Sanctuary

Sanctuary is a JavaScript functional programming library inspired by Haskell and PureScript. It's stricter than Ramda, and provides a similar suite of functions.

Sanctuary promotes programs composed of simple, pure functions. Such programs are easier to comprehend, test, and maintain – they are also a pleasure to write.

Sanctuary provides two data types, Maybe and Either, both of which are compatible with Fantasy Land. Thanks to these data types even Sanctuary functions which may fail, such as head, are composable.

Sanctuary makes it possible to write safe code without null checks. In JavaScript it's trivial to introduce a possible run-time type error:

words[0].toUpperCase()

If words is [] we'll get a familiar error at run-time:

TypeError: Cannot read property 'toUpperCase' of undefined

Sanctuary gives us a fighting chance of avoiding such errors. We might write:

S.map(S.toUpper, S.head(words))

Sanctuary is designed to work in Node.js and in ES5-compatible browsers.

Types

Sanctuary uses Haskell-like type signatures to describe the types of values, including functions. 'foo', for example, is a member of String; [1, 2, 3] is a member of Array Number. The double colon (::) is used to mean "is a member of", so one could write:

'foo' :: String
[1, 2, 3] :: Array Number

An identifier may appear to the left of the double colon:

Math.PI :: Number

The arrow (->) is used to express a function's type:

Math.abs :: Number -> Number

That states that Math.abs is a unary function which takes an argument of type Number and returns a value of type Number.

Some functions are parametrically polymorphic: their types are not fixed. Type variables are used in the representations of such functions:

S.I :: a -> a

a is a type variable. Type variables are not capitalized, so they are differentiable from type identifiers (which are always capitalized). By convention type variables have single-character names. The signature above states that S.I takes a value of any type and returns a value of the same type. Some signatures feature multiple type variables:

S.K :: a -> b -> a

It must be possible to replace all occurrences of a with a concrete type. The same applies for each other type variable. For the function above, the types with which a and b are replaced may be different, but needn't be.

Since all Sanctuary functions are curried (they accept their arguments one at a time), a binary function is represented as a unary function which returns a unary function: * -> * -> *. This aligns neatly with Haskell, which uses curried functions exclusively. In JavaScript, though, we may wish to represent the types of functions with arities less than or greater than one. The general form is (<input-types>) -> <output-type>, where <input-types> comprises zero or more comma–space (, ) -separated type representations:

• () -> String
• (a, b) -> a
• (a, b, c) -> d

Number -> Number can thus be seen as shorthand for (Number) -> Number.

The question mark (?) is used to represent types which include null and undefined as members. String?, for example, represents the type comprising null, undefined, and all strings.

Sanctuary embraces types. JavaScript doesn't support algebraic data types, but these can be simulated by providing a group of data constructors which return values with the same set of methods. A value of the Either type, for example, is created via the Left constructor or the Right constructor.

It's necessary to extend Haskell's notation to describe implicit arguments to the methods provided by Sanctuary's types. In x.map(y), for example, the map method takes an implicit argument x in addition to the explicit argument y. The type of the value upon which a method is invoked appears at the beginning of the signature, separated from the arguments and return value by a squiggly arrow (~>). The type of the map method of the Maybe type is written Maybe a ~> (a -> b) -> Maybe b. One could read this as:

When the map method is invoked on a value of type Maybe a (for any type a) with an argument of type a -> b (for any type b), it returns a value of type Maybe b.

The squiggly arrow is also used when representing non-function properties. Maybe a ~> Boolean, for example, represents a Boolean property of a value of type Maybe a.

Sanctuary supports type classes: constraints on type variables. Whereas a -> a implicitly supports every type, Functor f => (a -> b) -> f a -> f b requires that f be a type which satisfies the requirements of the Functor type class. Type-class constraints appear at the beginning of a type signature, separated from the rest of the signature by a fat arrow (=>).

Accessible pseudotype

What is the type of values which support property access? In other words, what is the type of which every value except null and undefined is a member? Object is close, but Object.create(null) produces a value which supports property access but which is not a member of the Object type.

Sanctuary uses the Accessible pseudotype to represent the set of values which support property access.

Integer pseudotype

The Integer pseudotype represents integers in the range (-2^53 .. 2^53). It is a pseudotype because each Integer is represented by a Number value. Sanctuary's run-time type checking asserts that a valid Number value is provided wherever an Integer value is required.

Type representatives

What is the type of Number? One answer is a -> Number, since it's a function which takes an argument of any type and returns a Number value. When provided as the first argument to is, though, Number is really the value-level representative of the Number type.

Sanctuary uses the TypeRep pseudotype to describe type representatives. For example:

Number :: TypeRep Number

Number is the sole inhabitant of the TypeRep Number type.

Type checking

Sanctuary functions are defined via sanctuary-def to provide run-time type checking. This is tremendously useful during development: type errors are reported immediately, avoiding circuitous stack traces (at best) and silent failures due to type coercion (at worst). For example:

S.add(2, true);
// ! TypeError: Invalid value
//
//   add :: FiniteNumber -> FiniteNumber -> FiniteNumber
//                          ^^^^^^^^^^^^
//                               1
//
//   1)  true :: Boolean
//
//   The value at position 1 is not a member of ‘FiniteNumber’.
//
//   See https://github.com/sanctuary-js/sanctuary-def/tree/v0.12.0#FiniteNumber for information about the sanctuary-def/FiniteNumber type.

Compare this to the behaviour of Ramda's unchecked equivalent:

R.add(2, true);
// => 3

There is a performance cost to run-time type checking. One may wish to disable type checking in certain contexts to avoid paying this cost. create facilitates the creation of a Sanctuary module which does not perform type checking.

In Node, one could use an environment variable to determine whether to perform type checking:

const {create, env} = require('sanctuary');

const checkTypes = process.env.NODE_ENV !== 'production';
const S = create({checkTypes, env});

API

create :: { checkTypes :: Boolean, env :: Array Type } -⁠> Module

Takes an options record and returns a Sanctuary module. checkTypes specifies whether to enable type checking. The module's polymorphic functions (such as I) require each value associated with a type variable to be a member of at least one type in the environment.

A well-typed application of a Sanctuary function will produce the same result regardless of whether type checking is enabled. If type checking is enabled, a badly typed application will produce an exception with a descriptive error message.

The following snippet demonstrates defining a custom type and using create to produce a Sanctuary module which is aware of that type:

const {create, env} = require('sanctuary');
const $ = require('sanctuary-def');
const type = require('sanctuary-type-identifiers');

//    Identity :: a -> Identity a
const Identity = function Identity(x) {
  if (!(this instanceof Identity)) return new Identity(x);
  this.value = x;
};

Identity['@@type'] = 'my-package/Identity@1';

Identity.prototype['fantasy-land/map'] = function(f) {
  return Identity(f(this.value));
};

//    IdentityType :: Type -> Type
const IdentityType = $.UnaryType(
  Identity['@@type'],
  'http://example.com/my-package#Identity',
  x => type(x) === Identity['@@type'],
  identity => [identity.value]
);

const S = create({
  checkTypes: process.env.NODE_ENV !== 'production',
  env: env.concat([IdentityType($.Unknown)]),
});

S.map(S.sub(1), Identity(43));
// => Identity(42)

See also env.

env :: Array Type

The default environment, which may be used as is or as the basis of a custom environment in conjunction with create.

Placeholder

Sanctuary functions are designed with partial application in mind. In many cases one can define a more specific function in terms of a more general one simply by applying the more general function to some (but not all) of its arguments. For example, one could define sum :: Foldable f => f Number -> Number as S.reduce(S.add, 0).

In some cases, though, there are multiple orders in which one may wish to provide a function's arguments. S.concat('prefix') is a function which prefixes its argument, but how would one define a function which suffixes its argument? It's possible with the help of __, the special placeholder value.

The placeholder indicates a hole to be filled at some future time. The following are all equivalent (_ represents the placeholder):

• f(x, y, z)
• f(_, y, z)(x)
• f(_, _, z)(x, y)
• f(_, _, z)(_, y)(x)

__ :: Placeholder

The special placeholder value.

> S.map(S.concat('@'), ['foo', 'bar', 'baz'])
['@foo', '@bar', '@baz']

> S.map(S.concat(S.__, '?'), ['foo', 'bar', 'baz'])
['foo?', 'bar?', 'baz?']

Classify

type :: Any -⁠> { namespace :: Maybe String, name :: String, version :: Integer }

Returns the result of parsing the type identifier of the given value.

> S.type(S.Just(42))
{namespace: Just('sanctuary'), name: 'Maybe', version: 0}

> S.type([1, 2, 3])
{namespace: Nothing, name: 'Array', version: 0}

is :: TypeRep a -⁠> Any -⁠> Boolean

Takes a type representative and a value of any type and returns true if the given value is of the specified type; false otherwise. Subtyping is not respected.

> S.is(Number, 42)
true

> S.is(Object, 42)
false

> S.is(String, 42)
false

Showable

toString :: Any -⁠> String

Alias of Z.toString.

> S.toString(-0)
'-0'

> S.toString(['foo', 'bar', 'baz'])
'["foo", "bar", "baz"]'

> S.toString({x: 1, y: 2, z: 3})
'{"x": 1, "y": 2, "z": 3}'

> S.toString(S.Left(S.Right(S.Just(S.Nothing))))
'Left(Right(Just(Nothing)))'

Fantasy Land

Sanctuary is compatible with the Fantasy Land specification.

equals :: Setoid a => a -⁠> a -⁠> Boolean

Curried version of Z.equals which requires two arguments of the same type.

To compare values of different types first use create to create a Sanctuary module with type checking disabled, then use that module's equals function.

> S.equals(0, -0)
true

> S.equals(NaN, NaN)
true

> S.equals(S.Just([1, 2, 3]), S.Just([1, 2, 3]))
true

> S.equals(S.Just([1, 2, 3]), S.Just([1, 2, 4]))
false

lt :: Ord a => a -⁠> (a -⁠> Boolean)

Flipped version of Z.lt intended for partial application.

See also lt_.

> S.filter(S.lt(3), [1, 2, 3, 4, 5])
[1, 2]

lt_ :: Ord a => a -⁠> a -⁠> Boolean

Curried version of Z.lt.

See also lt.

> S.lt_([1, 2, 3], [1, 2, 3])
false

> S.lt_([1, 2, 3], [1, 2, 4])
true

> S.lt_([1, 2, 3], [1, 2])
false

lte :: Ord a => a -⁠> (a -⁠> Boolean)

Flipped version of Z.lte intended for partial application.

See also lte_.

> S.filter(S.lte(3), [1, 2, 3, 4, 5])
[1, 2, 3]

lte_ :: Ord a => a -⁠> a -⁠> Boolean

Curried version of Z.lte.

See also lte.

> S.lte_([1, 2, 3], [1, 2, 3])
true

> S.lte_([1, 2, 3], [1, 2, 4])
true

> S.lte_([1, 2, 3], [1, 2])
false

gt :: Ord a => a -⁠> (a -⁠> Boolean)

Flipped version of Z.gt intended for partial application.

See also gt_.

> S.filter(S.gt(3), [1, 2, 3, 4, 5])
[4, 5]

gt_ :: Ord a => a -⁠> a -⁠> Boolean

Curried version of Z.gt.

See also gt.

> S.gt_([1, 2, 3], [1, 2, 3])
false

> S.gt_([1, 2, 3], [1, 2, 4])
false

> S.gt_([1, 2, 3], [1, 2])
true

gte :: Ord a => a -⁠> (a -⁠> Boolean)

Flipped version of Z.gte intended for partial application.

See also gte_.

> S.filter(S.gte(3), [1, 2, 3, 4, 5])
[3, 4, 5]

gte_ :: Ord a => a -⁠> a -⁠> Boolean

Curried version of Z.gte.

See also gte.

> S.gte_([1, 2, 3], [1, 2, 3])
true

> S.gte_([1, 2, 3], [1, 2, 4])
false

> S.gte_([1, 2, 3], [1, 2])
true

min :: Ord a => a -⁠> a -⁠> a

Returns the smaller of its two arguments (according to Z.lte).

See also max.

> S.min(10, 2)
2

> S.min(new Date('1999-12-31'), new Date('2000-01-01'))
new Date('1999-12-31')

> S.min('10', '2')
'10'

max :: Ord a => a -⁠> a -⁠> a

Returns the larger of its two arguments (according to Z.lte).

See also min.

> S.max(10, 2)
10

> S.max(new Date('1999-12-31'), new Date('2000-01-01'))
new Date('2000-01-01')

> S.max('10', '2')
'2'

id :: Category c => TypeRep c -⁠> c

Type-safe version of Z.id.

> S.id(Function)(42)
42

concat :: Semigroup a => a -⁠> a -⁠> a

Curried version of Z.concat.

> S.concat('abc', 'def')
'abcdef'

> S.concat([1, 2, 3], [4, 5, 6])
[1, 2, 3, 4, 5, 6]

> S.concat({x: 1, y: 2}, {y: 3, z: 4})
{x: 1, y: 3, z: 4}

> S.concat(S.Just([1, 2, 3]), S.Just([4, 5, 6]))
Just([1, 2, 3, 4, 5, 6])

empty :: Monoid a => TypeRep a -⁠> a

Type-safe version of Z.empty.

> S.empty(String)
''

> S.empty(Array)
[]

> S.empty(Object)
{}

map :: Functor f => (a -⁠> b) -⁠> f a -⁠> f b

Curried version of Z.map.

> S.map(Math.sqrt, [1, 4, 9])
[1, 2, 3]

> S.map(Math.sqrt, {x: 1, y: 4, z: 9})
{x: 1, y: 2, z: 3}

> S.map(Math.sqrt, S.Just(9))
Just(3)

> S.map(Math.sqrt, S.Right(9))
Right(3)

Replacing Functor f => f with Function x produces the B combinator from combinatory logic (i.e. compose):

Functor f => (a -> b) -> f a -> f b
(a -> b) -> Function x a -> Function x b
(a -> c) -> Function x a -> Function x c
(b -> c) -> Function x b -> Function x c
(b -> c) -> Function a b -> Function a c
(b -> c) -> (a -> b) -> (a -> c)

> S.map(Math.sqrt, S.add(1))(99)
10

bimap :: Bifunctor f => (a -⁠> b) -⁠> (c -⁠> d) -⁠> f a c -⁠> f b d

Curried version of Z.bimap.

> S.bimap(S.toUpper, Math.sqrt, S.Left('foo'))
Left('FOO')

> S.bimap(S.toUpper, Math.sqrt, S.Right(64))
Right(8)

promap :: Profunctor p => (a -⁠> b) -⁠> (c -⁠> d) -⁠> p b c -⁠> p a d

Curried version of Z.promap.

> S.promap(Math.abs, S.add(1), Math.sqrt)(-100)
11

alt :: Alt f => f a -⁠> f a -⁠> f a

Curried version of Z.alt.

> S.alt(S.Nothing, S.Just(1))
Just(1)

> S.alt(S.Just(2), S.Just(3))
Just(2)

> S.alt(S.Left('X'), S.Right(1))
Right(1)

> S.alt(S.Right(2), S.Right(3))
Right(2)

zero :: Plus f => TypeRep f -⁠> f a

Type-safe version of Z.zero.

> S.zero(Array)
[]

> S.zero(Object)
{}

> S.zero(S.Maybe)
Nothing

reduce :: Foldable f => (b -⁠> a -⁠> b) -⁠> b -⁠> f a -⁠> b

Takes a curried binary function, an initial value, and a Foldable, and applies the function to the initial value and the Foldable's first value, then applies the function to the result of the previous application and the Foldable's second value. Repeats this process until each of the Foldable's values has been used. Returns the initial value if the Foldable is empty; the result of the final application otherwise.

See also reduce_.

> S.reduce(S.add, 0, [1, 2, 3, 4, 5])
15

> S.reduce(xs => x => [x].concat(xs), [], [1, 2, 3, 4, 5])
[5, 4, 3, 2, 1]

reduce_ :: Foldable f => ((b, a) -⁠> b) -⁠> b -⁠> f a -⁠> b

Variant of reduce which takes an uncurried binary function.

traverse :: (Applicative f, Traversable t) => TypeRep f -⁠> (a -⁠> f b) -⁠> t a -⁠> f (t b)

Curried version of Z.traverse.

> S.traverse(Array, S.words, S.Just('foo bar baz'))
[Just('foo'), Just('bar'), Just('baz')]

> S.traverse(Array, S.words, S.Nothing)
[Nothing]

> S.traverse(S.Maybe, S.parseInt(16), ['A', 'B', 'C'])
Just([10, 11, 12])

> S.traverse(S.Maybe, S.parseInt(16), ['A', 'B', 'C', 'X'])
Nothing

> S.traverse(S.Maybe, S.parseInt(16), {a: 'A', b: 'B', c: 'C'})
Just({a: 10, b: 11, c: 12})

> S.traverse(S.Maybe, S.parseInt(16), {a: 'A', b: 'B', c: 'C', x: 'X'})
Nothing

sequence :: (Applicative f, Traversable t) => TypeRep f -⁠> t (f a) -⁠> f (t a)

Curried version of Z.sequence.

> S.sequence(Array, S.Just([1, 2, 3]))
[Just(1), Just(2), Just(3)]

> S.sequence(S.Maybe, [S.Just(1), S.Just(2), S.Just(3)])
Just([1, 2, 3])

> S.sequence(S.Maybe, [S.Just(1), S.Just(2), S.Nothing])
Nothing

> S.sequence(S.Maybe, {a: S.Just(1), b: S.Just(2), c: S.Just(3)})
Just({a: 1, b: 2, c: 3})

> S.sequence(S.Maybe, {a: S.Just(1), b: S.Just(2), c: S.Nothing})
Nothing

ap :: Apply f => f (a -⁠> b) -⁠> f a -⁠> f b

Curried version of Z.ap.

> S.ap([Math.sqrt, x => x * x], [1, 4, 9, 16, 25])
[1, 2, 3, 4, 5, 1, 16, 81, 256, 625]

> S.ap({x: Math.sqrt, y: S.add(1), z: S.sub(1)}, {w: 4, x: 4, y: 4})
{x: 2, y: 5}

> S.ap(S.Just(Math.sqrt), S.Just(64))
Just(8)

Replacing Apply f => f with Function x produces the S combinator from combinatory logic:

Apply f => f (a -> b) -> f a -> f b
Function x (a -> b) -> Function x a -> Function x b
Function x (a -> c) -> Function x a -> Function x c
Function x (b -> c) -> Function x b -> Function x c
Function a (b -> c) -> Function a b -> Function a c
(a -> b -> c) -> (a -> b) -> (a -> c)

> S.ap(s => n => s.slice(0, n), s => Math.ceil(s.length / 2))('Haskell')
'Hask'

lift2 :: Apply f => (a -⁠> b -⁠> c) -⁠> f a -⁠> f b -⁠> f c

Promotes a curried binary function to a function which operates on two Applys.

> S.lift2(S.add, S.Just(2), S.Just(3))
Just(5)

> S.lift2(S.add, S.Just(2), S.Nothing)
Nothing

> S.lift2(S.and, S.Just(true), S.Just(true))
Just(true)

> S.lift2(S.and, S.Just(true), S.Just(false))
Just(false)

lift3 :: Apply f => (a -⁠> b -⁠> c -⁠> d) -⁠> f a -⁠> f b -⁠> f c -⁠> f d

Promotes a curried ternary function to a function which operates on three Applys.

> S.lift3(S.reduce, S.Just(S.add), S.Just(0), S.Just([1, 2, 3]))
Just(6)

> S.lift3(S.reduce, S.Just(S.add), S.Just(0), S.Nothing)
Nothing

apFirst :: Apply f => f a -⁠> f b -⁠> f a

Curried version of Z.apFirst. Combines two effectful actions, keeping only the result of the first. Equivalent to Haskell's (<*) function.

See also apSecond.

> S.apFirst([1, 2], [3, 4])
[1, 1, 2, 2]

> S.apFirst(S.Just(1), S.Just(2))
Just(1)

apSecond :: Apply f => f a -⁠> f b -⁠> f b

Curried version of Z.apSecond. Combines two effectful actions, keeping only the result of the second. Equivalent to Haskell's (*>) function.

See also apFirst.

> S.apSecond([1, 2], [3, 4])
[3, 4, 3, 4]

> S.apSecond(S.Just(1), S.Just(2))
Just(2)

of :: Applicative f => TypeRep f -⁠> a -⁠> f a

Curried version of Z.of.

> S.of(Array, 42)
[42]

> S.of(Function, 42)(null)
42

> S.of(S.Maybe, 42)
Just(42)

> S.of(S.Either, 42)
Right(42)

chain :: Chain m => (a -⁠> m b) -⁠> m a -⁠> m b

Curried version of Z.chain.

> S.chain(x => [x, x], [1, 2, 3])
[1, 1, 2, 2, 3, 3]

> S.chain(n => s => s.slice(0, n), s => Math.ceil(s.length / 2))('slice')
'sli'

> S.chain(S.parseInt(10), S.Just('123'))
Just(123)

> S.chain(S.parseInt(10), S.Just('XXX'))
Nothing

join :: Chain m => m (m a) -⁠> m a

Type-safe version of Z.join. Removes one level of nesting from a nested monadic structure.

> S.join([[1], [2], [3]])
[1, 2, 3]

> S.join([[[1, 2, 3]]])
[[1, 2, 3]]

> S.join(S.Just(S.Just(1)))
S.Just(1)

Replacing Chain m => m with Function x produces the W combinator from combinatory logic:

Chain m => m (m a) -> m a
Function x (Function x a) -> Function x a
(x -> x -> a) -> (x -> a)

> S.join(S.concat)('abc')
'abcabc'

chainRec :: ChainRec m => TypeRep m -⁠> (a -⁠> m (Either a b)) -⁠> a -⁠> m b

Performs a chain-like computation with constant stack usage. Similar to Z.chainRec, but curried and more convenient due to the use of the Either type to indicate completion (via a Right).

> S.chainRec(Array,
.            s => s.length === 2 ? S.map(S.Right, [s + '!', s + '?'])
.                                : S.map(S.Left, [s + 'o', s + 'n']),
.            '')
['oo!', 'oo?', 'on!', 'on?', 'no!', 'no?', 'nn!', 'nn?']

extend :: Extend w => (w a -⁠> b) -⁠> w a -⁠> w b

Curried version of Z.extend.

> S.extend(S.joinWith(''), ['x', 'y', 'z'])
['xyz', 'yz', 'z']

extract :: Comonad w => w a -⁠> a

Type-safe version of Z.extract.

contramap :: Contravariant f => (b -⁠> a) -⁠> f a -⁠> f b

Type-safe version of Z.contramap.

> S.contramap(s => s.length, Math.sqrt)('Sanctuary')
3

filter :: (Applicative f, Foldable f, Monoid (f a)) => (a -⁠> Boolean) -⁠> f a -⁠> f a

Curried version of Z.filter.

See also filterM.

> S.filter(S.odd, [1, 2, 3, 4, 5])
[1, 3, 5]

filterM :: (Alternative m, Monad m) => (a -⁠> Boolean) -⁠> m a -⁠> m a

Curried version of Z.filterM.

See also filter.

> S.filterM(S.odd, [1, 2, 3, 4, 5])
[1, 3, 5]

> S.filterM(S.odd, S.Just(9))
Just(9)

> S.filterM(S.odd, S.Just(4))
Nothing

takeWhile :: (Foldable f, Alternative f) => (a -⁠> Boolean) -⁠> f a -⁠> f a

Discards the first inner value which does not satisfy the predicate, and all subsequent inner values.

> S.takeWhile(S.odd, [3, 3, 3, 7, 6, 3, 5, 4])
[3, 3, 3, 7]

> S.takeWhile(S.even, [3, 3, 3, 7, 6, 3, 5, 4])
[]

dropWhile :: (Foldable f, Alternative f) => (a -⁠> Boolean) -⁠> f a -⁠> f a

Retains the first inner value which does not satisfy the predicate, and all subsequent inner values.

> S.dropWhile(S.odd, [3, 3, 3, 7, 6, 3, 5, 4])
[6, 3, 5, 4]

> S.dropWhile(S.even, [3, 3, 3, 7, 6, 3, 5, 4])
[3, 3, 3, 7, 6, 3, 5, 4]

Combinator

I :: a -⁠> a

The I combinator. Returns its argument. Equivalent to Haskell's id function.

> S.I('foo')
'foo'

K :: a -⁠> b -⁠> a

The K combinator. Takes two values and returns the first. Equivalent to Haskell's const function.

> S.K('foo', 'bar')
'foo'

> S.map(S.K(42), S.range(0, 5))
[42, 42, 42, 42, 42]

A :: (a -⁠> b) -⁠> a -⁠> b

The A combinator. Takes a function and a value, and returns the result of applying the function to the value. Equivalent to Haskell's ($) function.

> S.A(S.add(1), 42)
43

> S.map(S.A(S.__, 100), [S.add(1), Math.sqrt])
[101, 10]

T :: a -⁠> (a -⁠> b) -⁠> b

The T (thrush) combinator. Takes a value and a function, and returns the result of applying the function to the value. Equivalent to Haskell's (&) function.

> S.T(42, S.add(1))
43

> S.map(S.T(100), [S.add(1), Math.sqrt])
[101, 10]

Function

curry2 :: ((a, b) -⁠> c) -⁠> a -⁠> b -⁠> c

Curries the given binary function.

> S.map(S.curry2(Math.pow)(10), [1, 2, 3])
[10, 100, 1000]

> S.map(S.curry2(Math.pow, 10), [1, 2, 3])
[10, 100, 1000]

curry3 :: ((a, b, c) -⁠> d) -⁠> a -⁠> b -⁠> c -⁠> d

Curries the given ternary function.

> global.replaceString = S.curry3((what, replacement, string) =>
.   string.replace(what, replacement)
. )
replaceString

> replaceString('banana')('orange')('banana icecream')
'orange icecream'

> replaceString('banana', 'orange', 'banana icecream')
'orange icecream'

curry4 :: ((a, b, c, d) -⁠> e) -⁠> a -⁠> b -⁠> c -⁠> d -⁠> e

Curries the given quaternary function.

> global.createRect = S.curry4((x, y, width, height) =>
.   ({x, y, width, height})
. )
createRect

> createRect(0)(0)(10)(10)
{x: 0, y: 0, width: 10, height: 10}

> createRect(0, 0, 10, 10)
{x: 0, y: 0, width: 10, height: 10}

curry5 :: ((a, b, c, d, e) -⁠> f) -⁠> a -⁠> b -⁠> c -⁠> d -⁠> e -⁠> f

Curries the given quinary function.

> global.toUrl = S.curry5((protocol, creds, hostname, port, pathname) =>
.   protocol + '//' +
.   S.maybe('', _ => _.username + ':' + _.password + '@', creds) +
.   hostname +
.   S.maybe('', S.concat(':'), port) +
.   pathname
. )
toUrl

> toUrl('https:')(S.Nothing)('example.com')(S.Just('443'))('/foo/bar')
'https://example.com:443/foo/bar'

> toUrl('https:', S.Nothing, 'example.com', S.Just('443'), '/foo/bar')
'https://example.com:443/foo/bar'

flip :: (a -⁠> b -⁠> c) -⁠> b -⁠> a -⁠> c

Takes a curried binary function and two values, and returns the result of applying the function to the values in reverse order.

This is the C combinator from combinatory logic.

> S.flip(S.concat, 'foo', 'bar')
'barfoo'

flip_ :: ((a, b) -⁠> c) -⁠> b -⁠> a -⁠> c

Variant of flip which takes an uncurried binary function.

Composition

compose :: Semigroupoid s => s b c -⁠> s a b -⁠> s a c

Curried version of Z.compose.

When specialized to Function, compose composes two unary functions, from right to left (this is the B combinator from combinatory logic).

The generalized type signature indicates that compose is compatible with any Semigroupoid.

See also pipe.

> S.compose(Math.sqrt, S.add(1))(99)
10

pipe :: [(a -⁠> b), (b -⁠> c), ..., (m -⁠> n)] -⁠> a -⁠> n

Takes an array of functions assumed to be unary and a value of any type, and returns the result of applying the sequence of transformations to the initial value.

In general terms, pipe performs left-to-right composition of an array of functions. pipe([f, g, h], x) is equivalent to h(g(f(x))).

> S.pipe([S.add(1), Math.sqrt, S.sub(1)], 99)
9

on :: (b -⁠> b -⁠> c) -⁠> (a -⁠> b) -⁠> a -⁠> a -⁠> c

Takes a binary function f, a unary function g, and two values x and y. Returns f(g(x))(g(y)).

This is the P combinator from combinatory logic.

See also on_.

> S.on(S.concat, S.reverse, [1, 2, 3], [4, 5, 6])
[3, 2, 1, 6, 5, 4]

on_ :: ((b, b) -⁠> c) -⁠> (a -⁠> b) -⁠> a -⁠> a -⁠> c

Variant of on which takes an uncurried binary function.

Maybe type

The Maybe type represents optional values: a value of type Maybe a is either a Just whose value is of type a or Nothing (with no value).

The Maybe type satisfies the Setoid, Monoid, Monad, Alternative, Traversable, and Extend specifications.

MaybeType :: Type -⁠> Type

A UnaryType for use with sanctuary-def.

Maybe :: TypeRep Maybe

The type representative for the Maybe type.

Nothing :: Maybe a

Nothing.

> S.Nothing
Nothing

Just :: a -⁠> Maybe a

Takes a value of any type and returns a Just with the given value.

> S.Just(42)
Just(42)

Maybe.@@type :: String

Maybe type identifier, 'sanctuary/Maybe'.

Maybe.fantasy-land/empty :: () -⁠> Maybe a

Returns Nothing.

> S.empty(S.Maybe)
Nothing

Maybe.fantasy-land/of :: a -⁠> Maybe a

Takes a value of any type and returns a Just with the given value.

> S.of(S.Maybe, 42)
Just(42)

Maybe.fantasy-land/zero :: () -⁠> Maybe a

Returns Nothing.

> S.zero(S.Maybe)
Nothing

Maybe#isNothing :: Maybe a ~> Boolean

true if this is Nothing; false if this is a Just.

> S.Nothing.isNothing
true

> S.Just(42).isNothing
false

Maybe#isJust :: Maybe a ~> Boolean

true if this is a Just; false if this is Nothing.

> S.Just(42).isJust
true

> S.Nothing.isJust
false

Maybe#toString :: Maybe a ~> () -⁠> String

Returns the string representation of the Maybe.

> S.toString(S.Nothing)
'Nothing'

> S.toString(S.Just([1, 2, 3]))
'Just([1, 2, 3])'

Maybe#inspect :: Maybe a ~> () -⁠> String

Returns the string representation of the Maybe. This method is used by util.inspect and the REPL to format a Maybe for display.

See also Maybe#toString.

> S.Nothing.inspect()
'Nothing'

> S.Just([1, 2, 3]).inspect()
'Just([1, 2, 3])'

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

Takes a value m of the same type and returns true if:

• this and m are both Nothing; or

• this and m are both Justs, and their values are equal according to Z.equals.

> S.equals(S.Nothing, S.Nothing)
true

> S.equals(S.Just([1, 2, 3]), S.Just([1, 2, 3]))
true

> S.equals(S.Just([1, 2, 3]), S.Just([3, 2, 1]))
false

> S.equals(S.Just([1, 2, 3]), S.Nothing)
false

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

Takes a value m of the same type and returns true if:

• this is Nothing; or

• this and m are both Justs and the value of this is less than or equal to the value of m according to Z.lte.

> S.lte_(S.Nothing, S.Nothing)
true

> S.lte_(S.Nothing, S.Just(0))
true

> S.lte_(S.Just(0), S.Nothing)
false

> S.lte_(S.Just(0), S.Just(1))
true

> S.lte_(S.Just(1), S.Just(0))
false

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

Returns the result of concatenating two Maybe values of the same type. a must have a Semigroup.

If this is Nothing and the argument is Nothing, this method returns Nothing.

If this is a Just and the argument is a Just, this method returns a Just whose value is the result of concatenating this Just's value and the given Just's value.

Otherwise, this method returns the Just.

> S.concat(S.Nothing, S.Nothing)
Nothing

> S.concat(S.Just([1, 2, 3]), S.Just([4, 5, 6]))
Just([1, 2, 3, 4, 5, 6])

> S.concat(S.Nothing, S.Just([1, 2, 3]))
Just([1, 2, 3])

> S.concat(S.Just([1, 2, 3]), S.Nothing)
Just([1, 2, 3])

Maybe#fantasy-land/map :: Maybe a ~> (a -⁠> b) -⁠> Maybe b

Takes a function and returns this if this is Nothing; otherwise it returns a Just whose value is the result of applying the function to this Just's value.

> S.map(Math.sqrt, S.Nothing)
Nothing

> S.map(Math.sqrt, S.Just(9))
Just(3)

Maybe#fantasy-land/ap :: Maybe a ~> Maybe (a -⁠> b) -⁠> Maybe b

Takes a Maybe and returns Nothing unless this is a Just and the argument is a Just, in which case it returns a Just whose value is the result of applying the given Just's value to this Just's value.

> S.ap(S.Nothing, S.Nothing)
Nothing

> S.ap(S.Nothing, S.Just(9))
Nothing

> S.ap(S.Just(Math.sqrt), S.Nothing)
Nothing

> S.ap(S.Just(Math.sqrt), S.Just(9))
Just(3)

Maybe#fantasy-land/chain :: Maybe a ~> (a -⁠> Maybe b) -⁠> Maybe b

Takes a function and returns this if this is Nothing; otherwise it returns the result of applying the function to this Just's value.

> S.chain(S.parseFloat, S.Nothing)
Nothing

> S.chain(S.parseFloat, S.Just('xxx'))
Nothing

> S.chain(S.parseFloat, S.Just('12.34'))
Just(12.34)

Maybe#fantasy-land/alt :: Maybe a ~> Maybe a -⁠> Maybe a

Chooses between this and the other Maybe provided as an argument. Returns this if this is a Just; the other Maybe otherwise.

> S.alt(S.Nothing, S.Nothing)
Nothing

> S.alt(S.Nothing, S.Just(1))
Just(1)

> S.alt(S.Just(2), S.Nothing)
Just(2)

> S.alt(S.Just(3), S.Just(4))
Just(3)

Maybe#fantasy-land/reduce :: Maybe a ~> ((b, a) -⁠> b, b) -⁠> b

Takes a function and an initial value of any type, and returns:

• the initial value if this is Nothing; otherwise

• the result of applying the function to the initial value and this Just's value.

> S.reduce_(Math.pow, 10, S.Nothing)
10

> S.reduce_(Math.pow, 10, S.Just(3))
1000

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

Takes the type representative of some Applicative and a function which returns a value of that Applicative, and returns:

• the result of applying the type representative's of function to this if this is Nothing; otherwise

• the result of mapping Just over the result of applying the first function to this Just's value.

> S.traverse(Array, S.words, S.Nothing)
[Nothing]

> S.traverse(Array, S.words, S.Just('foo bar baz'))
[Just('foo'), Just('bar'), Just('baz')]

Maybe#fantasy-land/extend :: Maybe a ~> (Maybe a -⁠> b) -⁠> Maybe b

Takes a function and returns this if this is Nothing; otherwise it returns a Just whose value is the result of applying the function to this.

> S.extend(x => x.value + 1, S.Nothing)
Nothing

> S.extend(x => x.value + 1, S.Just(42))
Just(43)

isNothing :: Maybe a -⁠> Boolean

Returns true if the given Maybe is Nothing; false if it is a Just.

> S.isNothing(S.Nothing)
true

> S.isNothing(S.Just(42))
false

isJust :: Maybe a -⁠> Boolean

Returns true if the given Maybe is a Just; false if it is Nothing.

> S.isJust(S.Just(42))
true

> S.isJust(S.Nothing)
false

fromMaybe :: a -⁠> Maybe a -⁠> a

Takes a default value and a Maybe, and returns the Maybe's value if the Maybe is a Just; the default value otherwise.

See also fromMaybe_ and maybeToNullable.

> S.fromMaybe(0, S.Just(42))
42

> S.fromMaybe(0, S.Nothing)
0

fromMaybe_ :: (() -⁠> a) -⁠> Maybe a -⁠> a

Variant of fromMaybe which takes a thunk so the default value is only computed if required.

> function fib(n) { return n <= 1 ? n : fib(n - 2) + fib(n - 1); }

> S.fromMaybe_(() => fib(30), S.Just(1000000))
1000000

> S.fromMaybe_(() => fib(30), S.Nothing)
832040

maybeToNullable :: Maybe a -⁠> Nullable a

Returns the given Maybe's value if the Maybe is a Just; null otherwise. Nullable is defined in sanctuary-def.

See also fromMaybe.

> S.maybeToNullable(S.Just(42))
42

> S.maybeToNullable(S.Nothing)
null

toMaybe :: a? -⁠> Maybe a

Takes a value and returns Nothing if the value is null or undefined; Just the value otherwise.

> S.toMaybe(null)
Nothing

> S.toMaybe(42)
Just(42)

maybe :: b -⁠> (a -⁠> b) -⁠> Maybe a -⁠> b

Takes a value of any type, a function, and a Maybe. If the Maybe is a Just, the return value is the result of applying the function to the Just's value. Otherwise, the first argument is returned.

See also maybe_.

> S.maybe(0, S.prop('length'), S.Just('refuge'))
6

> S.maybe(0, S.prop('length'), S.Nothing)
0

maybe_ :: (() -⁠> b) -⁠> (a -⁠> b) -⁠> Maybe a -⁠> b

Variant of maybe which takes a thunk so the default value is only computed if required.

> function fib(n) { return n <= 1 ? n : fib(n - 2) + fib(n - 1); }

> S.maybe_(() => fib(30), Math.sqrt, S.Just(1000000))
1000

> S.maybe_(() => fib(30), Math.sqrt, S.Nothing)
832040

justs :: Array (Maybe a) -⁠> Array a

Takes an array of Maybes and returns an array containing each Just's value. Equivalent to Haskell's catMaybes function.

See also lefts and rights.

> S.justs([S.Just('foo'), S.Nothing, S.Just('baz')])
['foo', 'baz']

mapMaybe :: (a -⁠> Maybe b) -⁠> Array a -⁠> Array b

Takes a function and an array, applies the function to each element of the array, and returns an array of "successful" results. If the result of applying the function to an element of the array is Nothing, the result is discarded; if the result is a Just, the Just's value is included in the output array.

In general terms, mapMaybe filters an array while mapping over it.

> S.mapMaybe(S.head, [[], [1, 2, 3], [], [4, 5, 6], []])
[1, 4]

encase :: (a -⁠> b) -⁠> a -⁠> Maybe b

Takes a unary function f which may throw and a value x of any type, and applies f to x inside a try block. If an exception is caught, the return value is Nothing; otherwise the return value is Just the result of applying f to x.

See also encaseEither.

> S.encase(eval, '1 + 1')
Just(2)

> S.encase(eval, '1 +')
Nothing

encase2 :: (a -⁠> b -⁠> c) -⁠> a -⁠> b -⁠> Maybe c

Binary version of encase.

See also encase2_.

encase2_ :: ((a, b) -⁠> c) -⁠> a -⁠> b -⁠> Maybe c

Variant of encase2 which takes an uncurried binary function.

encase3 :: (a -⁠> b -⁠> c -⁠> d) -⁠> a -⁠> b -⁠> c -⁠> Maybe d

Ternary version of encase.

See also encase3_.

encase3_ :: ((a, b, c) -⁠> d) -⁠> a -⁠> b -⁠> c -⁠> Maybe d

Variant of encase3 which takes an uncurried ternary function.

maybeToEither :: a -⁠> Maybe b -⁠> Either a b

Converts a Maybe to an Either. Nothing becomes a Left (containing the first argument); a Just becomes a Right.

See also eitherToMaybe.

> S.maybeToEither('Expecting an integer', S.parseInt(10, 'xyz'))
Left('Expecting an integer')

> S.maybeToEither('Expecting an integer', S.parseInt(10, '42'))
Right(42)

Either type

The Either type represents values with two possibilities: a value of type Either a b is either a Left whose value is of type a or a Right whose value is of type b.

The Either type satisfies the Setoid, Semigroup, Monad, Alt, Traversable, Extend, and Bifunctor specifications.

EitherType :: Type -⁠> Type -⁠> Type

A BinaryType for use with sanctuary-def.

Either :: TypeRep Either

The type representative for the Either type.

Left :: a -⁠> Either a b

Takes a value of any type and returns a Left with the given value.

> S.Left('Cannot divide by zero')
Left('Cannot divide by zero')

Right :: b -⁠> Either a b

Takes a value of any type and returns a Right with the given value.

> S.Right(42)
Right(42)

Either.@@type :: String

Either type identifier, 'sanctuary/Either'.

Either.fantasy-land/of :: b -⁠> Either a b

Takes a value of any type and returns a Right with the given value.

> S.of(S.Either, 42)
Right(42)

Either#isLeft :: Either a b ~> Boolean

true if this is a Left; false if this is a Right.

> S.Left('Cannot divide by zero').isLeft
true

> S.Right(42).isLeft
false

Either#isRight :: Either a b ~> Boolean

true if this is a Right; false if this is a Left.

> S.Right(42).isRight
true

> S.Left('Cannot divide by zero').isRight
false

Either#toString :: Either a b ~> () -⁠> String

Returns the string representation of the Either.

> S.toString(S.Left('Cannot divide by zero'))
'Left("Cannot divide by zero")'

> S.toString(S.Right([1, 2, 3]))
'Right([1, 2, 3])'

Either#inspect :: Either a b ~> () -⁠> String

Returns the string representation of the Either. This method is used by util.inspect and the REPL to format a Either for display.

See also Either#toString.

> S.Left('Cannot divide by zero').inspect()
'Left("Cannot divide by zero")'

> S.Right([1, 2, 3]).inspect()
'Right([1, 2, 3])'

Either#fantasy-land/equals :: (Setoid a, Setoid b) => Either a b ~> Either a b -⁠> Boolean

Takes a value e of the same type and returns true if:

• this and e are both Lefts or both Rights, and their values are equal according to Z.equals.

> S.equals(S.Right([1, 2, 3]), S.Right([1, 2, 3]))
true

> S.equals(S.Right([1, 2, 3]), S.Left([1, 2, 3]))
false

Either#fantasy-land/lte :: (Ord a, Ord b) => Either a b ~> Either a b -⁠> Boolean

Takes a value e of the same type and returns true if:

• this is a Left and e is a Right; or

• this and e are both Lefts or both Rights, and the value of this is less than or equal to the value of e according to Z.lte.

> S.lte_(S.Left(10), S.Right(0))
true

> S.lte_(S.Right(0), S.Left(10))
false

> S.lte_(S.Right(0), S.Right(1))
true

> S.lte_(S.Right(1), S.Right(0))
false

Either#fantasy-land/concat :: (Semigroup a, Semigroup b) => Either a b ~> Either a b -⁠> Either a b

Returns the result of concatenating two Either values of the same type. a must have a Semigroup, as must b.

If this is a Left and the argument is a Left, this method returns a Left whose value is the result of concatenating this Left's value and the given Left's value.

If this is a Right and the argument is a Right, this method returns a Right whose value is the result of concatenating this Right's value and the given Right's value.

Otherwise, this method returns the Right.

> S.concat(S.Left('abc'), S.Left('def'))
Left('abcdef')

> S.concat(S.Right([1, 2, 3]), S.Right([4, 5, 6]))
Right([1, 2, 3, 4, 5, 6])

> S.concat(S.Left('abc'), S.Right([1, 2, 3]))
Right([1, 2, 3])

> S.concat(S.Right([1, 2, 3]), S.Left('abc'))
Right([1, 2, 3])

Either#fantasy-land/map :: Either a b ~> (b -⁠> c) -⁠> Either a c

Takes a function and returns this if this is a Left; otherwise it returns a Right whose value is the result of applying the function to this Right's value.

See also Either#fantasy-land/bimap.

> S.map(Math.sqrt, S.Left('Cannot divide by zero'))
Left('Cannot divide by zero')

> S.map(Math.sqrt, S.Right(9))
Right(3)

Either#fantasy-land/bimap :: Either a b ~> (a -⁠> c, b -⁠> d) -⁠> Either c d

Takes two functions and returns:

• a Left whose value is the result of applying the first function to this Left's value if this is a Left; otherwise

• a Right whose value is the result of applying the second function to this Right's value.

Similar to Either#fantasy-land/map, but supports mapping over the left side as well as the right side.

> S.bimap(S.toUpper, S.add(1), S.Left('abc'))
Left('ABC')

> S.bimap(S.toUpper, S.add(1), S.Right(42))
Right(43)

Either#fantasy-land/ap :: Either a b ~> Either a (b -⁠> c) -⁠> Either a c

Takes an Either and returns a Left unless this is a Right and the argument is a Right, in which case it returns a Right whose value is the result of applying the given Right's value to this Right's value.

> S.ap(S.Left('No such function'), S.Left('Cannot divide by zero'))
Left('No such function')

> S.ap(S.Left('No such function'), S.Right(9))
Left('No such function')

> S.ap(S.Right(Math.sqrt), S.Left('Cannot divide by zero'))
Left('Cannot divide by zero')

> S.ap(S.Right(Math.sqrt), S.Right(9))
Right(3)

Either#fantasy-land/chain :: Either a b ~> (b -⁠> Either a c) -⁠> Either a c

Takes a function and returns this if this is a Left; otherwise it returns the result of applying the function to this Right's value.

> global.sqrt = n =>
.   n < 0 ? S.Left('Cannot represent square root of negative number')
.         : S.Right(Math.sqrt(n))
sqrt

> S.chain(sqrt, S.Left('Cannot divide by zero'))
Left('Cannot divide by zero')

> S.chain(sqrt, S.Right(-1))
Left('Cannot represent square root of negative number')

> S.chain(sqrt, S.Right(25))
Right(5)

Either#fantasy-land/alt :: Either a b ~> Either a b -⁠> Either a b

Chooses between this and the other Either provided as an argument. Returns this if this is a Right; the other Either otherwise.

> S.alt(S.Left('A'), S.Left('B'))
Left('B')

> S.alt(S.Left('C'), S.Right(1))
Right(1)

> S.alt(S.Right(2), S.Left('D'))
Right(2)

> S.alt(S.Right(3), S.Right(4))
Right(3)

Either#fantasy-land/reduce :: Either a b ~> ((c, b) -⁠> c, c) -⁠> c

Takes a function and an initial value of any type, and returns:

• the initial value if this is a Left; otherwise

• the result of applying the function to the initial value and this Right's value.

> S.reduce_(Math.pow, 10, S.Left('Cannot divide by zero'))
10

> S.reduce_(Math.pow, 10, S.Right(3))
1000

Either#fantasy-land/traverse :: Applicative f => Either a b ~> (TypeRep f, b -⁠> f c) -⁠> f (Either a c)

Takes the type representative of some Applicative and a function which returns a value of that Applicative, and returns:

• the result of applying the type representative's of function to this if this is a Left; otherwise

• the result of mapping Right over the result of applying the first function to this Right's value.

> S.traverse(Array, S.words, S.Left('Request failed'))
[Left('Request failed')]

> S.traverse(Array, S.words, S.Right('foo bar baz'))
[Right('foo'), Right('bar'), Right('baz')]

Either#fantasy-land/extend :: Either a b ~> (Either a b -⁠> c) -⁠> Either a c

Takes a function and returns this if this is a Left; otherwise it returns a Right whose value is the result of applying the function to this.

> S.extend(x => x.value + 1, S.Left('Cannot divide by zero'))
Left('Cannot divide by zero')

> S.extend(x => x.value + 1, S.Right(42))
Right(43)

isLeft :: Either a b -⁠> Boolean

Returns true if the given Either is a Left; false if it is a Right.

> S.isLeft(S.Left('Cannot divide by zero'))
true

> S.isLeft(S.Right(42))
false

isRight :: Either a b -⁠> Boolean

Returns true if the given Either is a Right; false if it is a Left.

> S.isRight(S.Right(42))
true

> S.isRight(S.Left('Cannot divide by zero'))
false

fromEither :: b -⁠> Either a b -⁠> b

Takes a default value and an Either, and returns the Right value if the Either is a Right; the default value otherwise.

> S.fromEither(0, S.Right(42))
42

> S.fromEither(0, S.Left(42))
0

toEither :: a -⁠> b? -⁠> Either a b

Converts an arbitrary value to an Either: a Left if the value is null or undefined; a Right otherwise. The first argument specifies the value of the Left in the "failure" case.

> S.toEither('XYZ', null)
Left('XYZ')

> S.toEither('XYZ', 'ABC')
Right('ABC')

> S.map(S.prop('0'), S.toEither('Invalid protocol', 'ftp://example.com/'.match(/^https?:/)))
Left('Invalid protocol')

> S.map(S.prop('0'), S.toEither('Invalid protocol', 'https://example.com/'.match(/^https?:/)))
Right('https:')

either :: (a -⁠> c) -⁠> (b -⁠> c) -⁠> Either a b -⁠> c

Takes two functions and an Either, and returns the result of applying the first function to the Left's value, if the Either is a Left, or the result of applying the second function to the Right's value, if the Either is a Right.

> S.either(S.toUpper, S.toString, S.Left('Cannot divide by zero'))
'CANNOT DIVIDE BY ZERO'

> S.either(S.toUpper, S.toString, S.Right(42))
'42'

lefts :: Array (Either a b) -⁠> Array a

Takes an array of Eithers and returns an array containing each Left's value.

See also rights.

> S.lefts([S.Right(20), S.Left('foo'), S.Right(10), S.Left('bar')])
['foo', 'bar']

rights :: Array (Either a b) -⁠> Array b

Takes an array of Eithers and returns an array containing each Right's value.

See also lefts.

> S.rights([S.Right(20), S.Left('foo'), S.Right(10), S.Left('bar')])
[20, 10]

tagBy :: (a -⁠> Boolean) -⁠> a -⁠> Either a a

Takes a predicate and a value, and returns a Right of the value if it satisfies the predicate; a Left of the value otherwise.

> S.tagBy(S.odd, 0)
Left(0)

> S.tagBy(S.odd, 1)
Right(1)

encaseEither :: (Error -⁠> l) -⁠> (a -⁠> r) -⁠> a -⁠> Either l r

Takes two unary functions, f and g, the second of which may throw, and a value x of any type. Applies g to x inside a try block. If an exception is caught, the return value is a Left containing the result of applying f to the caught Error object; otherwise the return value is a Right containing the result of applying g to x.

See also encase.

> S.encaseEither(S.I, JSON.parse, '["foo","bar","baz"]')
Right(['foo', 'bar', 'baz'])

> S.encaseEither(S.I, JSON.parse, '[')
Left(new SyntaxError('Unexpected end of JSON input'))

> S.encaseEither(S.prop('message'), JSON.parse, '[')
Left('Unexpected end of JSON input')

encaseEither2 :: (Error -⁠> l) -⁠> (a -⁠> b -⁠> r) -⁠> a -⁠> b -⁠> Either l r

Binary version of encaseEither.

See also encaseEither2_.

encaseEither2_ :: (Error -⁠> l) -⁠> ((a, b) -⁠> r) -⁠> a -⁠> b -⁠> Either l r

Variant of encaseEither2 which takes an uncurried binary function.

encaseEither3 :: (Error -⁠> l) -⁠> (a -⁠> b -⁠> c -⁠> r) -⁠> a -⁠> b -⁠> c -⁠> Either l r

Ternary version of encaseEither.

See also encaseEither3_.

encaseEither3_ :: (Error -⁠> l) -⁠> ((a, b, c) -⁠> r) -⁠> a -⁠> b -⁠> c -⁠> Either l r

Variant of encaseEither3 which takes an uncurried ternary function.

eitherToMaybe :: Either a b -⁠> Maybe b