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Sanctuary v0.15.0

Refuge from unsafe JavaScript

Overview

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.

Try changing words to [] in the REPL below. Hit return to re-evaluate.

["foo", "bar", "baz"]
"FOO"
Just ("FOO")

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

Ramda

Ramda provides several functions which return problematic values such as undefined, Infinity, or NaN when applied to unsuitable inputs. These are known as partial functions. Partial functions necessitate the use of guards or null checks. In order to safely use R.head, for example, one must ensure that the array is non-empty:

if (R.isEmpty (xs)) {
  // ...
} else {
  return f (R.head (xs));
}

Using the Maybe type renders such guards (and null checks) unnecessary. Changing functions such as R.head to return Maybe values was proposed in ramda/ramda#683, but was considered too much of a stretch for JavaScript programmers. Sanctuary was released the following month, in January 2015, as a companion library to Ramda.

In addition to broadening in scope in the years since its release, Sanctuary's philosophy has diverged from Ramda's in several respects.

Totality

Every Sanctuary function is defined for every value which is a member of the function's input type. Such functions are known as total functions. Ramda, on the other hand, contains a number of partial functions.

Information preservation

Certain Sanctuary functions preserve more information than their Ramda counterparts. Examples:

|> R.tail ([])                      |> S.tail ([])
[]                                  Nothing

|> R.tail (['foo'])                 |> S.tail (['foo'])
[]                                  Just ([])

|> R.replace (/^x/) ('') ('abc')    |> S.stripPrefix ('x') ('abc')
'abc'                               Nothing

|> R.replace (/^x/) ('') ('xabc')   |> S.stripPrefix ('x') ('xabc')
'abc'                               Just ('abc')

Invariants

Sanctuary performs rigorous type checking of inputs and outputs, and throws a descriptive error if a type error is encountered. This allows bugs to be caught and fixed early in the development cycle.

Ramda operates on the garbage in, garbage out principal. Functions are documented to take arguments of particular types, but these invariants are not enforced. The problem with this approach in a language as permissive as JavaScript is that there's no guarantee that garbage input will produce garbage output (ramda/ramda#1413). Ramda performs ad hoc type checking in some such cases (ramda/ramda#1419).

Sanctuary can be configured to operate in garbage in, garbage out mode. Ramda cannot be configured to enforce its invariants.

Currying

Sanctuary functions are curried. There is, for example, exactly one way to apply S.reduce to S.add, 0, and xs:

Ramda functions are also curried, but in a complex manner. There are four ways to apply R.reduce to R.add, 0, and xs:

Ramda supports all these forms because curried functions enable partial application, one of the library's tenets, but f(x)(y)(z) is considered too unfamiliar and too unattractive to appeal to JavaScript programmers.

Sanctuary's developers prefer a simple, unfamiliar construct to a complex, familiar one. Familiarity can be acquired; complexity is intrinsic.

The lack of breathing room in f(x)(y)(z) impairs readability. The simple solution to this problem, proposed in #438, is to include a space when applying a function: f (x) (y) (z).

Ramda also provides a special placeholder value, R.__, which removes the restriction that a function must be applied to its arguments in order. The following expressions are equivalent:

Variadic functions

Ramda provides several functions which take any number of arguments. These are known as variadic functions. Additionally, Ramda provides several functions which take variadic functions as arguments. Although natural in a dynamically typed language, variadic functions are at odds with the type notation Ramda and Sanctuary both use, leading to some indecipherable type signatures such as this one:

R.lift :: (*... -> *...) -> ([*]... -> [*])

Sanctuary has no variadic functions, nor any functions which take variadic functions as arguments. Sanctuary provides two "lift" functions, each with a helpful type signature:

S.lift2 :: Apply f => (a -> b -> c) -> f a -> f b -> f c
S.lift3 :: Apply f => (a -> b -> c -> d) -> f a -> f b -> f c -> f d

Implicit context

Ramda provides R.bind and R.invoker for working with methods. Additionally, many Ramda functions use Function#call or Function#apply to preserve context. Sanctuary makes no allowances for this.

Transducers

Several Ramda functions act as transducers. Sanctuary provides no support for transducers.

Modularity

Whereas Ramda has no dependencies, Sanctuary has a modular design: sanctuary-def provides type checking, sanctuary-type-classes provides Fantasy Land functions and type classes, sanctuary-show provides string representations, and algebraic data types are provided by sanctuary-either, sanctuary-maybe, and sanctuary-pair. Not only does this approach reduce the complexity of Sanctuary itself, but it allows these components to be reused in other contexts.

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:

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 fantasy-land/map method of the Maybe type is written Maybe a ~> (a -> b) -> Maybe b. One could read this as:

When the fantasy-land/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 (=>).

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:

! 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/v0.18.1#FiniteNumber for information about the sanctuary-def/FiniteNumber type.

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

3

There is a performance cost to run-time type checking. Type checking is disabled by default if process.env.NODE_ENV is 'production'. If this rule is unsuitable for a given program, one may use create to create a Sanctuary module based on a different rule. For example:

const S = sanctuary.create ({
  checkTypes: localStorage.getItem ('SANCTUARY_CHECK_TYPES') === 'true',
  env: sanctuary.env,
});

Occasionally one may wish to perform an operation which is not type safe, such as mapping over an object with heterogeneous values. This is possible via selective use of unchecked functions.

Installation

npm install sanctuary will install Sanctuary for use in Node.js.

Running Sanctuary in the browser is more involved. One must include a <script> for each dependency in addition to one for Sanctuary itself:

<script src="vendor/sanctuary-show.js"></script>
<script src="vendor/sanctuary-type-identifiers.js"></script>
<script src="vendor/sanctuary-type-classes.js"></script>
<script src="vendor/sanctuary-either.js"></script>
<script src="vendor/sanctuary-maybe.js"></script>
<script src="vendor/sanctuary-pair.js"></script>
<script src="vendor/sanctuary-def.js"></script>
<script src="vendor/sanctuary.js"></script>

To ensure compatibility one should use the dependency versions specified in package.json.

For convenience one could define aliases for various modules:

const S = window.sanctuary;
const $ = window.sanctuaryDef;
// ...

API

Configure

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 = x => {
  const identity = Object.create (Identity$prototype);
  identity.value = x;
  return identity;
};

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

const Identity$prototype = {
  'constructor': Identity,
  '@@show': function() { return `Identity (${S.show (this.value)})`; },
  '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 Sanctuary module's environment ((S.create ({checkTypes, env})).env is a reference to env). Useful in conjunction with create.

[Function, Arguments, (Array Unknown), Boolean, Date, Error, HtmlElement, Null, Number, Object, RegExp, (StrMap Unknown), String, Symbol, Undefined, FiniteNumber, NonZeroFiniteNumber, (Either Unknown Unknown), (Unknown -> Unknown), GlobalRegExp, NonGlobalRegExp, Integer, NonNegativeInteger, (Maybe Unknown), (Array2 Unknown Unknown), (Pair Unknown Unknown), RegexFlags, Type, TypeClass, ValidDate, ValidNumber]

unchecked :: Module

A complete Sanctuary module which performs no type checking. This is useful as it permits operations which Sanctuary's type checking would disallow, such as mapping over an object with heterogeneous values.

See also create.

{"x": "\"foo\"", "y": "true", "z": "42"}

Opting out of type checking may cause type errors to go unnoticed.

"22"

Classify

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

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

{"name": "Maybe", "namespace": Just ("sanctuary-maybe"), "version": 1}
{"name": "Array", "namespace": Nothing, "version": 0}

is :: Type -> Any -> Boolean

Returns true iff the given value is a member of the specified type. See $.test for details.

true
false

Showable

show :: Any -> String

Alias of show.

"-0"
"[\"foo\", \"bar\", \"baz\"]"
"{\"x\": 1, \"y\": 2, \"z\": 3}"
"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.

true
true
true
false

lt :: Ord a => a -> a -> Boolean

Returns true iff the second argument is less than the first according to Z.lt.

[1, 2]

lte :: Ord a => a -> a -> Boolean

Returns true iff the second argument is less than or equal to the first according to Z.lte.

[1, 2, 3]

gt :: Ord a => a -> a -> Boolean

Returns true iff the second argument is greater than the first according to Z.gt.

[4, 5]

gte :: Ord a => a -> a -> Boolean

Returns true iff the second argument is greater than or equal to the first according to Z.gte.

[3, 4, 5]

min :: Ord a => a -> a -> a

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

See also max.

2
new Date ("1999-12-31T00:00:00.000Z")
"10"

max :: Ord a => a -> a -> a

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

See also min.

10
new Date ("2000-01-01T00:00:00.000Z")
"2"

id :: Category c => TypeRep c -> c

Type-safe version of Z.id.

42

concat :: Semigroup a => a -> a -> a

Curried version of Z.concat.

"abcdef"
[1, 2, 3, 4, 5, 6]
{"x": 1, "y": 3, "z": 4}
Just ([1, 2, 3, 4, 5, 6])
Sum (42)

empty :: Monoid a => TypeRep a -> a

Type-safe version of Z.empty.

""
[]
{}
Sum (0)

invert :: Group g => g -> g

Type-safe version of Z.invert.

Sum (-5)

filter :: Filterable f => (a -> Boolean) -> f a -> f a

Curried version of Z.filter. Discards every element which does not satisfy the predicate.

See also reject.

[1, 3]
{"x": 1, "z": 3}
Nothing
Nothing
Just (1)

reject :: Filterable f => (a -> Boolean) -> f a -> f a

Curried version of Z.reject. Discards every element which satisfies the predicate.

See also filter.

[2]
{"y": 2}
Nothing
Just (0)
Nothing

takeWhile :: Filterable f => (a -> Boolean) -> f a -> f a

Curried version of Z.takeWhile. Discards the first element which does not satisfy the predicate, and all subsequent elements.

See also dropWhile.

[3, 3, 3, 7]
[]

dropWhile :: Filterable f => (a -> Boolean) -> f a -> f a

Curried version of Z.dropWhile. Retains the first element which does not satisfy the predicate, and all subsequent elements.

See also takeWhile.

[6, 3, 5, 4]
[3, 3, 3, 7, 6, 3, 5, 4]

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

Curried version of Z.map.

[1, 2, 3]
{"x": 1, "y": 2, "z": 3}
Just (3)
Right (3)
Pair (99980001) (9999)

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)
10

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

Curried version of Z.flip. Maps over the given functions, applying each to the given value.

Replacing Functor f => f with Function x produces the C combinator from combinatory logic:

Functor f => f (a -> b) -> a -> f b
Function x (a -> b) -> a -> Function x b
Function x (a -> c) -> a -> Function x c
Function x (b -> c) -> b -> Function x c
Function a (b -> c) -> b -> Function a c
(a -> b -> c) -> b -> a -> c
"foo!"
[1, 2]
{"ceil": 2, "floor": 1}
Cons (1) (Cons (2) (Nil))

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

Curried version of Z.bimap.

Pair ("FOO") (8)
Left ("FOO")
Right (8)

mapLeft :: Bifunctor f => (a -> b) -> f a c -> f b c

Curried version of Z.mapLeft. Maps the given function over the left side of a Bifunctor.

Pair ("FOO") (64)
Left ("FOO")
Right (64)

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

Curried version of Z.promap.

11

alt :: Alt f => f a -> f a -> f a

Curried version of Z.alt.

Just (1)
Just (2)
Right (1)
Right (2)

zero :: Plus f => TypeRep f -> f a

Type-safe version of Z.zero.

[]
{}
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.

15
[5, 4, 3, 2, 1]

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

Curried version of Z.traverse.

[Just ("foo"), Just ("bar"), Just ("baz")]
[Nothing]
Just ([10, 11, 12])
Nothing
Just ({"a": 10, "b": 11, "c": 12})
Nothing

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

Curried version of Z.sequence. Inverts the given t (f a) to produce an f (t a).

[Just (1), Just (2), Just (3)]
Just ([1, 2, 3])
Nothing
Just ({"a": 1, "b": 2, "c": 3})
Nothing

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

Curried version of Z.ap.

[1, 2, 3, 4, 5, 1, 16, 81, 256, 625]
{"x": 2, "y": 5}
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)
"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.

Just (5)
Nothing
Just (true)
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.

Just (6)
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.

[1, 1, 2, 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.

[3, 4, 3, 4]
Just (2)

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

Curried version of Z.of.

[42]
42
Just (42)
Right (42)

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

Curried version of Z.chain.

[1, 1, 2, 2, 3, 3]
"sli"
Just (123)
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.

[1, 2, 3]
[[1, 2, 3]]
Just (1)
Pair ("foobar") ("baz")

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)
"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).

["oo!", "oo?", "on!", "on?", "no!", "no?", "nn!", "nn?"]

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

Curried version of Z.extend.

["xyz", "yz", "z"]
[4, 3, 2, 1]

duplicate :: Extend w => w a -> w (w a)

Type-safe version of Z.duplicate. Adds one level of nesting to a comonadic structure.

Just (Just (1))
[[1]]
[[1, 2, 3], [2, 3], [3]]
[4, 3, 2, 1]

extract :: Comonad w => w a -> a

Type-safe version of Z.extract.

"bar"

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

Type-safe version of Z.contramap.

3

Combinator

I :: a -> a

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

"foo"

K :: a -> b -> a

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

"foo"
[42, 42, 42, 42, 42]

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.

43
[101, 10]

Function

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

Curries the given binary function.

[10, 100, 1000]

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

Curries the given ternary function.

undefined
"orange icecream"

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

Curries the given quaternary function.

undefined
{"height": 10, "width": 10, "x": 0, "y": 0}

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

Curries the given quinary function.

undefined
"https://example.com:443/foo/bar"

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.

10

pipe :: Foldable f => f (Any -> Any) -> a -> b

Takes a sequence 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 a sequence of functions. pipe ([f, g, h]) (x) is equivalent to h (g (f (x))).

9

pipeK :: (Foldable f, Chain m) => f (Any -> m Any) -> m a -> m b

Takes a sequence of functions assumed to be unary which return values with a Chain, and a value of that Chain, and returns the result of applying the sequence of transformations to the initial value.

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

Just (3)

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.

[3, 2, 1, 6, 5, 4]

Pair type

Pair is the canonical product type: a value of type Pair a b always contains exactly two values: one of type a; one of type b.

The implementation is provided by sanctuary-pair.

PairType :: Type -> Type -> Type

A BinaryType for use with sanctuary-def.

Pair :: a -> b -> Pair a b

Pair's sole data constructor. Additionally, it serves as the Pair type representative.

Pair ("foo") (42)

fst :: Pair a b -> a

fst (Pair (x) (y)) is equivalent to x.

"foo"

snd :: Pair a b -> b

snd (Pair (x) (y)) is equivalent to y.

42

swap :: Pair a b -> Pair b a

swap (Pair (x) (y)) is equivalent to Pair (y) (x).

Pair (42) ("foo")

Maybe type

The Maybe type represents optional values: a value of type Maybe a is either Nothing (the empty value) or a Just whose value is of type a.

The implementation is provided by sanctuary-maybe.

MaybeType :: Type -> Type

A UnaryType for use with sanctuary-def.

Maybe :: TypeRep Maybe

Maybe type representative.

Nothing :: Maybe a

The empty value of type Maybe a.

Nothing

Just :: a -> Maybe a

Constructs a value of type Maybe a from a value of type a.

Just (42)

isNothing :: Maybe a -> Boolean

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

true
false

isJust :: Maybe a -> Boolean

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

true
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.

42
0

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

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

undefined
1000000
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.

42
null

toMaybe :: a? -> Maybe a

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

Nothing
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_.

6
0

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

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

undefined
1000
832040

justs :: (Filterable f, Functor f) => f (Maybe a) -> f a

Discards each element which is Nothing, and unwraps each element which is a Just. Related to Haskell's catMaybes function.

See also lefts and rights.

["foo", "baz"]

mapMaybe :: (Filterable f, Functor f) => (a -> Maybe b) -> f a -> f b

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

[1, 4]
{"x": 1, "z": 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.

Just (2)
Nothing

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

Binary version of encase.

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

Ternary version of encase.

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.

Left ("Expecting an integer")
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 implementation is provided by sanctuary-either.

EitherType :: Type -> Type -> Type

A BinaryType for use with sanctuary-def.

Either :: TypeRep Either

Either type representative.

Left :: a -> Either a b

Constructs a value of type Either a b from a value of type a.

Left ("Cannot divide by zero")

Constructs a value of type Either a b from a value of type b.

Right (42)

isLeft :: Either a b -> Boolean

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

true
false

isRight :: Either a b -> Boolean

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

true
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.

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.

Left ("XYZ")
Right ("ABC")
Left ("Invalid protocol")
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.

"CANNOT DIVIDE BY ZERO"
"42"

lefts :: (Filterable f, Functor f) => f (Either a b) -> f a

Discards each element which is a Right, and unwraps each element which is a Left.

See also rights.

["foo", "bar"]

rights :: (Filterable f, Functor f) => f (Either a b) -> f b

Discards each element which is a Left, and unwraps each element which is a Right.

See also lefts.

[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.

Left (0)
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.

Right (["foo", "bar", "baz"])
Left (new SyntaxError ("Unexpected end of JSON input"))
Left ("Unexpected end of JSON input")

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

Binary version of encaseEither.

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

Ternary version of encaseEither.

eitherToMaybe :: Either a b -> Maybe b

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

See also maybeToEither.

Nothing
Just (42)

Logic

and :: Boolean -> Boolean -> Boolean

Boolean "and".

false
false
false
true

or :: Boolean -> Boolean -> Boolean

Boolean "or".

false
true
true
true

not :: Boolean -> Boolean

Boolean "not".

See also complement.

true
false

complement :: (a -> Boolean) -> a -> Boolean

Takes a unary predicate and a value of any type, and returns the logical negation of applying the predicate to the value.

See also 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.

See also when and unless.

1
4

when :: (a -> Boolean) -> (a -> a) -> a -> a

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

See also unless and ifElse.

4
-1

unless :: (a -> Boolean) -> (a -> a) -> a -> a

Takes a unary predicate, a unary function, and a value of any type, and returns the result of applying the function to the value if the value does not satisfy the predicate; the value otherwise.

See also when and ifElse.

4
-1

allPass :: Foldable f => f (a -> Boolean) -> a -> Boolean

Takes a structure containing zero or more predicates, and a value of any type. Returns true iff the value satisfies all of the predicates. None of the subsequent predicates will be applied after the first predicate not satisfied.

true
false

anyPass :: Foldable f => f (a -> Boolean) -> a -> Boolean

Takes a structure containing zero or more predicates, and a value of any type. Returns true iff the value satisfies any of the predicates. None of the subsequent predicates will be applied after the first predicate satisfied.

true
false

Array

slice :: Integer -> Integer -> Array a -> Maybe (Array a)

Takes a start index i, an end index j, and an array, and returns Just the [i,j) slice of the array if possible; Nothing otherwise. A negative index represents an offset from the length of the array.

See also take, drop, takeLast, and dropLast.

Just (["b", "c"])
Just (["c", "d"])
Nothing

at :: Integer -> Array a -> Maybe a

Returns Just the element of the given array at the specified index if the index is within the array's bounds; Nothing otherwise. A negative index represents an offset from the length of the array.

Just ("c")
Nothing
Just ("d")

Returns Just the first element of the given array if the array contains at least one element; Nothing otherwise.

Just (1)
Nothing

last :: Array a -> Maybe a

Returns Just the last element of the given array if the array contains at least one element; Nothing otherwise.

Just (3)
Nothing

tail :: Array a -> Maybe (Array a)

Returns Just all but the first of the given array's elements if the array contains at least one element; Nothing otherwise.

Just ([2, 3])
Nothing

init :: Array a -> Maybe (Array a)

Returns Just all but the last of the given array's elements if the array contains at least one element; Nothing otherwise.

Just ([1, 2])
Nothing

take :: Integer -> Array a -> Maybe (Array a)

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

Just (["a", "b"])
Just (["a", "b", "c", "d", "e"])
Nothing

takeLast :: Integer -> Array a -> Maybe (Array a)

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

Just (["d", "e"])
Just (["a", "b", "c", "d", "e"])
Nothing

drop :: Integer -> Array a -> Maybe (Array a)

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

Just (["c", "d", "e"])
Just ([])
Nothing

dropLast :: Integer -> Array a -> Maybe (Array a)

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

Just (["a", "b", "c"])
Just ([])
Nothing

size :: Foldable f => f a -> Integer

Returns the number of elements of the given structure.

0
3
0
3
0
1
1

append :: (Applicative f, Semigroup (f a)) => a -> f a -> f a

Returns the result of appending the first argument to the second.

See also prepend.

[1, 2, 3]
Cons (1) (Cons (2) (Cons (3) (Nil)))
Just ([1])
Just ([1, 2, 3])

prepend :: (Applicative f, Semigroup (f a)) => a -> f a -> f a

Returns the result of prepending the first argument to the second.

See also append.

[1, 2, 3]
Cons (1) (Cons (2) (Cons (3) (Nil)))
Just ([1])
Just ([1, 2, 3])

joinWith :: String -> Array String -> String

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

Properties:

See also splitOn.

"foo:bar:baz"

elem :: (Setoid a, Foldable f) => a -> f a -> Boolean

Takes a value and a structure and returns true iff the value is an element of the structure.

See also find.

true
false
true
false
true
false
false

find :: Foldable f => (a -> Boolean) -> f a -> Maybe a

Takes a predicate and a structure and returns Just the leftmost element of the structure which satisfies the predicate; Nothing if there is no such element.

See also elem.

Just (-2)
Nothing

foldMap :: (Monoid m, Foldable f) => TypeRep m -> (a -> m) -> f a -> m

Curried version of Z.foldMap. Deconstructs a foldable by mapping every element to a monoid and concatenating the results.

"sincostan"

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:

[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.

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
[-5, -4, -3, -2, -1]
[]

groupBy :: (a -> a -> Boolean) -> Array a -> Array (Array a)

Splits its array argument into an array of arrays of equal, adjacent elements. Equality is determined by the function provided as the first argument. Its behaviour can be surprising for functions that aren't reflexive, transitive, and symmetric (see equivalence relation).

Properties:

[[1, 1], [2], [1, 1]]
[[2], [-3, 3, 3, 3], [4, -4], [4]]

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

Reverses the elements of the given structure.

[3, 2, 1]
Cons (3) (Cons (2) (Cons (1) (Nil)))
"cba"

sort :: (Ord a, Applicative m, Foldable m, Monoid (m a)) => m a -> m a

Performs a stable sort of the elements of the given structure, using Z.lte for comparisons.

Properties:

See also sortBy.

["bar", "baz", "foo"]
[Left (2), Left (4), Right (1), Right (3)]

sortBy :: (Ord b, Applicative m, Foldable m, Monoid (m a)) => (a -> b) -> m a -> m a

Performs a stable sort of the elements of the given structure, using Z.lte to compare the values produced by applying the given function to each element of the structure.

Properties:

See also sort.

[{"rank": 2, "suit": "hearts"}, {"rank": 5, "suit": "hearts"}, {"rank": 5, "suit": "spades"}, {"rank": 7, "suit": "spades"}]
[{"rank": 5, "suit": "hearts"}, {"rank": 2, "suit": "hearts"}, {"rank": 7, "suit": "spades"}, {"rank": 5, "suit": "spades"}]

If descending order is desired, one may use Descending:

[121, 117, 116, 114, 110, 99, 97, 97, 83]

zip :: Array a -> Array b -> Array (Pair a b)

Returns an array of pairs of corresponding elements from the given arrays. The length of the resulting array is equal to the length of the shorter input array.

See also zipWith.

[Pair ("a") ("x"), Pair ("b") ("y")]
[Pair (1) (2), Pair (3) (4)]

zipWith :: (a -> b -> c) -> Array a -> Array b -> Array c

Returns the result of combining, pairwise, the given arrays using the given binary function. The length of the resulting array is equal to the length of the shorter input array.

See also zip.

["ax", "by"]
[[1, 2], [3, 4]]

Object

prop :: 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.

1

props :: 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.

1

get :: (Any -> Boolean) -> String -> a -> Maybe b

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.

See also gets and prop.

Just (1)
Nothing
Nothing
Just ([1, 2, 3])
Nothing

gets :: (Any -> Boolean) -> Array String -> a -> Maybe b

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.

See also get.

Just (42)
Nothing
Nothing

StrMap

StrMap is an abbreviation of string map. A string map is an object, such as {foo: 1, bar: 2, baz: 3}, whose values are all members of the same type. Formally, a value is a member of type StrMap a if its type identifier is 'Object' and the values of its enumerable own properties are all members of type a.

singleton :: String -> a -> StrMap a

Takes a string and a value of any type, and returns a string map with a single entry (mapping the key to the value).

{"foo": 42}

insert :: String -> a -> StrMap a -> StrMap a

Takes a string, a value of any type, and a string map, and returns a string map comprising all the entries of the given string map plus the entry specified by the first two arguments (which takes precedence).

Equivalent to Haskell's insert function. Similar to Clojure's assoc function.

{"a": 1, "b": 2, "c": 3}
{"a": 4, "b": 2}

remove :: String -> StrMap a -> StrMap a

Takes a string and a string map, and returns a string map comprising all the entries of the given string map except the one whose key matches the given string (if such a key exists).

Equivalent to Haskell's delete function. Similar to Clojure's dissoc function.

{"a": 1, "b": 2}
{}

keys :: StrMap a -> Array String

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

["a", "b", "c"]

values :: StrMap a -> Array a

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

[1, 2, 3]

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

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

[Pair ("a") (1), Pair ("b") (2), Pair ("c") (3)]

fromPairs :: Foldable f => f (Pair String a) -> StrMap a

Returns a string map containing the key–value pairs specified by the given Foldable. If a key appears in multiple pairs, the rightmost pair takes precedence.

{"a": 1, "b": 2, "c": 3}
{"x": 2}

Number

negate :: ValidNumber -> ValidNumber

Negates its argument.

-12.5
42

add :: FiniteNumber -> FiniteNumber -> FiniteNumber

Returns the sum of two (finite) numbers.

2

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

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

15
0
42
0

sub :: FiniteNumber -> FiniteNumber -> FiniteNumber

Takes a finite number n and returns the subtract n function.

[0, 1, 2]

mult :: FiniteNumber -> FiniteNumber -> FiniteNumber

Returns the product of two (finite) numbers.

8

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

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

120
1
42
1

div :: NonZeroFiniteNumber -> FiniteNumber -> FiniteNumber

Takes a non-zero finite number n and returns the divide by n function.

[0, 0.5, 1, 1.5]

pow :: FiniteNumber -> FiniteNumber -> FiniteNumber

Takes a finite number n and returns the power of n function.

[9, 4, 1, 0, 1, 4, 9]
[1, 2, 3, 4, 5]

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

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

Just (3)
Nothing
Just (42)
Nothing

Integer

even :: Integer -> Boolean

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

true
false

odd :: Integer -> Boolean

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

true
false

Parse

parseDate :: String -> Maybe ValidDate

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

Just (new Date ("2011-01-19T17:40:00.000Z"))
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.

Just (-123.45)
Nothing

parseInt :: Radix -> 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.

Just (-42)
Just (255)
Nothing

parseJson :: (Any -> Boolean) -> String -> Maybe a

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.

Nothing
Nothing
Nothing
Just ([1, 2, 3])

RegExp

regex :: RegexFlags -> String -> RegExp

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

/:\d+:/g

regexEscape :: String -> String

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

Properties:

"\\-=\\*\\{XYZ\\}\\*=\\-"

test :: RegExp -> String -> Boolean

Takes a pattern and a string, and returns true iff the pattern matches the string.

true
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:

See also matchAll.

Just ({"groups": [Just ("good")], "match": "goodbye"})
Just ({"groups": [Nothing], "match": "bye"})

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.

See also match.

[]
[{"groups": [Just ("foo")], "match": "@foo"}, {"groups": [Just ("bar")], "match": "@bar"}, {"groups": [Just ("baz")], "match": "@baz"}]

String

toUpper :: String -> String

Returns the upper-case equivalent of its argument.

See also toLower.

"ABC DEF 123"

toLower :: String -> String

Returns the lower-case equivalent of its argument.

See also toUpper.

"abc def 123"

trim :: String -> String

Strips leading and trailing whitespace characters.

"foo bar"

stripPrefix :: String -> String -> Maybe String

Returns Just the portion of the given string (the second argument) left after removing the given prefix (the first argument) if the string starts with the prefix; Nothing otherwise.

See also stripSuffix.

Just ("sanctuary.js.org")
Nothing

stripSuffix :: String -> String -> Maybe String

Returns Just the portion of the given string (the second argument) left after removing the given suffix (the first argument) if the string ends with the suffix; Nothing otherwise.

See also stripPrefix.

Just ("README")
Nothing

words :: String -> Array String

Takes a string and returns the array of words the string contains (words are delimited by whitespace characters).

See also unwords.

["foo", "bar", "baz"]

unwords :: Array String -> String

Takes an array of words and returns the result of joining the words with separating spaces.

See also words.

"foo bar baz"

lines :: String -> Array String

Takes a string and returns the array of lines the string contains (lines are delimited by newlines: '\n' or '\r\n' or '\r'). The resulting strings do not contain newlines.

See also unlines.

["foo", "bar", "baz"]

unlines :: Array String -> String

Takes an array of lines and returns the result of joining the lines after appending a terminating line feed ('\n') to each.

See also lines.

"foo\nbar\nbaz\n"

splitOn :: String -> String -> Array String

Returns the substrings of its second argument separated by occurrences of its first argument.

See also joinWith and splitOnRegex.

["foo", "bar", "baz"]

splitOnRegex :: GlobalRegExp -> String -> Array String

Takes a pattern and a string, and returns the result of splitting the string at every non-overlapping occurrence of the pattern.

Properties:

See also splitOn.

["foo", "bar", "baz"]
["foo", "bar", "baz"]