fantasyland/fantasy-land

---

fantasyland/fantasy-land.json

{
  "createdAt": "2013-04-12T03:10:22Z",
  "defaultBranch": "master",
  "description": "Specification for interoperability of common algebraic structures in JavaScript",
  "fullName": "fantasyland/fantasy-land",
  "homepage": null,
  "language": "JavaScript",
  "name": "fantasy-land",
  "pushedAt": "2024-11-10T21:50:53Z",
  "stargazersCount": 10224,
  "topics": [
    "fantasy-land",
    "functional",
    "javascript",
    "specification"
  ],
  "updatedAt": "2025-11-23T19:26:01Z",
  "url": "https://github.com/fantasyland/fantasy-land"
}

Fantasy Land Specification

(aka “Algebraic JavaScript Specification”)

This project specifies interoperability of common algebraic structures:

 Setoid   Semigroupoid  Semigroup   Foldable        Functor      Contravariant  Filterable
(equals)    (compose)    (concat)   (reduce)         (map)        (contramap)    (filter)
    |           |           |           \         / | | | | \
    |           |           |            \       /  | | | |  \
    |           |           |             \     /   | | | |   \
    |           |           |              \   /    | | | |    \
    |           |           |               \ /     | | | |     \
   Ord      Category     Monoid         Traversable | | | |      \
  (lte)       (id)       (empty)        (traverse)  / | | \       \
                            |                      /  | |  \       \
                            |                     /   / \   \       \
                            |             Profunctor /   \ Bifunctor \
                            |              (promap) /     \ (bimap)   \
                            |                      /       \           \
                          Group                   /         \           \
                         (invert)               Alt        Apply      Extend
                                               (alt)        (ap)     (extend)
                                                /           / \           \
                                               /           /   \           \
                                              /           /     \           \
                                             /           /       \           \
                                            /           /         \           \
                                          Plus    Applicative    Chain      Comonad
                                         (zero)       (of)      (chain)    (extract)
                                            \         / \         / \
                                             \       /   \       /   \
                                              \     /     \     /     \
                                               \   /       \   /       \
                                                \ /         \ /         \
                                            Alternative    Monad     ChainRec
                                                                    (chainRec)

General

An algebra is a set of values, a set of operators that it is closed under and some laws it must obey.

Each Fantasy Land algebra is a separate specification. An algebra may have dependencies on other algebras which must be implemented.

Terminology

1. “value” is any JavaScript value, including any which have the structures defined below.
2. “equivalent” is an appropriate definition of equivalence for the given value. The definition should ensure that the two values can be safely swapped out in a program that respects abstractions. For example:
   • Two lists are equivalent if they are equivalent at all indices.
   • Two plain old JavaScript objects, interpreted as dictionaries, are equivalent when they are equivalent for all keys.
   • Two promises are equivalent when they yield equivalent values.
   • Two functions are equivalent if they yield equivalent outputs for equivalent inputs.

Type signature notation

The type signature notation used in this document is described below:^{[1]!(#sanctuary-types)}

• :: “is a member of”.
  • e :: t can be read as: “the expression e is a member of type t”.
  • true :: Boolean - “true is a member of type Boolean”.
  • 42 :: Integer, Number - “42 is a member of the Integer and Number types”.
• New types can be created via type constructors.
  • Type constructors can take zero or more type arguments.
  • Array is a type constructor which takes one type argument.
  • Array String is the type of all arrays of strings. Each of the following has type Array String: [], ['foo', 'bar', 'baz'].
  • Array (Array String) is the type of all arrays of arrays of strings. Each of the following has type Array (Array String): [], [ [], [] ], [ [], ['foo'], ['bar', 'baz'] ].
• Lowercase letters stand for type variables.
  • Type variables can take any type unless they have been restricted by means of type constraints (see fat arrow below).
• -> (arrow) Function type constructor.
  • -> is an infix type constructor that takes two type arguments where left argument is the input type and the right argument is the output type.
  • ->’s input type can be a grouping of types to create the type of a function which accepts zero or more arguments. The syntax is: (<input-types>) -> <output-type>, where <input-types> comprises zero or more comma–space (, )-separated type representations and parens may be omitted for unary functions.
  • String -> Array String is a type satisfied by functions which take a String and return an Array String.
  • String -> Array String -> Array String is a type satisfied by functions which take a String and return a function which takes an Array String and returns an Array String.
  • (String, Array String) -> Array String is a type satisfied by functions which take a String and an Array String as arguments and return an Array String.
  • () -> Number is a type satisfied by functions which do not take arguments and return a Number.
• ~> (squiggly arrow) Method type constructor.
  • When a function is a property of an Object, it is called a method. All methods have an implicit parameter type - the type of which they are a property.
  • a ~> a -> a is a type satisfied by methods on Objects of type a which take a type a as an argument and return a value of type a.
• => (fat arrow) Expresses constraints on type variables.
  • In a ~> a -> a (see squiggly arrow above), a can be of any type. Semigroup a => a ~> a -> a adds a constraint such that the type a must now satisfy the Semigroup typeclass. To satisfy a typeclass means to lawfully implement all functions/methods specified by that typeclass.

For example:

fantasy-land/traverse :: Applicative f, Traversable t => t a ~> (TypeRep f, a -> f b) -> f (t b)
'-------------------'    '--------------------------'    '-'    '-------------------'    '-----'
 '                        '                               '      '                        '
 '                        ' - type constraints            '      ' - argument types       ' - return type
 '                                                        '
 '- method name                                           ' - method target type

---

1. See the Types section in Sanctuary’s docs for more info. [↩]!(#sanctuary-types-return)

Type representatives

Certain behaviours are defined from the perspective of a member of a type. Other behaviours do not require a member. Thus certain algebras require a type to provide a value-level representative (with certain properties). The Identity type, for example, could provide Id as its type representative: Id :: TypeRep Identity.

If a type provides a type representative, each member of the type must have a constructor property which is a reference to the type representative.

Algebras

Setoid

1. a['fantasy-land/equals']!(a) === true (reflexivity)
2. a['fantasy-land/equals']!(b) === b['fantasy-land/equals']!(a) (symmetry)
3. If a['fantasy-land/equals']!(b) and b['fantasy-land/equals']!(c), then a['fantasy-land/equals']!(c) (transitivity)

fantasy-land/equals method

fantasy-land/equals :: Setoid a => a ~> a -> Boolean

A value which has a Setoid must provide a fantasy-land/equals method. The fantasy-land/equals method takes one argument:

a['fantasy-land/equals']!(b)

1. b must be a value of the same Setoid

   1. If b is not the same Setoid, behaviour of fantasy-land/equals is unspecified (returning false is recommended).

2. fantasy-land/equals must return a boolean (true or false).

Ord

A value that implements the Ord specification must also implement the [Setoid]!(#setoid) specification.

1. a['fantasy-land/lte']!(b) or b['fantasy-land/lte']!(a) (totality)
2. If a['fantasy-land/lte']!(b) and b['fantasy-land/lte']!(a), then a['fantasy-land/equals']!(b) (antisymmetry)
3. If a['fantasy-land/lte']!(b) and b['fantasy-land/lte']!(c), then a['fantasy-land/lte']!(c) (transitivity)

fantasy-land/lte method

fantasy-land/lte :: Ord a => a ~> a -> Boolean

A value which has an Ord must provide a fantasy-land/lte method. The fantasy-land/lte method takes one argument:

 a['fantasy-land/lte']!(b)

1. b must be a value of the same Ord

   1. If b is not the same Ord, behaviour of fantasy-land/lte is unspecified (returning false is recommended).

2. fantasy-land/lte must return a boolean (true or false).

Semigroupoid

1. a['fantasy-land/compose']!(b)['fantasy-land/compose']!(c) === a['fantasy-land/compose']!(b['fantasy-land/compose']!(c)) (associativity)

fantasy-land/compose method

fantasy-land/compose :: Semigroupoid c => c i j ~> c j k -> c i k

A value which has a Semigroupoid must provide a fantasy-land/compose method. The fantasy-land/compose method takes one argument:

a['fantasy-land/compose']!(b)

1. b must be a value of the same Semigroupoid

   1. If b is not the same semigroupoid, behaviour of fantasy-land/compose is unspecified.

2. fantasy-land/compose must return a value of the same Semigroupoid.

Category

A value that implements the Category specification must also implement the [Semigroupoid]!(#semigroupoid) specification.

1. a['fantasy-land/compose']!(C['fantasy-land/id']!()) is equivalent to a (right identity)
2. C['fantasy-land/id']!()['fantasy-land/compose']!(a) is equivalent to a (left identity)

fantasy-land/id method

fantasy-land/id :: Category c => () -> c a a

A value which has a Category must provide a fantasy-land/id function on its [type representative]!(#type-representatives):

C['fantasy-land/id']!()

Given a value c, one can access its type representative via the constructor property:

c.constructor['fantasy-land/id']!()

1. fantasy-land/id must return a value of the same Category

Semigroup

1. a['fantasy-land/concat']!(b)['fantasy-land/concat']!(c) is equivalent to a['fantasy-land/concat']!(b['fantasy-land/concat']!(c)) (associativity)

fantasy-land/concat method

fantasy-land/concat :: Semigroup a => a ~> a -> a

A value which has a Semigroup must provide a fantasy-land/concat method. The fantasy-land/concat method takes one argument:

s['fantasy-land/concat']!(b)

1. b must be a value of the same Semigroup

   1. If b is not the same semigroup, behaviour of fantasy-land/concat is unspecified.

2. fantasy-land/concat must return a value of the same Semigroup.

Monoid

A value that implements the Monoid specification must also implement the [Semigroup]!(#semigroup) specification.

1. m['fantasy-land/concat']!(M['fantasy-land/empty']!()) is equivalent to m (right identity)
2. M['fantasy-land/empty']!()['fantasy-land/concat']!(m) is equivalent to m (left identity)

fantasy-land/empty method

fantasy-land/empty :: Monoid m => () -> m

A value which has a Monoid must provide a fantasy-land/empty function on its [type representative]!(#type-representatives):

M['fantasy-land/empty']!()

Given a value m, one can access its type representative via the constructor property:

m.constructor['fantasy-land/empty']!()

1. fantasy-land/empty must return a value of the same Monoid

Group

A value that implements the Group specification must also implement the [Monoid]!(#monoid) specification.

1. g['fantasy-land/concat']!(g['fantasy-land/invert']!()) is equivalent to g.constructor['fantasy-land/empty']!() (right inverse)
2. g['fantasy-land/invert']!()['fantasy-land/concat']!(g) is equivalent to g.constructor['fantasy-land/empty']!() (left inverse)

fantasy-land/invert method

fantasy-land/invert :: Group g => g ~> () -> g

A value which has a Group must provide a fantasy-land/invert method. The fantasy-land/invert method takes no arguments:

g['fantasy-land/invert']!()

1. fantasy-land/invert must return a value of the same Group.

Filterable

1. v['fantasy-land/filter']!(x => p(x) && q(x)) is equivalent to v['fantasy-land/filter']!(p)['fantasy-land/filter']!(q) (distributivity)
2. v['fantasy-land/filter']!(x => true) is equivalent to v (identity)
3. v['fantasy-land/filter']!(x => false) is equivalent to w['fantasy-land/filter']!(x => false) if v and w are values of the same Filterable (annihilation)

fantasy-land/filter method

fantasy-land/filter :: Filterable f => f a ~> (a -> Boolean) -> f a

A value which has a Filterable must provide a fantasy-land/filter method. The fantasy-land/filter method takes one argument:

v['fantasy-land/filter']!(p)

1. p must be a function.

   1. If p is not a function, the behaviour of fantasy-land/filter is unspecified.
   2. p must return either true or false. If it returns any other value, the behaviour of fantasy-land/filter is unspecified.

2. fantasy-land/filter must return a value of the same Filterable.

Functor

1. u['fantasy-land/map']!(a => a) is equivalent to u (identity)
2. u['fantasy-land/map']!(x => f(g(x))) is equivalent to u['fantasy-land/map']!(g)['fantasy-land/map']!(f) (composition)

fantasy-land/map method

fantasy-land/map :: Functor f => f a ~> (a -> b) -> f b

A value which has a Functor must provide a fantasy-land/map method. The fantasy-land/map method takes one argument:

u['fantasy-land/map']!(f)

1. f must be a function,

   1. If f is not a function, the behaviour of fantasy-land/map is unspecified.
   2. f can return any value.
   3. No parts of f’s return value should be checked.

2. fantasy-land/map must return a value of the same Functor

Contravariant

1. u['fantasy-land/contramap']!(a => a) is equivalent to u (identity)
2. u['fantasy-land/contramap']!(x => f(g(x))) is equivalent to u['fantasy-land/contramap']!(f)['fantasy-land/contramap']!(g) (composition)

fantasy-land/contramap method

fantasy-land/contramap :: Contravariant f => f a ~> (b -> a) -> f b

A value which has a Contravariant must provide a fantasy-land/contramap method. The fantasy-land/contramap method takes one argument:

u['fantasy-land/contramap']!(f)

1. f must be a function,

   1. If f is not a function, the behaviour of fantasy-land/contramap is unspecified.
   2. f can return any value.
   3. No parts of f’s return value should be checked.

2. fantasy-land/contramap must return a value of the same Contravariant

Apply

A value that implements the Apply specification must also implement the [Functor]!(#functor) specification.

1. v['fantasy-land/ap']!(u['fantasy-land/ap']!(a['fantasy-land/map']!(f => g => x => f(g(x))))) is equivalent to v['fantasy-land/ap']!(u)['fantasy-land/ap']!(a) (composition)

fantasy-land/ap method

fantasy-land/ap :: Apply f => f a ~> f (a -> b) -> f b

A value which has an Apply must provide a fantasy-land/ap method. The fantasy-land/ap method takes one argument:

a['fantasy-land/ap']!(b)

1. b must be an Apply of a function

   1. If b does not represent a function, the behaviour of fantasy-land/ap is unspecified.
   2. b must be same Apply as a.

2. a must be an Apply of any value

3. fantasy-land/ap must apply the function in Apply b to the value in Apply a

   1. No parts of return value of that function should be checked.

4. The Apply returned by fantasy-land/ap must be the same as a and b

Applicative

A value that implements the Applicative specification must also implement the [Apply]!(#apply) specification.

1. v['fantasy-land/ap']!(A['fantasy-land/of']!(x => x)) is equivalent to v (identity)
2. A['fantasy-land/of']!(x)['fantasy-land/ap']!(A['fantasy-land/of']!(f)) is equivalent to A['fantasy-land/of']!(f(x)) (homomorphism)
3. A['fantasy-land/of']!(y)['fantasy-land/ap']!(u) is equivalent to u['fantasy-land/ap']!(A['fantasy-land/of']!(f => f(y))) (interchange)

fantasy-land/of method

fantasy-land/of :: Applicative f => a -> f a

A value which has an Applicative must provide a fantasy-land/of function on its [type representative]!(#type-representatives). The fantasy-land/of function takes one argument:

F['fantasy-land/of']!(a)

Given a value f, one can access its type representative via the constructor property:

f.constructor['fantasy-land/of']!(a)

1. fantasy-land/of must provide a value of the same Applicative

   1. No parts of a should be checked

Alt

A value that implements the Alt specification must also implement the [Functor]!(#functor) specification.

1. a['fantasy-land/alt']!(b)['fantasy-land/alt']!(c) is equivalent to a['fantasy-land/alt']!(b['fantasy-land/alt']!(c)) (associativity)
2. a['fantasy-land/alt']!(b)['fantasy-land/map']!(f) is equivalent to a['fantasy-land/map']!(f)['fantasy-land/alt']!(b['fantasy-land/map']!(f)) (distributivity)

fantasy-land/alt method

fantasy-land/alt :: Alt f => f a ~> f a -> f a

A value which has a Alt must provide a fantasy-land/alt method. The fantasy-land/alt method takes one argument:

a['fantasy-land/alt']!(b)

1. b must be a value of the same Alt

   1. If b is not the same Alt, behaviour of fantasy-land/alt is unspecified.
   2. a and b can contain any value of same type.
   3. No parts of a’s and b’s containing value should be checked.

2. fantasy-land/alt must return a value of the same Alt.

Plus

A value that implements the Plus specification must also implement the [Alt]!(#alt) specification.

1. x['fantasy-land/alt']!(A['fantasy-land/zero']!()) is equivalent to x (right identity)
2. A['fantasy-land/zero']!()['fantasy-land/alt']!(x) is equivalent to x (left identity)
3. A['fantasy-land/zero']!()['fantasy-land/map']!(f) is equivalent to A['fantasy-land/zero']!() (annihilation)

fantasy-land/zero method

fantasy-land/zero :: Plus f => () -> f a

A value which has a Plus must provide a fantasy-land/zero function on its [type representative]!(#type-representatives):

A['fantasy-land/zero']!()

Given a value x, one can access its type representative via the constructor property:

x.constructor['fantasy-land/zero']!()

1. fantasy-land/zero must return a value of the same Plus

Alternative

A value that implements the Alternative specification must also implement the [Applicative]!(#applicative) and [Plus]!(#plus) specifications.

1. x['fantasy-land/ap']!(f['fantasy-land/alt']!(g)) is equivalent to x['fantasy-land/ap']!(f)['fantasy-land/alt']!(x['fantasy-land/ap']!(g)) (distributivity)
2. x['fantasy-land/ap']!(A['fantasy-land/zero']!()) is equivalent to A['fantasy-land/zero']!() (annihilation)

Foldable

1. u['fantasy-land/reduce'] is equivalent to u['fantasy-land/reduce']!((acc, x) => acc.concat([x]), []).reduce

fantasy-land/reduce method

fantasy-land/reduce :: Foldable f => f a ~> ((b, a) -> b, b) -> b

A value which has a Foldable must provide a fantasy-land/reduce method. The fantasy-land/reduce method takes two arguments:

u['fantasy-land/reduce']!(f, x)

1. f must be a binary function

   1. if f is not a function, the behaviour of fantasy-land/reduce is unspecified.
   2. The first argument to f must be the same type as x.
   3. f must return a value of the same type as x.
   4. No parts of f’s return value should be checked.

2. x is the initial accumulator value for the reduction

   1. No parts of x should be checked.

Traversable

A value that implements the Traversable specification must also implement the [Functor]!(#functor) and [Foldable]!(#foldable) specifications.

1. t(u['fantasy-land/traverse']!(F, x => x)) is equivalent to u['fantasy-land/traverse']!(G, t) for any t such that t(a)['fantasy-land/map']!(f) is equivalent to t(a['fantasy-land/map']!(f)) (naturality)

2. u['fantasy-land/traverse']!(F, F['fantasy-land/of']) is equivalent to F['fantasy-land/of']!(u) for any Applicative F (identity)

3. u['fantasy-land/traverse']!(Compose, x => new Compose(x)) is equivalent to new Compose(u['fantasy-land/traverse']!(F, x => x)['fantasy-land/map']!(x => x['fantasy-land/traverse']!(G, x => x))) for Compose defined below and any Applicatives F and G (composition)

function Compose(c) {
  this.c = c;
}

Compose['fantasy-land/of'] = function(x) {
  return new Compose(F['fantasy-land/of']!(G['fantasy-land/of']!(x)));
};

Compose.prototype['fantasy-land/ap'] = function(f) {
  return new Compose(this.c['fantasy-land/ap']!(f.c['fantasy-land/map']!(u => y => y['fantasy-land/ap']!(u))));
};

Compose.prototype['fantasy-land/map'] = function(f) {
  return new Compose(this.c['fantasy-land/map']!(y => y['fantasy-land/map']!(f)));
};

fantasy-land/traverse method

fantasy-land/traverse :: Applicative f, Traversable t => t a ~> (TypeRep f, a -> f b) -> f (t b)

A value which has a Traversable must provide a fantasy-land/traverse method. The fantasy-land/traverse method takes two arguments:

u['fantasy-land/traverse']!(A, f)

1. A must be the [type representative]!(#type-representatives) of an Applicative.

2. f must be a function which returns a value

   1. If f is not a function, the behaviour of fantasy-land/traverse is unspecified.
   2. f must return a value of the type represented by A.

3. fantasy-land/traverse must return a value of the type represented by A.

Chain

A value that implements the Chain specification must also implement the [Apply]!(#apply) specification.

1. m['fantasy-land/chain']!(f)['fantasy-land/chain']!(g) is equivalent to m['fantasy-land/chain']!(x => f(x)['fantasy-land/chain']!(g)) (associativity)

fantasy-land/chain method

fantasy-land/chain :: Chain m => m a ~> (a -> m b) -> m b

A value which has a Chain must provide a fantasy-land/chain method. The fantasy-land/chain method takes one argument:

m['fantasy-land/chain']!(f)

1. f must be a function which returns a value

   1. If f is not a function, the behaviour of fantasy-land/chain is unspecified.
   2. f must return a value of the same Chain

2. fantasy-land/chain must return a value of the same Chain

ChainRec

A value that implements the ChainRec specification must also implement the [Chain]!(#chain) specification.

1. M['fantasy-land/chainRec']!((next, done, v) => p(v) ? d(v)['fantasy-land/map']!(done) : n(v)['fantasy-land/map']!(next), i) is equivalent to (function step(v) { return p(v) ? d(v) : n(v)['fantasy-land/chain']!(step); }(i)) (equivalence)
2. Stack usage of M['fantasy-land/chainRec']!(f, i) must be at most a constant multiple of the stack usage of f itself.

fantasy-land/chainRec method

fantasy-land/chainRec :: ChainRec m => ((a -> c, b -> c, a) -> m c, a) -> m b

A Type which has a ChainRec must provide a fantasy-land/chainRec function on its [type representative]!(#type-representatives). The fantasy-land/chainRec function takes two arguments:

M['fantasy-land/chainRec']!(f, i)

Given a value m, one can access its type representative via the constructor property:

m.constructor['fantasy-land/chainRec']!(f, i)

1. f must be a function which returns a value
   1. If f is not a function, the behaviour of fantasy-land/chainRec is unspecified.
   2. f takes three arguments next, done, value
      1. next is a function which takes one argument of same type as i and can return any value
      2. done is a function which takes one argument and returns the same type as the return value of next
      3. value is some value of the same type as i
   3. f must return a value of the same ChainRec which contains a value returned from either done or next
2. fantasy-land/chainRec must return a value of the same ChainRec which contains a value of same type as argument of done

Monad

A value that implements the Monad specification must also implement the [Applicative]!(#applicative) and [Chain]!(#chain) specifications.

1. M['fantasy-land/of']!(a)['fantasy-land/chain']!(f) is equivalent to f(a) (left identity)
2. m['fantasy-land/chain']!(M['fantasy-land/of']) is equivalent to m (right identity)

Extend

A value that implements the Extend specification must also implement the [Functor]!(#functor) specification.

1. w['fantasy-land/extend']!(g)['fantasy-land/extend']!(f) is equivalent to w['fantasy-land/extend']!(_w => f(_w['fantasy-land/extend']!(g)))

fantasy-land/extend method

fantasy-land/extend :: Extend w => w a ~> (w a -> b) -> w b

An Extend must provide a fantasy-land/extend method. The fantasy-land/extend method takes one argument:

 w['fantasy-land/extend']!(f)

1. f must be a function which returns a value

   1. If f is not a function, the behaviour of fantasy-land/extend is unspecified.
   2. f must return a value of type v, for some variable v contained in w.
   3. No parts of f’s return value should be checked.

2. fantasy-land/extend must return a value of the same Extend.

Comonad

A value that implements the Comonad specification must also implement the [Extend]!(#extend) specification.

1. w['fantasy-land/extend']!(_w => _w['fantasy-land/extract']!()) is equivalent to w (left identity)
2. w['fantasy-land/extend']!(f)['fantasy-land/extract']!() is equivalent to f(w) (right identity)

fantasy-land/extract method

fantasy-land/extract :: Comonad w => w a ~> () -> a

A value which has a Comonad must provide a fantasy-land/extract method on itself. The fantasy-land/extract method takes no arguments:

w['fantasy-land/extract']!()

1. fantasy-land/extract must return a value of type v, for some variable v contained in w.
   1. v must have the same type that f returns in fantasy-land/extend.

Bifunctor

A value that implements the Bifunctor specification must also implement the [Functor]!(#functor) specification.

1. p['fantasy-land/bimap']!(a => a, b => b) is equivalent to p (identity)
2. p['fantasy-land/bimap']!(a => f(g(a)), b => h(i(b))) is equivalent to p['fantasy-land/bimap']!(g, i)['fantasy-land/bimap']!(f, h) (composition)

fantasy-land/bimap method

fantasy-land/bimap :: Bifunctor f => f a c ~> (a -> b, c -> d) -> f b d

A value which has a Bifunctor must provide a fantasy-land/bimap method. The fantasy-land/bimap method takes two arguments:

c['fantasy-land/bimap']!(f, g)

1. f must be a function which returns a value

   1. If f is not a function, the behaviour of fantasy-land/bimap is unspecified.
   2. f can return any value.
   3. No parts of f’s return value should be checked.

2. g must be a function which returns a value

   1. If g is not a function, the behaviour of fantasy-land/bimap is unspecified.
   2. g can return any value.
   3. No parts of g’s return value should be checked.

3. fantasy-land/bimap must return a value of the same Bifunctor.

Profunctor

A value that implements the Profunctor specification must also implement the [Functor]!(#functor) specification.

1. p['fantasy-land/promap']!(a => a, b => b) is equivalent to p (identity)
2. p['fantasy-land/promap']!(a => f(g(a)), b => h(i(b))) is equivalent to p['fantasy-land/promap']!(f, i)['fantasy-land/promap']!(g, h) (composition)

fantasy-land/promap method

fantasy-land/promap :: Profunctor p => p b c ~> (a -> b, c -> d) -> p a d

A value which has a Profunctor must provide a fantasy-land/promap method.

The fantasy-land/promap method takes two arguments:

c['fantasy-land/promap']!(f, g)

1. f must be a function which returns a value

   1. If f is not a function, the behaviour of fantasy-land/promap is unspecified.
   2. f can return any value.
   3. No parts of f’s return value should be checked.

2. g must be a function which returns a value

   1. If g is not a function, the behaviour of fantasy-land/promap is unspecified.
   2. g can return any value.
   3. No parts of g’s return value should be checked.

3. fantasy-land/promap must return a value of the same Profunctor

Derivations

When creating data types which satisfy multiple algebras, authors may choose to implement certain methods then derive the remaining methods. Derivations:

• [fantasy-land/equals][] may be derived from [fantasy-land/lte][] !:

  function equals(other) { return this['fantasy-land/lte']!(other) && other['fantasy-land/lte']!(this); }

• [fantasy-land/map][] may be derived from [fantasy-land/ap][] and [fantasy-land/of][] !:

  function map(f) { return this['fantasy-land/ap']!(this.constructor['fantasy-land/of']!(f)); }

• [fantasy-land/map][] may be derived from [fantasy-land/chain][] and [fantasy-land/of][] !:

  function map(f) { return this['fantasy-land/chain']!(a => this.constructor['fantasy-land/of']!(f(a))); }

• [fantasy-land/map][] may be derived from [fantasy-land/bimap][] !:

  function map(f) { return this['fantasy-land/bimap']!(a => a, f); }

• [fantasy-land/map][] may be derived from [fantasy-land/promap][] !:

  function map(f) { return this['fantasy-land/promap']!(a => a, f); }

• [fantasy-land/ap][] may be derived from [fantasy-land/chain][] !:

  function ap(m) { return m['fantasy-land/chain']!(f => this['fantasy-land/map']!(f)); }

• [fantasy-land/reduce][] may be derived as follows:

  function reduce(f, acc) {
    function Const(value) {
      this.value = value;
    }
    Const['fantasy-land/of'] = function(_) {
      return new Const(acc);
    };
    Const.prototype['fantasy-land/map'] = function(_) {
      return this;
    };
    Const.prototype['fantasy-land/ap'] = function(b) {
      return new Const(f(b.value, this.value));
    };
    return this['fantasy-land/traverse']!(x => new Const(x), Const['fantasy-land/of']).value;
  }

• [fantasy-land/map][] may be derived as follows:

  function map(f) {
    function Id(value) {
      this.value = value;
    }
    Id['fantasy-land/of'] = function(x) {
      return new Id(x);
    };
    Id.prototype['fantasy-land/map'] = function(f) {
      return new Id(f(this.value));
    };
    Id.prototype['fantasy-land/ap'] = function(b) {
      return new Id(this.value(b.value));
    };
    return this['fantasy-land/traverse']!(x => Id['fantasy-land/of']!(f(x)), Id['fantasy-land/of']).value;
  }

• [fantasy-land/filter][] may be derived from [fantasy-land/of][], [fantasy-land/chain][], and [fantasy-land/zero][] !:

  function filter(pred) {
    var F = this.constructor;
    return this['fantasy-land/chain']!(x => pred(x) ? F['fantasy-land/of']!(x) : F['fantasy-land/zero']!());
  }

• [fantasy-land/filter][] may be derived from [fantasy-land/concat][], [fantasy-land/of][], [fantasy-land/zero][], and [fantasy-land/reduce][] !:

  function filter(pred) {
    var F = this.constructor;
    return this['fantasy-land/reduce']!((f, x) => pred(x) ? f['fantasy-land/concat']!(F['fantasy-land/of']!(x)) : f, F['fantasy-land/zero']!());
  }

If a data type provides a method which could be derived, its behaviour must be equivalent to that of the derivation (or derivations).

Notes

1. If there’s more than a single way to implement the methods and laws, the implementation should choose one and provide wrappers for other uses.
2. It’s discouraged to overload the specified methods. It can easily result in broken and buggy behaviour.
3. It is recommended to throw an exception on unspecified behaviour.
4. An Identity container which implements many of the methods is provided by sanctuary-identity.

[fantasy-land/ap] !: #ap-method [fantasy-land/bimap] !: #bimap-method [fantasy-land/chain] !: #chain-method [fantasy-land/concat] !: #concat-method [fantasy-land/equals] !: #equals-method [fantasy-land/filter] !: #filter-method [fantasy-land/lte] !: #lte-method [fantasy-land/map] !: #map-method [fantasy-land/of] !: #of-method [fantasy-land/promap] !: #promap-method [fantasy-land/reduce] !: #reduce-method [fantasy-land/zero] !: #zero-method

Alternatives

There also exists Static Land Specification with exactly the same ideas as Fantasy Land but based on static methods instead of instance methods.