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.inc('XXX');
// ! TypeError: Invalid value
//
//   inc :: FiniteNumber -> FiniteNumber
//          ^^^^^^^^^^^^
//               1
//
//   1)  "XXX" :: String
//
//   The value at position 1 is not a member of ‘FiniteNumber’.

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

R.inc('XXX');
// => NaN

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: checkTypes, env: 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');

//    identityTypeIdent :: String
const identityTypeIdent = 'my-package/Identity';

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

Identity['@@type'] = identityTypeIdent;

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

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

//    isIdentity :: a -> Boolean
const isIdentity = x => type(x) === identityTypeIdent;

//    identityToArray :: Identity a -> Array a
const identityToArray = identity => [identity.value];

//    IdentityType :: Type
const IdentityType =
$.UnaryType(identityTypeIdent, isIdentity, identityToArray);

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

`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 -> String`

Takes a value, x, of any type and returns its type identifier.

> S.type(S.Just(42))
'sanctuary/Maybe'

> S.type([1, 2, 3])
'Array'

`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)
false

> S.equals(NaN, NaN)
true

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

`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.inc)(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.inc, 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.

`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

`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

`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(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.

`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.

`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(xs => xs.length, ['foo', 'bar', 'baz', 'quux'])
[4]

`extract :: Comonad w => w a -> a`

Type-safe version of Z.extract.

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

Curried version of Z.filter.

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

Curried version of Z.filterM.

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.inc, 42)
43

> S.map(S.A(S.__, 100), [S.inc, 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.inc)
43

> S.map(S.T(100), [S.inc, 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 :: (b -> c) -> (a -> b) -> a -> c`

Composes two unary functions, from right to left. Equivalent to Haskell's (.) function.

This is the B combinator from combinatory logic.

`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.inc, Math.sqrt, S.dec])(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)).

`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.

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

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

it is Nothing and this is Nothing; or
it is a Just and this is a Just, and their values are equal according to 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/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 two functions which both return values of the same Applicative, (the second of which must be that type's of function) and returns:

the result of applying of 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.

`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.

`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.

`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.

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

Binary version of encase.

`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.

`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.

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.

`Either#fantasy-land/equals :: Either a b ~> Either a b -> Boolean`

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

it is a Left and this is a Left, and their values are equal according to equals; or
it is a Right and this is a Right, and their values are equal according to 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/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.

`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.inc, S.Left('abc'))
Left('ABC')

> S.bimap(S.toUpper, S.inc, 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 two functions which both return values of the same Applicative, (the second of which must be that type's of function) and returns:

the result of applying of 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.

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

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

`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.

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

Binary version of encaseEither.

`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.

`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`

Converts an Either to a Maybe. A Left becomes Nothing; a Right becomes a Just.

Logic

`and :: Boolean -> Boolean -> Boolean`

Boolean "and".

> S.and(false, false)
false

> S.and(false, true)
false

> S.and(true, false)
false

> S.and(true, true)
true

`or :: Boolean -> Boolean -> Boolean`

Boolean "or".

> S.or(false, false)
false

> S.or(false, true)
true

> S.or(true, false)
true

> S.or(true, true)
true

`not :: Boolean -> Boolean`

Boolean "not".

> S.not(false)
true

> S.not(true)
false

`ifElse :: (a -> Boolean) -> (a -> b) -> (a -> b) -> a -> b`

Takes a unary predicate, a unary "if" function, a unary "else" function, and a value of any type, and returns the result of applying the "if" function to the value if the value satisfies the predicate; the result of applying the "else" function to the value otherwise.

> S.ifElse(x => x < 0, Math.abs, Math.sqrt, -1)
1

> S.ifElse(x => x < 0, Math.abs, Math.sqrt, 16)
4

`allPass :: Array (a -> Boolean) -> a -> Boolean`

Takes an array of unary predicates and a value of any type and returns true if all the predicates pass; false otherwise. None of the subsequent predicates will be evaluated after the first failed predicate.

> S.allPass([S.test(/q/), S.test(/u/), S.test(/i/)], 'quiessence')
true

> S.allPass([S.test(/q/), S.test(/u/), S.test(/i/)], 'fissiparous')
false

`anyPass :: Array (a -> Boolean) -> a -> Boolean`

Takes an array of unary predicates and a value of any type and returns true if any of the predicates pass; false otherwise. None of the subsequent predicates will be evaluated after the first passed predicate.

> S.anyPass([S.test(/q/), S.test(/u/), S.test(/i/)], 'incandescent')
true

> S.anyPass([S.test(/q/), S.test(/u/), S.test(/i/)], 'empathy')
false

List

The List type constructor enables type signatures to describe ad hoc polymorphic functions which operate on either Array or String values.

Mental gymnastics are required to treat arrays and strings similarly. [1, 2, 3] is a list containing 1, 2, and 3. 'abc' is a list containing 'a', 'b', and 'c'. But what is the type of 'a'? String, since JavaScript has no Char type! Thus:

'abc' :: String, List String, List (List String), ...

Every member of String is also a member of List String! This affects the interpretation of type signatures. Consider the type of indexOf:

a -> List a -> Maybe Integer

Assume the second argument is 'hello' :: List String. a must then be replaced with String:

String -> List String -> Maybe Integer

Since List String and String are interchangeable, the former can be replaced with the latter:

String -> String -> Maybe Integer

It's then apparent that the first argument needn't be a single-character string; the correspondence between arrays and strings does not hold.

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

Concatenates two (homogeneous) arrays, two strings, or two values of any other type which satisfies the Semigroup specification.

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

> S.concat('foo', 'bar')
'foobar'

> S.concat(S.Just('foo'), S.Just('bar'))
Just('foobar')

`slice :: Integer -> Integer -> List a -> Maybe (List a)`

Returns Just a list containing the elements from the supplied list from a beginning index (inclusive) to an end index (exclusive). Returns Nothing unless the start interval is less than or equal to the end interval, and the list contains both (half-open) intervals. Accepts negative indices, which indicate an offset from the end of the list.

> S.slice(1, 3, ['a', 'b', 'c', 'd', 'e'])
Just(['b', 'c'])

> S.slice(-2, -0, ['a', 'b', 'c', 'd', 'e'])
Just(['d', 'e'])

> S.slice(2, -0, ['a', 'b', 'c', 'd', 'e'])
Just(['c', 'd', 'e'])

> S.slice(1, 6, ['a', 'b', 'c', 'd', 'e'])
Nothing

> S.slice(2, 6, 'banana')
Just('nana')

`at :: Integer -> List a -> Maybe a`

Takes an index and a list and returns Just the element of the list at the index if the index is within the list's bounds; Nothing otherwise. A negative index represents an offset from the length of the list.

> S.at(2, ['a', 'b', 'c', 'd', 'e'])
Just('c')

> S.at(5, ['a', 'b', 'c', 'd', 'e'])
Nothing

> S.at(-2, ['a', 'b', 'c', 'd', 'e'])
Just('d')

`head :: List a -> Maybe a`

Takes a list and returns Just the first element of the list if the list contains at least one element; Nothing if the list is empty.

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

> S.head([])
Nothing

`last :: List a -> Maybe a`

Takes a list and returns Just the last element of the list if the list contains at least one element; Nothing if the list is empty.

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

> S.last([])
Nothing

`tail :: List a -> Maybe (List a)`

Takes a list and returns Just a list containing all but the first of the list's elements if the list contains at least one element; Nothing if the list is empty.

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

> S.tail([])
Nothing

`init :: List a -> Maybe (List a)`

Takes a list and returns Just a list containing all but the last of the list's elements if the list contains at least one element; Nothing if the list is empty.

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

> S.init([])
Nothing

`take :: Integer -> List a -> Maybe (List a)`

Returns Just the first N elements of the given collection if N is greater than or equal to zero and less than or equal to the length of the collection; Nothing otherwise.

> S.take(2, ['a', 'b', 'c', 'd', 'e'])
Just(['a', 'b'])

> S.take(4, 'abcdefg')
Just('abcd')

> S.take(4, ['a', 'b', 'c'])
Nothing

`takeLast :: Integer -> List a -> Maybe (List a)`

Returns Just the last N elements of the given collection if N is greater than or equal to zero and less than or equal to the length of the collection; Nothing otherwise.

> S.takeLast(2, ['a', 'b', 'c', 'd', 'e'])
Just(['d', 'e'])

> S.takeLast(4, 'abcdefg')
Just('defg')

> S.takeLast(4, ['a', 'b', 'c'])
Nothing

`drop :: Integer -> List a -> Maybe (List a)`

Returns Just all but the first N elements of the given collection if N is greater than or equal to zero and less than or equal to the length of the collection; Nothing otherwise.

> S.drop(2, ['a', 'b', 'c', 'd', 'e'])
Just(['c', 'd', 'e'])

> S.drop(4, 'abcdefg')
Just('efg')

> S.drop(4, 'abc')
Nothing

`dropLast :: Integer -> List a -> Maybe (List a)`

Returns Just all but the last N elements of the given collection if N is greater than or equal to zero and less than or equal to the length of the collection; Nothing otherwise.

> S.dropLast(2, ['a', 'b', 'c', 'd', 'e'])
Just(['a', 'b', 'c'])

> S.dropLast(4, 'abcdefg')
Just('abc')

> S.dropLast(4, 'abc')
Nothing

`reverse :: List a -> List a`

Returns the elements of the given list in reverse order.

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

> S.reverse('abc')
'cba'

`indexOf :: a -> List a -> Maybe Integer`

Takes a value of any type and a list, and returns Just the index of the first occurrence of the value in the list, if applicable; Nothing otherwise.

> S.indexOf('a', ['b', 'a', 'n', 'a', 'n', 'a'])
Just(1)

> S.indexOf('x', ['b', 'a', 'n', 'a', 'n', 'a'])
Nothing

> S.indexOf('an', 'banana')
Just(1)

> S.indexOf('ax', 'banana')
Nothing

`lastIndexOf :: a -> List a -> Maybe Integer`

Takes a value of any type and a list, and returns Just the index of the last occurrence of the value in the list, if applicable; Nothing otherwise.

> S.lastIndexOf('a', ['b', 'a', 'n', 'a', 'n', 'a'])
Just(5)

> S.lastIndexOf('x', ['b', 'a', 'n', 'a', 'n', 'a'])
Nothing

> S.lastIndexOf('an', 'banana')
Just(3)

> S.lastIndexOf('ax', 'banana')
Nothing

Array

`append :: a -> Array a -> Array a`

Takes a value of any type and an array of values of that type, and returns the result of appending the value to the array.

`prepend :: a -> Array a -> Array a`

Takes a value of any type and an array of values of that type, and returns the result of prepending the value to the array.

`joinWith :: String -> Array String -> String`

Joins the strings of the second argument separated by the first argument.

Properties:

forall s :: String, t :: String. S.joinWith(s, S.splitOn(s, t)) = t

`find :: (a -> Boolean) -> Array a -> Maybe a`

Takes a predicate and an array and returns Just the leftmost element of the array which satisfies the predicate; Nothing if none of the array's elements satisfies the predicate.

> S.find(n => n < 0, [1, -2, 3, -4, 5])
Just(-2)

> S.find(n => n < 0, [1, 2, 3, 4, 5])
Nothing

`pluck :: Accessible a => String -> Array a -> Array b`

Combines map and prop. pluck(k, xs) is equivalent to map(prop(k), xs).

> S.pluck('x', [{x: 1}, {x: 2}, {x: 3}])
[1, 2, 3]

`unfoldr :: (b -> Maybe (Pair a b)) -> b -> Array a`

Takes a function and a seed value, and returns an array generated by applying the function repeatedly. The array is initially empty. The function is initially applied to the seed value. Each application of the function should result in either:

Nothing, in which case the array is returned; or
Just a pair, in which case the first element is appended to the array and the function is applied to the second element.

> S.unfoldr(n => n < 5 ? S.Just([n, n + 1]) : S.Nothing, 1)
[1, 2, 3, 4]

`range :: Integer -> Integer -> Array Integer`

Returns an array of consecutive integers starting with the first argument and ending with the second argument minus one. Returns [] if the second argument is less than or equal to the first argument.

> S.range(0, 10)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

> S.range(-5, 0)
[-5, -4, -3, -2, -1]

> S.range(0, -5)
[]

Object

`prop :: Accessible a => String -> a -> b`

Takes a property name and an object with known properties and returns the value of the specified property. If for some reason the object lacks the specified property, a type error is thrown.

For accessing properties of uncertain objects, use get instead.

`props :: Accessible a => Array String -> a -> b`

Takes a property path (an array of property names) and an object with known structure and returns the value at the given path. If for some reason the path does not exist, a type error is thrown.

For accessing property paths of uncertain objects, use gets instead.

> S.props(['a', 'b', 'c'], {a: {b: {c: 1}}})
1

`get :: Accessible a => (b -> Boolean) -> String -> a -> Maybe c`

Takes a predicate, a property name, and an object and returns Just the value of the specified object property if it exists and the value satisfies the given predicate; Nothing otherwise.

`gets :: Accessible a => (b -> Boolean) -> Array String -> a -> Maybe c`

Takes a predicate, a property path (an array of property names), and an object and returns Just the value at the given path if such a path exists and the value satisfies the given predicate; Nothing otherwise.

`keys :: StrMap a -> Array String`

Returns the keys of the given string map, in arbitrary order.

> S.keys({b: 2, c: 3, a: 1}).sort()
['a', 'b', 'c']

`values :: StrMap a -> Array a`

Returns the values of the given string map, in arbitrary order.

> S.values({a: 1, c: 3, b: 2}).sort()
[1, 2, 3]

`pairs :: StrMap a -> Array (Pair String a)`

Returns the key–value pairs of the given string map, in arbitrary order.

> S.pairs({b: 2, a: 1, c: 3}).sort()
[['a', 1], ['b', 2], ['c', 3]]

Number

`negate :: ValidNumber -> ValidNumber`

Negates its argument.

> S.negate(12.5)
-12.5

> S.negate(-42)
42

`add :: FiniteNumber -> FiniteNumber -> FiniteNumber`

Returns the sum of two (finite) numbers.

> S.add(1, 1)
2

`sum :: Foldable f => f FiniteNumber -> FiniteNumber`

Returns the sum of the given array of (finite) numbers.

> S.sum([1, 2, 3, 4, 5])
15

> S.sum([])
0

> S.sum(S.Just(42))
42

> S.sum(S.Nothing)
0

`sub :: FiniteNumber -> FiniteNumber -> FiniteNumber`

Returns the difference between two (finite) numbers.

> S.sub(4, 2)
2

`inc :: FiniteNumber -> FiniteNumber`

Increments a (finite) number by one.

> S.inc(1)
2

`dec :: FiniteNumber -> FiniteNumber`

Decrements a (finite) number by one.

> S.dec(2)
1

`mult :: FiniteNumber -> FiniteNumber -> FiniteNumber`

Returns the product of two (finite) numbers.

> S.mult(4, 2)
8

`product :: Foldable f => f FiniteNumber -> FiniteNumber`

Returns the product of the given array of (finite) numbers.

> S.product([1, 2, 3, 4, 5])
120

> S.product([])
1

> S.product(S.Just(42))
42

> S.product(S.Nothing)
1

`div :: FiniteNumber -> NonZeroFiniteNumber -> FiniteNumber`

Returns the result of dividing its first argument (a finite number) by its second argument (a non-zero finite number).

> S.div(7, 2)
3.5

`mean :: Foldable f => f FiniteNumber -> Maybe FiniteNumber`

Returns the mean of the given array of (finite) numbers.

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

> S.mean([])
Nothing

> S.mean(S.Just(42))
Just(42)

> S.mean(S.Nothing)
Nothing

`min :: Ord a => a -> a -> a`

Returns the smaller of its two arguments.

Strings are compared lexicographically. Specifically, the Unicode code point value of each character in the first string is compared to the value of the corresponding character in the second string.

`max :: Ord a => a -> a -> a`

Returns the larger of its two arguments.

Strings are compared lexicographically. Specifically, the Unicode code point value of each character in the first string is compared to the value of the corresponding character in the second string.

Integer

`even :: Integer -> Boolean`

Returns true if the given integer is even; false if it is odd.

> S.even(42)
true

> S.even(99)
false

`odd :: Integer -> Boolean`

Returns true if the given integer is odd; false if it is even.

> S.odd(99)
true

> S.odd(42)
false

Parse

`parseDate :: String -> Maybe Date`

Takes a string and returns Just the date represented by the string if it does in fact represent a date; Nothing otherwise.

> S.parseDate('2011-01-19T17:40:00Z')
Just(new Date('2011-01-19T17:40:00.000Z'))

> S.parseDate('today')
Nothing

`parseFloat :: String -> Maybe Number`

Takes a string and returns Just the number represented by the string if it does in fact represent a number; Nothing otherwise.

> S.parseFloat('-123.45')
Just(-123.45)

> S.parseFloat('foo.bar')
Nothing

`parseInt :: Integer -> String -> Maybe Integer`

Takes a radix (an integer between 2 and 36 inclusive) and a string, and returns Just the number represented by the string if it does in fact represent a number in the base specified by the radix; Nothing otherwise.

This function is stricter than parseInt: a string is considered to represent an integer only if all its non-prefix characters are members of the character set specified by the radix.

> S.parseInt(10, '-42')
Just(-42)

> S.parseInt(16, '0xFF')
Just(255)

> S.parseInt(16, '0xGG')
Nothing

`parseJson :: (a -> Boolean) -> String -> Maybe b`

Takes a predicate and a string which may or may not be valid JSON, and returns Just the result of applying JSON.parse to the string if the result satisfies the predicate; Nothing otherwise.

> S.parseJson(S.is(Array), '["foo","bar","baz"]')
Just(['foo', 'bar', 'baz'])

> S.parseJson(S.is(Array), '[')
Nothing

> S.parseJson(S.is(Object), '["foo","bar","baz"]')
Nothing

RegExp

`regex :: RegexFlags -> String -> RegExp`

Takes a RegexFlags and a pattern, and returns a RegExp.

> S.regex('g', ':\\d+:')
/:\d+:/g

`regexEscape :: String -> String`

Takes a string which may contain regular expression metacharacters, and returns a string with those metacharacters escaped.

Properties:

forall s :: String. S.test(S.regex('', S.regexEscape(s)), s) = true

> S.regexEscape('-=*{XYZ}*=-')
'\\-=\\*\\{XYZ\\}\\*=\\-'

`test :: RegExp -> String -> Boolean`

Takes a pattern and a string, and returns true if the pattern matches the string; false otherwise.

> S.test(/^a/, 'abacus')
true

> S.test(/^a/, 'banana')
false

`match :: NonGlobalRegExp -> String -> Maybe { match :: String, groups :: Array (Maybe String) }`

Takes a pattern and a string, and returns Just a match record if the pattern matches the string; Nothing otherwise.

groups :: Array (Maybe String) acknowledges the existence of optional capturing groups.

Properties:

forall p :: Pattern, s :: String. S.head(S.matchAll(S.regex("g", p), s)) = S.match(S.regex("", p), s)

`matchAll :: GlobalRegExp -> String -> Array { match :: String, groups :: Array (Maybe String) }`

Takes a pattern and a string, and returns an array of match records.

groups :: Array (Maybe String) acknowledges the existence of optional capturing groups.

String

`toUpper :: String -> String`

Returns the upper-case equivalent of its argument.

`toLower :: String -> String`

Returns the lower-case equivalent of its argument.

`trim :: String -> String`

Strips leading and trailing whitespace characters.

> S.trim('\t\t foo bar \n')
'foo bar'

Name		Name	Last commit message	Last commit date
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scripts		scripts
test		test
.gitignore		.gitignore
CONTRIBUTING.md		CONTRIBUTING.md
LICENSE		LICENSE
Makefile		Makefile
README.md		README.md
bower.json		bower.json
circle.yml		circle.yml
index.js		index.js
karma.conf.js		karma.conf.js
package.json		package.json

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