# ❑ Sanctuary [![npm](https://img.shields.io/npm/v/sanctuary.svg)](https://www.npmjs.com/package/sanctuary) [![CircleCI](https://img.shields.io/circleci/project/github/sanctuary-js/sanctuary/master.svg)](https://circleci.com/gh/sanctuary-js/sanctuary/tree/master) [![Gitter](https://img.shields.io/gitter/room/badges/shields.svg)](https://gitter.im/sanctuary-js/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 that may fail, such as [`head`](#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. ## ❑ Folktale [Folktale][], like Sanctuary, is a standard library for functional programming in JavaScript. It is well designed and well documented. Whereas Sanctuary treats JavaScript as a member of the ML language family, Folktale embraces JavaScript's object-oriented programming model. Programming with Folktale resembles programming with Scala. ## ❑ Ramda [Ramda][] provides several functions that 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 that 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][GIGO] principle. 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`: - `S.reduce (S.add) (0) (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`: - `R.reduce (R.add) (0) (xs)` - `R.reduce (R.add) (0, xs)` - `R.reduce (R.add, 0) (xs)` - `R.reduce (R.add, 0, 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.__`][], that removes the restriction that a function must be applied to its arguments in order. The following expressions are equivalent: - `R.reduce (R.__, 0, xs) (R.add)` - `R.reduce (R.add, R.__, xs) (0)` - `R.reduce (R.__, 0) (R.add) (xs)` - `R.reduce (R.__, 0) (R.add, xs)` - `R.reduce (R.__, R.__, xs) (R.add) (0)` - `R.reduce (R.__, R.__, xs) (R.add, 0)` ### ❑ Variadic functions Ramda provides several functions that take any number of arguments. These are known as [variadic functions][]. Additionally, Ramda provides several functions that 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 that 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 that 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 that 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 `() -> `, where `` 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`. Sanctuary embraces types. JavaScript doesn't support algebraic data types, but these can be simulated by providing a group of data constructors that 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 that 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: ```javascript S.add (2) (true); // ! TypeError: Invalid value // // add :: FiniteNumber -> FiniteNumber -> FiniteNumber // ^^^^^^^^^^^^ // 1 // // 1) true :: Boolean // // The value at position 1 is not a member of ‘FiniteNumber’. // // See https://github.com/sanctuary-js/sanctuary-def/tree/v0.22.0#FiniteNumber for information about the FiniteNumber type. ``` Compare this to the behaviour of Ramda's unchecked equivalent: ```javascript R.add (2) (true); // => 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`](#create) to create a Sanctuary module based on a different rule. For example: ```javascript const S = sanctuary.create ({ checkTypes: localStorage.getItem ('SANCTUARY_CHECK_TYPES') === 'true', env: sanctuary.env, }); ``` Occasionally one may wish to perform an operation that is not type safe, such as mapping over an object with heterogeneous values. This is possible via selective use of [`unchecked`](#unchecked) functions. ## ❑ Installation `npm install sanctuary` will install Sanctuary for use in Node.js. To add Sanctuary to a website, add the following ` ``` Optionally, define aliases for various modules: ```javascript 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`](#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 that is aware of that type: ```javascript 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$prototype); identity.value = x; return identity; }; // identityTypeIdent :: String const identityTypeIdent = 'my-package/Identity@1'; const Identity$prototype = { '@@type': identityTypeIdent, '@@show': function() { return `Identity (${S.show (this.value)})`; }, 'fantasy-land/map': function(f) { return Identity (f (this.value)); }, }; // IdentityType :: Type -> Type const IdentityType = $.UnaryType ('Identity') ('http://example.com/my-package#Identity') ([]) (x => type (x) === identityTypeIdent) (identity => [identity.value]); const S = create ({ checkTypes: process.env.NODE_ENV !== 'production', env: env.concat ([IdentityType ($.Unknown)]), }); S.map (S.sub (1)) (Identity (43)); // => Identity (42) ``` See also [`env`](#env). #### `env :: Array Type` The Sanctuary module's environment (`(S.create ({checkTypes, env})).env` is a reference to `env`). Useful in conjunction with [`create`](#create). ```javascript > S.env [ $.AnyFunction, . $.Arguments, . $.Array ($.Unknown), . $.Array2 ($.Unknown) ($.Unknown), . $.Boolean, . $.Buffer, . $.Date, . $.Descending ($.Unknown), . $.Either ($.Unknown) ($.Unknown), . $.Error, . $.Fn ($.Unknown) ($.Unknown), . $.HtmlElement, . $.Identity ($.Unknown), . $.JsMap ($.Unknown) ($.Unknown), . $.JsSet ($.Unknown), . $.Maybe ($.Unknown), . $.Module, . $.Null, . $.Number, . $.Object, . $.Pair ($.Unknown) ($.Unknown), . $.RegExp, . $.StrMap ($.Unknown), . $.String, . $.Symbol, . $.Type, . $.TypeClass, . $.Undefined ] ``` #### `unchecked :: Module` A complete Sanctuary module that performs no type checking. This is useful as it permits operations that Sanctuary's type checking would disallow, such as mapping over an object with heterogeneous values. See also [`create`](#create). ```javascript > S.unchecked.map (S.show) ({x: 'foo', y: true, z: 42}) {x: '"foo"', y: 'true', z: '42'} ``` Opting out of type checking may cause type errors to go unnoticed. ```javascript > S.unchecked.add (2) ('2') '22' ``` ### ❑ Classify #### `type :: Any -⁠> { namespace :: Maybe String, name :: String, version :: NonNegativeInteger }` Returns the result of parsing the [type identifier][] of the given value. ```javascript > S.type (S.Just (42)) {namespace: Just ('sanctuary-maybe'), name: 'Maybe', version: 1} > S.type ([1, 2, 3]) {namespace: Nothing, name: 'Array', version: 0} ``` #### `is :: Type -⁠> Any -⁠> Boolean` Returns `true` [iff][] the given value is a member of the specified type. See [`$.test`][] for details. ```javascript > S.is ($.Array ($.Integer)) ([1, 2, 3]) true > S.is ($.Array ($.Integer)) ([1, 2, 3.14]) false ``` ### ❑ Showable #### `show :: Any -⁠> String` Alias of [`show`][]. ```javascript > S.show (-0) '-0' > S.show (['foo', 'bar', 'baz']) '["foo", "bar", "baz"]' > S.show ({x: 1, y: 2, z: 3}) '{"x": 1, "y": 2, "z": 3}' > S.show (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`][] that requires two arguments of the same type. To compare values of different types first use [`create`](#create) to create a Sanctuary module with type checking disabled, then use that module's `equals` function. ```javascript > S.equals (0) (-0) true > S.equals (NaN) (NaN) true > S.equals (S.Just ([1, 2, 3])) (S.Just ([1, 2, 3])) true > S.equals (S.Just ([1, 2, 3])) (S.Just ([1, 2, 4])) false ``` #### `lt :: Ord a => a -⁠> a -⁠> Boolean` Returns `true` [iff][] the *second* argument is less than the first according to [`Z.lt`][]. ```javascript > S.filter (S.lt (3)) ([1, 2, 3, 4, 5]) [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`][]. ```javascript > S.filter (S.lte (3)) ([1, 2, 3, 4, 5]) [1, 2, 3] ``` #### `gt :: Ord a => a -⁠> a -⁠> Boolean` Returns `true` [iff][] the *second* argument is greater than the first according to [`Z.gt`][]. ```javascript > S.filter (S.gt (3)) ([1, 2, 3, 4, 5]) [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`][]. ```javascript > S.filter (S.gte (3)) ([1, 2, 3, 4, 5]) [3, 4, 5] ``` #### `min :: Ord a => a -⁠> a -⁠> a` Returns the smaller of its two arguments (according to [`Z.lte`][]). See also [`max`](#max). ```javascript > S.min (10) (2) 2 > S.min (new Date ('1999-12-31')) (new Date ('2000-01-01')) new Date ('1999-12-31') > S.min ('10') ('2') '10' ``` #### `max :: Ord a => a -⁠> a -⁠> a` Returns the larger of its two arguments (according to [`Z.lte`][]). See also [`min`](#min). ```javascript > S.max (10) (2) 10 > S.max (new Date ('1999-12-31')) (new Date ('2000-01-01')) new Date ('2000-01-01') > S.max ('10') ('2') '2' ``` #### `clamp :: Ord a => a -⁠> a -⁠> a -⁠> a` Takes a lower bound, an upper bound, and a value of the same type. Returns the value if it is within the bounds; the nearer bound otherwise. See also [`min`](#min) and [`max`](#max). ```javascript > S.clamp (0) (100) (42) 42 > S.clamp (0) (100) (-1) 0 > S.clamp ('A') ('Z') ('~') 'Z' ``` #### `id :: Category c => TypeRep c -⁠> c` [Type-safe][sanctuary-def] version of [`Z.id`][]. ```javascript > S.id (Function) (42) 42 ``` #### `concat :: Semigroup a => a -⁠> a -⁠> a` Curried version of [`Z.concat`][]. ```javascript > 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]) > S.concat (Sum (18)) (Sum (24)) Sum (42) ``` #### `empty :: Monoid a => TypeRep a -⁠> a` [Type-safe][sanctuary-def] version of [`Z.empty`][]. ```javascript > S.empty (String) '' > S.empty (Array) [] > S.empty (Object) {} > S.empty (Sum) Sum (0) ``` #### `invert :: Group g => g -⁠> g` [Type-safe][sanctuary-def] version of [`Z.invert`][]. ```javascript > S.invert (Sum (5)) Sum (-5) ``` #### `filter :: Filterable f => (a -⁠> Boolean) -⁠> f a -⁠> f a` Curried version of [`Z.filter`][]. Discards every element that does not satisfy the predicate. See also [`reject`](#reject). ```javascript > S.filter (S.odd) ([1, 2, 3]) [1, 3] > S.filter (S.odd) ({x: 1, y: 2, z: 3}) {x: 1, z: 3} > S.filter (S.odd) (S.Nothing) Nothing > S.filter (S.odd) (S.Just (0)) Nothing > S.filter (S.odd) (S.Just (1)) Just (1) ``` #### `reject :: Filterable f => (a -⁠> Boolean) -⁠> f a -⁠> f a` Curried version of [`Z.reject`][]. Discards every element that satisfies the predicate. See also [`filter`](#filter). ```javascript > S.reject (S.odd) ([1, 2, 3]) [2] > S.reject (S.odd) ({x: 1, y: 2, z: 3}) {y: 2} > S.reject (S.odd) (S.Nothing) Nothing > S.reject (S.odd) (S.Just (0)) Just (0) > S.reject (S.odd) (S.Just (1)) Nothing ``` #### `map :: Functor f => (a -⁠> b) -⁠> f a -⁠> f b` Curried version of [`Z.map`][]. ```javascript > 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) > S.map (Math.sqrt) (S.Pair (99980001) (99980001)) Pair (99980001) (9999) ``` Replacing `Functor f => f` with `Function x` produces the B combinator from combinatory logic (i.e. [`compose`](#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) ```javascript > S.map (Math.sqrt) (S.add (1)) (99) 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 ```javascript > S.flip (S.concat) ('!') ('foo') 'foo!' > S.flip ([Math.floor, Math.ceil]) (1.5) [1, 2] > S.flip ({floor: Math.floor, ceil: Math.ceil}) (1.5) {floor: 1, ceil: 2} > S.flip (Cons (Math.floor) (Cons (Math.ceil) (Nil))) (1.5) Cons (1) (Cons (2) (Nil)) ``` #### `bimap :: Bifunctor f => (a -⁠> b) -⁠> (c -⁠> d) -⁠> f a c -⁠> f b d` Curried version of [`Z.bimap`][]. ```javascript > S.bimap (S.toUpper) (Math.sqrt) (S.Pair ('foo') (64)) Pair ('FOO') (8) > S.bimap (S.toUpper) (Math.sqrt) (S.Left ('foo')) Left ('FOO') > S.bimap (S.toUpper) (Math.sqrt) (S.Right (64)) 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. ```javascript > S.mapLeft (S.toUpper) (S.Pair ('foo') (64)) Pair ('FOO') (64) > S.mapLeft (S.toUpper) (S.Left ('foo')) Left ('FOO') > S.mapLeft (S.toUpper) (S.Right (64)) Right (64) ``` #### `promap :: Profunctor p => (a -⁠> b) -⁠> (c -⁠> d) -⁠> p b c -⁠> p a d` Curried version of [`Z.promap`][]. ```javascript > S.promap (Math.abs) (S.add (1)) (Math.sqrt) (-100) 11 ``` #### `alt :: Alt f => f a -⁠> f a -⁠> f a` Curried version of [`Z.alt`][] with arguments flipped to facilitate partial application. ```javascript > S.alt (S.Just ('default')) (S.Nothing) Just ('default') > S.alt (S.Just ('default')) (S.Just ('hello')) Just ('hello') > S.alt (S.Right (0)) (S.Left ('X')) Right (0) > S.alt (S.Right (0)) (S.Right (1)) Right (1) ``` #### `zero :: Plus f => TypeRep f -⁠> f a` [Type-safe][sanctuary-def] version of [`Z.zero`][]. ```javascript > S.zero (Array) [] > S.zero (Object) {} > S.zero (S.Maybe) Nothing ``` #### `reduce :: Foldable f => (b -⁠> a -⁠> b) -⁠> b -⁠> f a -⁠> b` Takes a curried binary function, an initial value, and a [Foldable][], and applies the function to the initial value and the Foldable's first value, then applies the function to the result of the previous application and the Foldable's second value. Repeats this process until each of the Foldable's values has been used. Returns the initial value if the Foldable is empty; the result of the final application otherwise. See also [`reduce_`](#reduce_). ```javascript > S.reduce (S.add) (0) ([1, 2, 3, 4, 5]) 15 > S.reduce (xs => x => S.prepend (x) (xs)) ([]) ([1, 2, 3, 4, 5]) [5, 4, 3, 2, 1] ``` #### `reduce_ :: Foldable f => (a -⁠> b -⁠> b) -⁠> b -⁠> f a -⁠> b` Variant of [`reduce`](#reduce) that takes a reducing function with arguments flipped. ```javascript > S.reduce_ (S.append) ([]) (Cons (1) (Cons (2) (Cons (3) (Nil)))) [1, 2, 3] > S.reduce_ (S.prepend) ([]) (Cons (1) (Cons (2) (Cons (3) (Nil)))) [3, 2, 1] ``` #### `traverse :: (Applicative f, Traversable t) => TypeRep f -⁠> (a -⁠> f b) -⁠> t a -⁠> f (t b)` Curried version of [`Z.traverse`][]. ```javascript > S.traverse (Array) (S.words) (S.Just ('foo bar baz')) [Just ('foo'), Just ('bar'), Just ('baz')] > S.traverse (Array) (S.words) (S.Nothing) [Nothing] > S.traverse (S.Maybe) (S.parseInt (16)) (['A', 'B', 'C']) Just ([10, 11, 12]) > S.traverse (S.Maybe) (S.parseInt (16)) (['A', 'B', 'C', 'X']) Nothing > S.traverse (S.Maybe) (S.parseInt (16)) ({a: 'A', b: 'B', c: 'C'}) Just ({a: 10, b: 11, c: 12}) > S.traverse (S.Maybe) (S.parseInt (16)) ({a: 'A', b: 'B', c: 'C', x: 'X'}) Nothing ``` #### `sequence :: (Applicative f, Traversable t) => TypeRep f -⁠> t (f a) -⁠> f (t a)` Curried version of [`Z.sequence`][]. Inverts the given `t (f a)` to produce an `f (t a)`. ```javascript > S.sequence (Array) (S.Just ([1, 2, 3])) [Just (1), Just (2), Just (3)] > S.sequence (S.Maybe) ([S.Just (1), S.Just (2), S.Just (3)]) Just ([1, 2, 3]) > S.sequence (S.Maybe) ([S.Just (1), S.Just (2), S.Nothing]) Nothing > S.sequence (S.Maybe) ({a: S.Just (1), b: S.Just (2), c: S.Just (3)}) Just ({a: 1, b: 2, c: 3}) > S.sequence (S.Maybe) ({a: S.Just (1), b: S.Just (2), c: S.Nothing}) Nothing ``` #### `ap :: Apply f => f (a -⁠> b) -⁠> f a -⁠> f b` Curried version of [`Z.ap`][]. ```javascript > S.ap ([Math.sqrt, x => x * x]) ([1, 4, 9, 16, 25]) [1, 2, 3, 4, 5, 1, 16, 81, 256, 625] > S.ap ({x: Math.sqrt, y: S.add (1), z: S.sub (1)}) ({w: 4, x: 4, y: 4}) {x: 2, y: 5} > S.ap (S.Just (Math.sqrt)) (S.Just (64)) Just (8) ``` Replacing `Apply f => f` with `Function x` produces the S combinator from combinatory logic: Apply f => f (a -> b) -> f a -> f b Function x (a -> b) -> Function x a -> Function x b Function x (a -> c) -> Function x a -> Function x c Function x (b -> c) -> Function x b -> Function x c Function a (b -> c) -> Function a b -> Function a c (a -> b -> c) -> (a -> b) -> (a -> c) ```javascript > 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 that operates on two [Apply][]s. ```javascript > 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 that operates on three [Apply][]s. ```javascript > S.lift3 (S.reduce) (S.Just (S.add)) (S.Just (0)) (S.Just ([1, 2, 3])) Just (6) > S.lift3 (S.reduce) (S.Just (S.add)) (S.Just (0)) (S.Nothing) Nothing ``` #### `apFirst :: Apply f => f a -⁠> f b -⁠> f a` Curried version of [`Z.apFirst`][]. Combines two effectful actions, keeping only the result of the first. Equivalent to Haskell's `(<*)` function. See also [`apSecond`](#apSecond). ```javascript > S.apFirst ([1, 2]) ([3, 4]) [1, 1, 2, 2] > S.apFirst (S.Just (1)) (S.Just (2)) Just (1) ``` #### `apSecond :: Apply f => f a -⁠> f b -⁠> f b` Curried version of [`Z.apSecond`][]. Combines two effectful actions, keeping only the result of the second. Equivalent to Haskell's `(*>)` function. See also [`apFirst`](#apFirst). ```javascript > S.apSecond ([1, 2]) ([3, 4]) [3, 4, 3, 4] > S.apSecond (S.Just (1)) (S.Just (2)) Just (2) ``` #### `of :: Applicative f => TypeRep f -⁠> a -⁠> f a` Curried version of [`Z.of`][]. ```javascript > 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`][]. ```javascript > 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][sanctuary-def] version of [`Z.join`][]. Removes one level of nesting from a nested monadic structure. ```javascript > S.join ([[1], [2], [3]]) [1, 2, 3] > S.join ([[[1, 2, 3]]]) [[1, 2, 3]] > S.join (S.Just (S.Just (1))) Just (1) > S.join (S.Pair ('foo') (S.Pair ('bar') ('baz'))) 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) ```javascript > S.join (S.concat) ('abc') 'abcabc' ``` #### `chainRec :: ChainRec m => TypeRep m -⁠> (a -⁠> m (Either a b)) -⁠> a -⁠> m b` Performs a [`chain`](#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). ```javascript > 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`][]. ```javascript > S.extend (S.joinWith ('')) (['x', 'y', 'z']) ['xyz', 'yz', 'z'] > S.extend (f => f ([3, 4])) (S.reverse) ([1, 2]) [4, 3, 2, 1] ``` #### `duplicate :: Extend w => w a -⁠> w (w a)` [Type-safe][sanctuary-def] version of [`Z.duplicate`][]. Adds one level of nesting to a comonadic structure. ```javascript > S.duplicate (S.Just (1)) Just (Just (1)) > S.duplicate ([1]) [[1]] > S.duplicate ([1, 2, 3]) [[1, 2, 3], [2, 3], [3]] > S.duplicate (S.reverse) ([1, 2]) ([3, 4]) [4, 3, 2, 1] ``` #### `extract :: Comonad w => w a -⁠> a` [Type-safe][sanctuary-def] version of [`Z.extract`][]. ```javascript > S.extract (S.Pair ('foo') ('bar')) 'bar' ``` #### `contramap :: Contravariant f => (b -⁠> a) -⁠> f a -⁠> f b` [Type-safe][sanctuary-def] version of [`Z.contramap`][]. ```javascript > S.contramap (s => s.length) (Math.sqrt) ('Sanctuary') 3 ``` ### ❑ Combinator #### `I :: a -⁠> a` The I combinator. Returns its argument. Equivalent to Haskell's `id` function. ```javascript > S.I ('foo') 'foo' ``` #### `K :: a -⁠> b -⁠> a` The K combinator. Takes two values and returns the first. Equivalent to Haskell's `const` function. ```javascript > S.K ('foo') ('bar') 'foo' > S.map (S.K (42)) (S.range (0) (5)) [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. ```javascript > S.T (42) (S.add (1)) 43 > S.map (S.T (100)) ([S.add (1), Math.sqrt]) [101, 10] ``` ### ❑ Function #### `curry2 :: ((a, b) -⁠> c) -⁠> a -⁠> b -⁠> c` Curries the given binary function. ```javascript > 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. ```javascript > const replaceString = S.curry3 ((what, replacement, string) => . string.replace (what, replacement) . ) > replaceString ('banana') ('orange') ('banana icecream') 'orange icecream' ``` #### `curry4 :: ((a, b, c, d) -⁠> e) -⁠> a -⁠> b -⁠> c -⁠> d -⁠> e` Curries the given quaternary function. ```javascript > const createRect = S.curry4 ((x, y, width, height) => . ({x, y, width, height}) . ) > 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. ```javascript > const toUrl = S.curry5 ((protocol, creds, hostname, port, pathname) => . protocol + '//' + . S.maybe ('') (S.flip (S.concat) ('@')) (creds) + . hostname + . S.maybe ('') (S.concat (':')) (port) + . pathname . ) > toUrl ('https:') (S.Nothing) ('example.com') (S.Just ('443')) ('/foo/bar') '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`](#pipe). ```javascript > S.compose (Math.sqrt) (S.add (1)) (99) 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)))`. ```javascript > S.pipe ([S.add (1), Math.sqrt, S.sub (1)]) (99) 9 ``` #### `pipeK :: (Foldable f, Chain m) => f (Any -⁠> m Any) -⁠> m a -⁠> m b` Takes a sequence of functions assumed to be unary that 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)))`. ```javascript > S.pipeK ([S.tail, S.tail, S.head]) (S.Just ([1, 2, 3, 4])) 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. ```javascript > S.on (S.concat) (S.reverse) ([1, 2, 3]) ([4, 5, 6]) [3, 2, 1, 6, 5, 4] ``` ### ❑ Pair 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][]. #### `Pair :: a -⁠> b -⁠> Pair a b` Pair's sole data constructor. Additionally, it serves as the Pair [type representative][]. ```javascript > S.Pair ('foo') (42) Pair ('foo') (42) ``` #### `pair :: (a -⁠> b -⁠> c) -⁠> Pair a b -⁠> c` Case analysis for the `Pair a b` type. ```javascript > S.pair (S.concat) (S.Pair ('foo') ('bar')) 'foobar' ``` #### `fst :: Pair a b -⁠> a` `fst (Pair (x) (y))` is equivalent to `x`. ```javascript > S.fst (S.Pair ('foo') (42)) 'foo' ``` #### `snd :: Pair a b -⁠> b` `snd (Pair (x) (y))` is equivalent to `y`. ```javascript > S.snd (S.Pair ('foo') (42)) 42 ``` #### `swap :: Pair a b -⁠> Pair b a` `swap (Pair (x) (y))` is equivalent to `Pair (y) (x)`. ```javascript > S.swap (S.Pair ('foo') (42)) Pair (42) ('foo') ``` ### ❑ Maybe 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][]. #### `Maybe :: TypeRep Maybe` Maybe [type representative][]. #### `Nothing :: Maybe a` The empty value of type `Maybe a`. ```javascript > S.Nothing Nothing ``` #### `Just :: a -⁠> Maybe a` Constructs a value of type `Maybe a` from a value of type `a`. ```javascript > S.Just (42) Just (42) ``` #### `isNothing :: Maybe a -⁠> Boolean` Returns `true` if the given Maybe is Nothing; `false` if it is a Just. ```javascript > 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. ```javascript > S.isJust (S.Just (42)) true > S.isJust (S.Nothing) false ``` #### `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_`](#maybe_) and [`fromMaybe`](#fromMaybe). ```javascript > S.maybe (0) (S.prop ('length')) (S.Just ('refuge')) 6 > S.maybe (0) (S.prop ('length')) (S.Nothing) 0 ``` #### `maybe_ :: (() -⁠> b) -⁠> (a -⁠> b) -⁠> Maybe a -⁠> b` Variant of [`maybe`](#maybe) that takes a thunk so the default value is only computed if required. ```javascript > 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 ``` #### `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 [`maybe`](#maybe), [`fromMaybe_`](#fromMaybe_), and [`maybeToNullable`](#maybeToNullable). ```javascript > S.fromMaybe (0) (S.Just (42)) 42 > S.fromMaybe (0) (S.Nothing) 0 ``` #### `fromMaybe_ :: (() -⁠> a) -⁠> Maybe a -⁠> a` Variant of [`fromMaybe`](#fromMaybe) that takes a thunk so the default value is only computed if required. ```javascript > 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 ``` #### `justs :: (Filterable f, Functor f) => f (Maybe a) -⁠> f a` Discards each element that is Nothing, and unwraps each element that is a Just. Related to Haskell's `catMaybes` function. See also [`lefts`](#lefts) and [`rights`](#rights). ```javascript > S.justs ([S.Just ('foo'), S.Nothing, S.Just ('baz')]) ['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. ```javascript > S.mapMaybe (S.head) ([[], [1, 2, 3], [], [4, 5, 6], []]) [1, 4] > S.mapMaybe (S.head) ({x: [1, 2, 3], y: [], z: [4, 5, 6]}) {x: 1, z: 4} ``` #### `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`](#fromMaybe). ```javascript > S.maybeToNullable (S.Just (42)) 42 > S.maybeToNullable (S.Nothing) null ``` #### `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`](#eitherToMaybe). ```javascript > S.maybeToEither ('Expecting an integer') (S.parseInt (10) ('xyz')) Left ('Expecting an integer') > S.maybeToEither ('Expecting an integer') (S.parseInt (10) ('42')) Right (42) ``` ### ❑ Either 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][]. #### `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`. ```javascript > S.Left ('Cannot divide by zero') Left ('Cannot divide by zero') ``` #### `Right :: b -⁠> Either a b` Constructs a value of type `Either a b` from a value of type `b`. ```javascript > S.Right (42) Right (42) ``` #### `isLeft :: Either a b -⁠> Boolean` Returns `true` if the given Either is a Left; `false` if it is a Right. ```javascript > 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. ```javascript > S.isRight (S.Right (42)) true > S.isRight (S.Left ('Cannot divide by zero')) false ``` #### `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. See also [`fromLeft`](#fromLeft) and [`fromRight`](#fromRight). ```javascript > S.either (S.toUpper) (S.show) (S.Left ('Cannot divide by zero')) 'CANNOT DIVIDE BY ZERO' > S.either (S.toUpper) (S.show) (S.Right (42)) '42' ``` #### `fromLeft :: a -⁠> Either a b -⁠> a` Takes a default value and an Either, and returns the Left value if the Either is a Left; the default value otherwise. See also [`either`](#either) and [`fromRight`](#fromRight). ```javascript > S.fromLeft ('abc') (S.Left ('xyz')) 'xyz' > S.fromLeft ('abc') (S.Right (123)) 'abc' ``` #### `fromRight :: 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. See also [`either`](#either) and [`fromLeft`](#fromLeft). ```javascript > S.fromRight (123) (S.Right (789)) 789 > S.fromRight (123) (S.Left ('abc')) 123 ``` #### `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. The behaviour of `fromEither` is likely to change in a future release. Please use [`fromRight`](#fromRight) instead. ```javascript > S.fromEither (0) (S.Right (42)) 42 > S.fromEither (0) (S.Left (42)) 0 ``` #### `lefts :: (Filterable f, Functor f) => f (Either a b) -⁠> f a` Discards each element that is a Right, and unwraps each element that is a Left. See also [`rights`](#rights). ```javascript > S.lefts ([S.Right (20), S.Left ('foo'), S.Right (10), S.Left ('bar')]) ['foo', 'bar'] ``` #### `rights :: (Filterable f, Functor f) => f (Either a b) -⁠> f b` Discards each element that is a Left, and unwraps each element that is a Right. See also [`lefts`](#lefts). ```javascript > S.rights ([S.Right (20), S.Left ('foo'), S.Right (10), S.Left ('bar')]) [20, 10] ``` #### `tagBy :: (a -⁠> Boolean) -⁠> a -⁠> Either a a` Takes a predicate and a value, and returns a Right of the value if it satisfies the predicate; a Left of the value otherwise. ```javascript > S.tagBy (S.odd) (0) Left (0) > S.tagBy (S.odd) (1) Right (1) ``` #### `encase :: Throwing e a b -⁠> a -⁠> Either e b` Takes a function that may throw and returns a pure function. ```javascript > S.encase (JSON.parse) ('["foo","bar","baz"]') Right (['foo', 'bar', 'baz']) > S.encase (JSON.parse) ('[') Left (new SyntaxError ('Unexpected end of JSON input')) ``` #### `eitherToMaybe :: Either a b -⁠> Maybe b` Converts an Either to a Maybe. A Left becomes Nothing; a Right becomes a Just. See also [`maybeToEither`](#maybeToEither). ```javascript > S.eitherToMaybe (S.Left ('Cannot divide by zero')) Nothing > S.eitherToMaybe (S.Right (42)) Just (42) ``` ### ❑ Logic #### `and :: Boolean -⁠> Boolean -⁠> Boolean` Boolean "and". ```javascript > 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". ```javascript > 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". See also [`complement`](#complement). ```javascript > S.not (false) true > S.not (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`](#not). ```javascript > Number.isInteger (42) true > S.complement (Number.isInteger) (42) false ``` #### `boolean :: a -⁠> a -⁠> Boolean -⁠> a` Case analysis for the `Boolean` type. `boolean (x) (y) (b)` evaluates to `x` if `b` is `false`; to `y` if `b` is `true`. ```javascript > S.boolean ('no') ('yes') (false) 'no' > S.boolean ('no') ('yes') (true) 'yes' ``` #### `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`](#when) and [`unless`](#unless). ```javascript > S.ifElse (x => x < 0) (Math.abs) (Math.sqrt) (-1) 1 > S.ifElse (x => x < 0) (Math.abs) (Math.sqrt) (16) 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`](#unless) and [`ifElse`](#ifElse). ```javascript > S.when (x => x >= 0) (Math.sqrt) (16) 4 > S.when (x => x >= 0) (Math.sqrt) (-1) -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`](#when) and [`ifElse`](#ifElse). ```javascript > S.unless (x => x < 0) (Math.sqrt) (16) 4 > S.unless (x => x < 0) (Math.sqrt) (-1) -1 ``` ### ❑ Array #### `array :: b -⁠> (a -⁠> Array a -⁠> b) -⁠> Array a -⁠> b` Case analysis for the `Array a` type. ```javascript > S.array (S.Nothing) (head => tail => S.Just (head)) ([]) Nothing > S.array (S.Nothing) (head => tail => S.Just (head)) ([1, 2, 3]) Just (1) > S.array (S.Nothing) (head => tail => S.Just (tail)) ([]) Nothing > S.array (S.Nothing) (head => tail => S.Just (tail)) ([1, 2, 3]) Just ([2, 3]) ``` #### `head :: Foldable f => f a -⁠> Maybe a` Returns Just the first element of the given structure if the structure contains at least one element; Nothing otherwise. ```javascript > S.head ([1, 2, 3]) Just (1) > S.head ([]) Nothing > S.head (Cons (1) (Cons (2) (Cons (3) (Nil)))) Just (1) > S.head (Nil) Nothing ``` #### `last :: Foldable f => f a -⁠> Maybe a` Returns Just the last element of the given structure if the structure contains at least one element; Nothing otherwise. ```javascript > S.last ([1, 2, 3]) Just (3) > S.last ([]) Nothing > S.last (Cons (1) (Cons (2) (Cons (3) (Nil)))) Just (3) > S.last (Nil) Nothing ``` #### `tail :: (Applicative f, Foldable f, Monoid (f a)) => f a -⁠> Maybe (f a)` Returns Just all but the first of the given structure's elements if the structure contains at least one element; Nothing otherwise. ```javascript > S.tail ([1, 2, 3]) Just ([2, 3]) > S.tail ([]) Nothing > S.tail (Cons (1) (Cons (2) (Cons (3) (Nil)))) Just (Cons (2) (Cons (3) (Nil))) > S.tail (Nil) Nothing ``` #### `init :: (Applicative f, Foldable f, Monoid (f a)) => f a -⁠> Maybe (f a)` Returns Just all but the last of the given structure's elements if the structure contains at least one element; Nothing otherwise. ```javascript > S.init ([1, 2, 3]) Just ([1, 2]) > S.init ([]) Nothing > S.init (Cons (1) (Cons (2) (Cons (3) (Nil)))) Just (Cons (1) (Cons (2) (Nil))) > S.init (Nil) Nothing ``` #### `take :: (Applicative f, Foldable f, Monoid (f a)) => Integer -⁠> f a -⁠> Maybe (f a)` Returns Just the first N elements of the given structure if N is non-negative and less than or equal to the size of the structure; Nothing otherwise. ```javascript > S.take (0) (['foo', 'bar']) Just ([]) > S.take (1) (['foo', 'bar']) Just (['foo']) > S.take (2) (['foo', 'bar']) Just (['foo', 'bar']) > S.take (3) (['foo', 'bar']) Nothing > S.take (3) (Cons (1) (Cons (2) (Cons (3) (Cons (4) (Cons (5) (Nil)))))) Just (Cons (1) (Cons (2) (Cons (3) (Nil)))) ``` #### `drop :: (Applicative f, Foldable f, Monoid (f a)) => Integer -⁠> f a -⁠> Maybe (f a)` Returns Just all but the first N elements of the given structure if N is non-negative and less than or equal to the size of the structure; Nothing otherwise. ```javascript > S.drop (0) (['foo', 'bar']) Just (['foo', 'bar']) > S.drop (1) (['foo', 'bar']) Just (['bar']) > S.drop (2) (['foo', 'bar']) Just ([]) > S.drop (3) (['foo', 'bar']) Nothing > S.drop (3) (Cons (1) (Cons (2) (Cons (3) (Cons (4) (Cons (5) (Nil)))))) Just (Cons (4) (Cons (5) (Nil))) ``` #### `takeLast :: (Applicative f, Foldable f, Monoid (f a)) => Integer -⁠> f a -⁠> Maybe (f a)` Returns Just the last N elements of the given structure if N is non-negative and less than or equal to the size of the structure; Nothing otherwise. ```javascript > S.takeLast (0) (['foo', 'bar']) Just ([]) > S.takeLast (1) (['foo', 'bar']) Just (['bar']) > S.takeLast (2) (['foo', 'bar']) Just (['foo', 'bar']) > S.takeLast (3) (['foo', 'bar']) Nothing > S.takeLast (3) (Cons (1) (Cons (2) (Cons (3) (Cons (4) (Nil))))) Just (Cons (2) (Cons (3) (Cons (4) (Nil)))) ``` #### `dropLast :: (Applicative f, Foldable f, Monoid (f a)) => Integer -⁠> f a -⁠> Maybe (f a)` Returns Just all but the last N elements of the given structure if N is non-negative and less than or equal to the size of the structure; Nothing otherwise. ```javascript > S.dropLast (0) (['foo', 'bar']) Just (['foo', 'bar']) > S.dropLast (1) (['foo', 'bar']) Just (['foo']) > S.dropLast (2) (['foo', 'bar']) Just ([]) > S.dropLast (3) (['foo', 'bar']) Nothing > S.dropLast (3) (Cons (1) (Cons (2) (Cons (3) (Cons (4) (Nil))))) Just (Cons (1) (Nil)) ``` #### `takeWhile :: (a -⁠> Boolean) -⁠> Array a -⁠> Array a` Discards the first element that does not satisfy the predicate, and all subsequent elements. See also [`dropWhile`](#dropWhile). ```javascript > S.takeWhile (S.odd) ([3, 3, 3, 7, 6, 3, 5, 4]) [3, 3, 3, 7] > S.takeWhile (S.even) ([3, 3, 3, 7, 6, 3, 5, 4]) [] ``` #### `dropWhile :: (a -⁠> Boolean) -⁠> Array a -⁠> Array a` Retains the first element that does not satisfy the predicate, and all subsequent elements. See also [`takeWhile`](#takeWhile). ```javascript > S.dropWhile (S.odd) ([3, 3, 3, 7, 6, 3, 5, 4]) [6, 3, 5, 4] > S.dropWhile (S.even) ([3, 3, 3, 7, 6, 3, 5, 4]) [3, 3, 3, 7, 6, 3, 5, 4] ``` #### `size :: Foldable f => f a -⁠> NonNegativeInteger` Returns the number of elements of the given structure. ```javascript > S.size ([]) 0 > S.size (['foo', 'bar', 'baz']) 3 > S.size (Nil) 0 > S.size (Cons ('foo') (Cons ('bar') (Cons ('baz') (Nil)))) 3 > S.size (S.Nothing) 0 > S.size (S.Just ('quux')) 1 > S.size (S.Pair ('ignored!') ('counted!')) 1 ``` #### `all :: Foldable f => (a -⁠> Boolean) -⁠> f a -⁠> Boolean` Returns `true` [iff][] all the elements of the structure satisfy the predicate. See also [`any`](#any) and [`none`](#none). ```javascript > S.all (S.odd) ([]) true > S.all (S.odd) ([1, 3, 5]) true > S.all (S.odd) ([1, 2, 3]) false ``` #### `any :: Foldable f => (a -⁠> Boolean) -⁠> f a -⁠> Boolean` Returns `true` [iff][] any element of the structure satisfies the predicate. See also [`all`](#all) and [`none`](#none). ```javascript > S.any (S.odd) ([]) false > S.any (S.odd) ([2, 4, 6]) false > S.any (S.odd) ([1, 2, 3]) true ``` #### `none :: Foldable f => (a -⁠> Boolean) -⁠> f a -⁠> Boolean` Returns `true` [iff][] none of the elements of the structure satisfies the predicate. Properties: - `forall p :: a -> Boolean, xs :: Foldable f => f a. S.none (p) (xs) = S.not (S.any (p) (xs))` - `forall p :: a -> Boolean, xs :: Foldable f => f a. S.none (p) (xs) = S.all (S.complement (p)) (xs)` See also [`all`](#all) and [`any`](#any). ```javascript > S.none (S.odd) ([]) true > S.none (S.odd) ([2, 4, 6]) true > S.none (S.odd) ([1, 2, 3]) false ``` #### `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`](#prepend). ```javascript > S.append (3) ([1, 2]) [1, 2, 3] > S.append (3) (Cons (1) (Cons (2) (Nil))) Cons (1) (Cons (2) (Cons (3) (Nil))) > S.append ([1]) (S.Nothing) Just ([1]) > S.append ([3]) (S.Just ([1, 2])) 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`](#append). ```javascript > S.prepend (1) ([2, 3]) [1, 2, 3] > S.prepend (1) (Cons (2) (Cons (3) (Nil))) Cons (1) (Cons (2) (Cons (3) (Nil))) > S.prepend ([1]) (S.Nothing) Just ([1]) > S.prepend ([1]) (S.Just ([2, 3])) Just ([1, 2, 3]) ``` #### `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` See also [`splitOn`](#s

	Stars	Issues	Version	Updated	Created
folktale	2,034	30	2.3.2	5 years ago	9 years ago
immer			10.1.1	a month ago	8 years ago
immutable	32,872	122	4.3.6	20 days ago	11 years ago
lodash	59,055	85	4.17.21	3 years ago	12 years ago
ramda	23,621	149	0.30.0	a month ago	10 years ago
sanctuary	3,022	38	3.1.0	4 years ago	9 years ago

folktale vs immer vs immutable vs lodash vs ramda vs sanctuary

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