe[α] => { Nothing: Void → β, Just: α → β } → β
*/
Maybe.prototype.cata = unimplemented
Nothing.prototype.cata = function(pattern) {
return pattern.Nothing()
}
Just.prototype.cata = function(pattern) {
return pattern.Just(this.value);
}
/**
* JSON serialisation
*
* @method
* @summary @Maybe[α] => Void → Object
*/
Maybe.prototype.toJSON = unimplemented
Nothing.prototype.toJSON = function() {
return { '#type': 'folktale:Maybe.Nothing' }
}
Just.prototype.toJSON = function() {
return { '#type': 'folktale:Maybe.Just'
, value: this.value }
}