Need help with functional-programming-jargon?

Click the “chat” button below for chat support from the developer who created it, or find similar developers for support.

Jargon from the functional programming world in simple terms!

13.8K Stars 810 Forks MIT License 264 Commits 24 Opened issues

Readme

Functional programming (FP) provides many advantages, and its popularity has been increasing as a result. However, each programming paradigm comes with its own unique jargon and FP is no exception. By providing a glossary, we hope to make learning FP easier.

Examples are presented in JavaScript (ES2015). Why JavaScript?

Where applicable, this document uses terms defined in the Fantasy Land spec

**Translations**
* Portuguese
* Spanish
* Chinese
* Bahasa Indonesia
* Python World
* Scala World
* Rust World
* Korean
* Haskell Turkish

**Table of Contents**
<!-- RM(noparent,notop) -->

- Arity
- Higher-Order Functions (HOF)
- Closure
- Partial Application
- Currying
- Auto Currying
- Function Composition
- Continuation
- Purity
- Side effects
- Idempotent
- Point-Free Style
- Predicate
- Contracts
- Category
- Value
- Constant
- Functor
- Pointed Functor
- Lift
- Referential Transparency
- Equational Reasoning
- Lambda
- Lambda Calculus
- Lazy evaluation
- Monoid
- Monad
- Comonad
- Applicative Functor
- Morphism
- Setoid
- Semigroup
- Foldable
- Lens
- Type Signatures
- Algebraic data type
- Option
- Function
- Partial function
- Functional Programming Libraries in JavaScript

The number of arguments a function takes. From words like unary, binary, ternary, etc. This word has the distinction of being composed of two suffixes, "-ary" and "-ity." Addition, for example, takes two arguments, and so it is defined as a binary function or a function with an arity of two. Such a function may sometimes be called "dyadic" by people who prefer Greek roots to Latin. Likewise, a function that takes a variable number of arguments is called "variadic," whereas a binary function must be given two and only two arguments, currying and partial application notwithstanding (see below).

const sum = (a, b) => a + bconst arity = sum.length console.log(arity) // 2

// The arity of sum is 2

A function which takes a function as an argument and/or returns a function.

const filter = (predicate, xs) => xs.filter(predicate)

const is = (type) => (x) => Object(x) instanceof type

filter(is(Number), [0, '1', 2, null]) // [0, 2]

A closure is a way of accessing a variable outside its scope. Formally, a closure is a technique for implementing lexically scoped named binding. It is a way of storing a function with an environment.

A closure is a scope which captures local variables of a function for access even after the execution has moved out of the block in which it is defined. ie. they allow referencing a scope after the block in which the variables were declared has finished executing.

const addTo = x => y => x + y; var addToFive = addTo(5); addToFive(3); //returns 8

The function

addTo()returns a function(internally called

add()), lets store it in a variable called

addToFivewith a curried call having parameter 5.

Ideally, when the function

addTofinishes execution, its scope, with local variables add, x, y should not be accessible. But, it returns 8 on calling

addToFive(). This means that the state of the function

addTois saved even after the block of code has finished executing, otherwise there is no way of knowing that

addTowas called as

addTo(5)and the value of x was set to 5.

Lexical scoping is the reason why it is able to find the values of x and add - the private variables of the parent which has finished executing. This value is called a Closure.

The stack along with the lexical scope of the function is stored in form of reference to the parent. This prevents the closure and the underlying variables from being garbage collected(since there is at least one live reference to it).

Lambda Vs Closure: A lambda is essentially a function that is defined inline rather than the standard method of declaring functions. Lambdas can frequently be passed around as objects.

A closure is a function that encloses its surrounding state by referencing fields external to its body. The enclosed state remains across invocations of the closure.

**Further reading/Sources**
* Lambda Vs Closure
* JavaScript Closures highly voted discussion

Partially applying a function means creating a new function by pre-filling some of the arguments to the original function.

// Helper to create partially applied functions // Takes a function and some arguments const partial = (f, ...args) => // returns a function that takes the rest of the arguments (...moreArgs) => // and calls the original function with all of them f(...args, ...moreArgs)// Something to apply const add3 = (a, b, c) => a + b + c

// Partially applying

`2`

and`3`

to`add3`

gives you a one-argument function const fivePlus = partial(add3, 2, 3) // (c) => 2 + 3 + cfivePlus(4) // 9

You can also use

Function.prototype.bindto partially apply a function in JS:

const add1More = add3.bind(null, 2, 3) // (c) => 2 + 3 + c

Partial application helps create simpler functions from more complex ones by baking in data when you have it. Curried functions are automatically partially applied.

The process of converting a function that takes multiple arguments into a function that takes them one at a time.

Each time the function is called it only accepts one argument and returns a function that takes one argument until all arguments are passed.

const sum = (a, b) => a + bconst curriedSum = (a) => (b) => a + b

curriedSum(40)(2) // 42.

const add2 = curriedSum(2) // (b) => 2 + b

add2(10) // 12

Transforming a function that takes multiple arguments into one that if given less than its correct number of arguments returns a function that takes the rest. When the function gets the correct number of arguments it is then evaluated.

lodash & Ramda have a

curryfunction that works this way.

const add = (x, y) => x + yconst curriedAdd = _.curry(add) curriedAdd(1, 2) // 3 curriedAdd(1) // (y) => 1 + y curriedAdd(1)(2) // 3

**Further reading**
* Favoring Curry
* Hey Underscore, You're Doing It Wrong!

The act of putting two functions together to form a third function where the output of one function is the input of the other.

const compose = (f, g) => (a) => f(g(a)) // Definition const floorAndToString = compose((val) => val.toString(), Math.floor) // Usage floorAndToString(121.212121) // '121'

At any given point in a program, the part of the code that's yet to be executed is known as a continuation.

const printAsString = (num) => console.log(`Given ${num}`)const addOneAndContinue = (num, cc) => { const result = num + 1 cc(result) }

addOneAndContinue(2, printAsString) // 'Given 3'

Continuations are often seen in asynchronous programming when the program needs to wait to receive data before it can continue. The response is often passed off to the rest of the program, which is the continuation, once it's been received.

const continueProgramWith = (data) => { // Continues program with data }readFileAsync('path/to/file', (err, response) => { if (err) { // handle error return } continueProgramWith(response) })

A function is pure if the return value is only determined by its input values, and does not produce side effects.

const greet = (name) => `Hi, ${name}`greet('Brianne') // 'Hi, Brianne'

As opposed to each of the following:

window.name = 'Brianne'const greet = () =>

`Hi, ${window.name}`

greet() // "Hi, Brianne"

The above example's output is based on data stored outside of the function...

let greetingconst greet = (name) => { greeting =

`Hi, ${name}`

}greet('Brianne') greeting // "Hi, Brianne"

... and this one modifies state outside of the function.

A function or expression is said to have a side effect if apart from returning a value, it interacts with (reads from or writes to) external mutable state.

const differentEveryTime = new Date()

console.log('IO is a side effect!')

A function is idempotent if reapplying it to its result does not produce a different result.

f(f(x)) ≍ f(x)

Math.abs(Math.abs(10))

sort(sort(sort([2, 1])))

Writing functions where the definition does not explicitly identify the arguments used. This style usually requires currying or other Higher-Order functions. A.K.A Tacit programming.

// Given const map = (fn) => (list) => list.map(fn) const add = (a) => (b) => a + b// Then

// Not points-free -

`numbers`

is an explicit argument const incrementAll = (numbers) => map(add(1))(numbers)// Points-free - The list is an implicit argument const incrementAll2 = map(add(1))

incrementAllidentifies and uses the parameter

numbers, so it is not points-free.

incrementAll2is written just by combining functions and values, making no mention of its arguments. It

Points-free function definitions look just like normal assignments without

functionor

=>.

A predicate is a function that returns true or false for a given value. A common use of a predicate is as the callback for array filter.

const predicate = (a) => a > 2;[1, 2, 3, 4].filter(predicate) // [3, 4]

A contract specifies the obligations and guarantees of the behavior from a function or expression at runtime. This acts as a set of rules that are expected from the input and output of a function or expression, and errors are generally reported whenever a contract is violated.

// Define our contract : int -> boolean const contract = (input) => { if (typeof input === 'number') return true throw new Error('Contract violated: expected int -> boolean') }const addOne = (num) => contract(num) && num + 1

addOne(2) // 3 addOne('some string') // Contract violated: expected int -> boolean

A category in category theory is a collection of objects and morphisms between them. In programming, typically types act as the objects and functions as morphisms.

To be a valid category 3 rules must be met:

- There must be an identity morphism that maps an object to itself.
Where
a

is an object in some category, there must be a function froma -> a

. - Morphisms must compose.
Where
a

,b

, andc

are objects in some category, andf

is a morphism froma -> b

, andg

is a morphism fromb -> c

;g(f(x))

must be equivalent to(g • f)(x)

. - Composition must be associative
f • (g • h)

is the same as(f • g) • h

Since these rules govern composition at very abstract level, category theory is great at uncovering new ways of composing things.

**Further reading**

Anything that can be assigned to a variable.

5 Object.freeze({name: 'John', age: 30}) // The `freeze` function enforces immutability. ;(a) => a ;[1] undefined

A variable that cannot be reassigned once defined.

const five = 5 const john = Object.freeze({name: 'John', age: 30})

Constants are referentially transparent. That is, they can be replaced with the values that they represent without affecting the result.

With the above two constants the following expression will always return

true.

john.age + five === ({name: 'John', age: 30}).age + (5)

An object that implements a

mapfunction which, while running over each value in the object to produce a new object, adheres to two rules:

**Preserves identity**

object.map(x => x) ≍ object

**Composable**

object.map(compose(f, g)) ≍ object.map(g).map(f)

(

f,

gare arbitrary functions)

A common functor in JavaScript is

Arraysince it abides to the two functor rules:

;[1, 2, 3].map(x => x) // = [1, 2, 3]

and

const f = x => x + 1 const g = x => x * 2;[1, 2, 3].map(x => f(g(x))) // = [3, 5, 7] ;[1, 2, 3].map(g).map(f) // = [3, 5, 7]

An object with an

offunction that puts

ES2015 adds

Array.ofmaking arrays a pointed functor.

Array.of(1) // [1]

Lifting is when you take a value and put it into an object like a functor. If you lift a function into an Applicative Functor then you can make it work on values that are also in that functor.

Some implementations have a function called

lift, or

liftA2to make it easier to run functions on functors.

const liftA2 = (f) => (a, b) => a.map(f).ap(b) // note it's `ap` and not `map`.const mult = a => b => a * b

const liftedMult = liftA2(mult) // this function now works on functors like array

liftedMult([1, 2], [3]) // [3, 6] liftA2(a => b => a + b)([1, 2], [3, 4]) // [4, 5, 5, 6]

Lifting a one-argument function and applying it does the same thing as

map.

const increment = (x) => x + 1lift(increment)([2]) // [3] ;[2].map(increment) // [3]

An expression that can be replaced with its value without changing the behavior of the program is said to be referentially transparent.

Say we have function greet:

const greet = () => 'Hello World!'

Any invocation of

greet()can be replaced with

Hello World!hence greet is referentially transparent.

When an application is composed of expressions and devoid of side effects, truths about the system can be derived from the parts.

An anonymous function that can be treated like a value.

;(function (a) { return a + 1 });(a) => a + 1

Lambdas are often passed as arguments to Higher-Order functions.

;[1, 2].map((a) => a + 1) // [2, 3]

You can assign a lambda to a variable.

const add1 = (a) => a + 1

A branch of mathematics that uses functions to create a universal model of computation.

Lazy evaluation is a call-by-need evaluation mechanism that delays the evaluation of an expression until its value is needed. In functional languages, this allows for structures like infinite lists, which would not normally be available in an imperative language where the sequencing of commands is significant.

const rand = function*() { while (1 < 2) { yield Math.random() } }

const randIter = rand() randIter.next() // Each execution gives a random value, expression is evaluated on need.

An object with a function that "combines" that object with another of the same type (semigroup) which has an "identity" value.

One simple monoid is the addition of numbers:

1 + 1 // 2

In this case number is the object and

+is the function.

When any value is combined with the "identity" value the result must be the original value. The identity must also be commutative.

The identity value for addition is

0.

js 1 + 0 // 1 0 + 1 // 1 1 + 0 === 0 + 1

It's also required that the grouping of operations will not affect the result (associativity):

1 + (2 + 3) === (1 + 2) + 3 // true

Array concatenation also forms a monoid:

;[1, 2].concat([3, 4]) // [1, 2, 3, 4]

The identity value is empty array

[]

;[1, 2].concat([]) // [1, 2]

As a counterexample, subtraction does not form a monoid because there is no commutative identity value:

0 - 4 === 4 - 0 // false

ofand

chainfunctions.

chainis like

mapexcept it un-nests the resulting nested object.

// Implementation Array.prototype.chain = function (f) { return this.reduce((acc, it) => acc.concat(f(it)), []) }// Usage Array.of('cat,dog', 'fish,bird').chain((a) => a.split(',')) // ['cat', 'dog', 'fish', 'bird']

// Contrast to map Array.of('cat,dog', 'fish,bird').map((a) => a.split(',')) // [['cat', 'dog'], ['fish', 'bird']]

ofis also known as

returnin other functional languages.

chainis also known as

flatmapand

bindin other languages.

An object that has

extractand

extendfunctions.

const CoIdentity = (v) => ({ val: v, extract () { return this.val }, extend (f) { return CoIdentity(f(this)) } })

Extract takes a value out of a functor.

CoIdentity(1).extract() // 1

Extend runs a function on the comonad. The function should return the same type as the comonad.

CoIdentity(1).extend((co) => co.extract() + 1) // CoIdentity(2)

An applicative functor is an object with an

apfunction.

apapplies a function in the object to a value in another object of the same type.

// Implementation Array.prototype.ap = function (xs) { return this.reduce((acc, f) => acc.concat(xs.map(f)), []) }// Example usage ;[(a) => a + 1].ap([1]) // [2]

This is useful if you have two objects and you want to apply a binary function to their contents.

// Arrays that you want to combine const arg1 = [1, 3] const arg2 = [4, 5]// combining function - must be curried for this to work const add = (x) => (y) => x + y

const partiallyAppliedAdds = [add].ap(arg1) // [(y) => 1 + y, (y) => 3 + y]

This gives you an array of functions that you can call

apon to get the result:

partiallyAppliedAdds.ap(arg2) // [5, 6, 7, 8]

A transformation function.

A function where the input type is the same as the output.

// uppercase :: String -> String const uppercase = (str) => str.toUpperCase()// decrement :: Number -> Number const decrement = (x) => x - 1

A pair of transformations between 2 types of objects that is structural in nature and no data is lost.

For example, 2D coordinates could be stored as an array

[2,3]or object

{x: 2, y: 3}.

// Providing functions to convert in both directions makes them isomorphic. const pairToCoords = (pair) => ({x: pair[0], y: pair[1]})const coordsToPair = (coords) => [coords.x, coords.y]

coordsToPair(pairToCoords([1, 2])) // [1, 2]

pairToCoords(coordsToPair({x: 1, y: 2})) // {x: 1, y: 2}

A homomorphism is just a structure preserving map. In fact, a functor is just a homomorphism between categories as it preserves the original category's structure under the mapping.

A.of(f).ap(A.of(x)) == A.of(f(x))Either.of(

.toUpper).ap(Either.of("oreos")) == Either.of(.toUpper("oreos"))

A

reduceRightfunction that applies a function against an accumulator and each value of the array (from right-to-left) to reduce it to a single value.

const sum = xs => xs.reduceRight((acc, x) => acc + x, 0)sum([1, 2, 3, 4, 5]) // 15

An

unfoldfunction. An

unfoldis the opposite of

fold(

reduce). It generates a list from a single value.

const unfold = (f, seed) => { function go(f, seed, acc) { const res = f(seed); return res ? go(f, res[1], acc.concat([res[0]])) : acc; } return go(f, seed, []) }

const countDown = n => unfold((n) => { return n <= 0 ? undefined : [n, n - 1] }, n)countDown(5) // [5, 4, 3, 2, 1]

The combination of anamorphism and catamorphism.

A function just like

reduceRight. However, there's a difference:

In paramorphism, your reducer's arguments are the current value, the reduction of all previous values, and the list of values that formed that reduction.

// Obviously not safe for lists containing `undefined`, // but good enough to make the point. const para = (reducer, accumulator, elements) => { if (elements.length === 0) return accumulatorconst head = elements[0] const tail = elements.slice(1)

return reducer(head, tail, para(reducer, accumulator, tail)) }

const suffixes = list => para( (x, xs, suffxs) => [xs, ... suffxs], [], list )

suffixes([1, 2, 3, 4, 5]) // [[2, 3, 4, 5], [3, 4, 5], [4, 5], [5], []]

The third parameter in the reducer (in the above example,

[x, ... xs]) is kind of like having a history of what got you to your current acc value.

it's the opposite of paramorphism, just as anamorphism is the opposite of catamorphism. Whereas with paramorphism, you combine with access to the accumulator and what has been accumulated, apomorphism lets you

unfoldwith the potential to return early.

An object that has an

equalsfunction which can be used to compare other objects of the same type.

Make array a setoid:

Array.prototype.equals = function (arr) { const len = this.length if (len !== arr.length) { return false } for (let i = 0; i < len; i++) { if (this[i] !== arr[i]) { return false } } return true };[1, 2].equals([1, 2]) // true ;[1, 2].equals([0]) // false

An object that has a

concatfunction that combines it with another object of the same type.

;[1].concat([2]) // [1, 2]

An object that has a

reducefunction that applies a function against an accumulator and each element in the array (from left to right) to reduce it to a single value.

const sum = (list) => list.reduce((acc, val) => acc + val, 0) sum([1, 2, 3]) // 6

A lens is a structure (often an object or function) that pairs a getter and a non-mutating setter for some other data structure.

// Using [Ramda's lens](http://ramdajs.com/docs/#lens) const nameLens = R.lens( // getter for name property on an object (obj) => obj.name, // setter for name property (val, obj) => Object.assign({}, obj, {name: val}) )

Having the pair of get and set for a given data structure enables a few key features.

const person = {name: 'Gertrude Blanch'}// invoke the getter R.view(nameLens, person) // 'Gertrude Blanch'

// invoke the setter R.set(nameLens, 'Shafi Goldwasser', person) // {name: 'Shafi Goldwasser'}

// run a function on the value in the structure R.over(nameLens, uppercase, person) // {name: 'GERTRUDE BLANCH'}

Lenses are also composable. This allows easy immutable updates to deeply nested data.

// This lens focuses on the first item in a non-empty array const firstLens = R.lens( // get first item in array xs => xs[0], // non-mutating setter for first item in array (val, [__, ...xs]) => [val, ...xs] )const people = [{name: 'Gertrude Blanch'}, {name: 'Shafi Goldwasser'}]

// Despite what you may assume, lenses compose left-to-right. R.over(compose(firstLens, nameLens), uppercase, people) // [{'name': 'GERTRUDE BLANCH'}, {'name': 'Shafi Goldwasser'}]

Other implementations: * partial.lenses - Tasty syntax sugar and a lot of powerful features * nanoscope - Fluent-interface

Often functions in JavaScript will include comments that indicate the types of their arguments and return values.

There's quite a bit of variance across the community but they often follow the following patterns:

// functionName :: firstArgType -> secondArgType -> returnType// add :: Number -> Number -> Number const add = (x) => (y) => x + y

// increment :: Number -> Number const increment = (x) => x + 1

If a function accepts another function as an argument it is wrapped in parentheses.

// call :: (a -> b) -> a -> b const call = (f) => (x) => f(x)

The letters

a,

b,

c,

dare used to signify that the argument can be of any type. The following version of

maptakes a function that transforms a value of some type

ainto another type

b, an array of values of type

a, and returns an array of values of type

b.

// map :: (a -> b) -> [a] -> [b] const map = (f) => (list) => list.map(f)

**Further reading**
* Ramda's type signatures
* Mostly Adequate Guide
* What is Hindley-Milner? on Stack Overflow

A composite type made from putting other types together. Two common classes of algebraic types are sum and product.

A Sum type is the combination of two types together into another one. It is called sum because the number of possible values in the result type is the sum of the input types.

JavaScript doesn't have types like this but we can use

Sets to pretend: ```js // imagine that rather than sets here we have types that can only have these values const bools = new Set([true, false]) const halfTrue = new Set(['half-true'])

// The weakLogic type contains the sum of the values from bools and halfTrue const weakLogicValues = new Set([...bools, ...halfTrue]) ```

Sum types are sometimes called union types, discriminated unions, or tagged unions.

There's a couple libraries in JS which help with defining and using union types.

Flow includes union types and TypeScript has Enums to serve the same role.

A **product** type combines types together in a way you're probably more familiar with:

// point :: (Number, Number) -> {x: Number, y: Number} const point = (x, y) => ({ x, y })

It's called a product because the total possible values of the data structure is the product of the different values. Many languages have a tuple type which is the simplest formulation of a product type.

See also Set theory.

Option is a sum type with two cases often called

Someand

None.

Option is useful for composing functions that might not return a value.

// Naive definitionconst Some = (v) => ({ val: v, map (f) { return Some(f(this.val)) }, chain (f) { return f(this.val) } })

const None = () => ({ map (f) { return this }, chain (f) { return this } })

// maybeProp :: (String, {a}) -> Option a const maybeProp = (key, obj) => typeof obj[key] === 'undefined' ? None() : Some(obj[key])

Use

chainto sequence functions that return

Options ```js

// getItem :: Cart -> Option CartItem const getItem = (cart) => maybeProp('item', cart)

// getPrice :: Item -> Option Number const getPrice = (item) => maybeProp('price', item)

// getNestedPrice :: cart -> Option a const getNestedPrice = (cart) => getItem(cart).chain(getPrice)

getNestedPrice({}) // None() getNestedPrice({item: {foo: 1}}) // None() getNestedPrice({item: {price: 9.99}}) // Some(9.99) ```

Optionis also known as

Maybe.

Someis sometimes called

Just.

Noneis sometimes called

Nothing.

A **function**

f :: A => Bis an expression - often called arrow or lambda expression - with

Aand

B. That value depends entirely on the argument, making functions context-independant, or referentially transparent. What is implied here is that a function must not produce any hidden side effects - a function is always pure, by definition. These properties make functions pleasant to work with: they are entirely deterministic and therefore predictable. Functions enable working with code as data, abstracting over behaviour:

// times2 :: Number -> Number const times2 = n => n * 2[1, 2, 3].map(times2) // [2, 4, 6]

A partial function is a function which is not defined for all arguments - it might return an unexpected result or may never terminate. Partial functions add cognitive overhead, they are harder to reason about and can lead to runtime errors. Some examples: ```js // example 1: sum of the list // sum :: [Number] -> Number const sum = arr => arr.reduce((a, b) => a + b) sum([1, 2, 3]) // 6 sum([]) // TypeError: Reduce of empty array with no initial value

// example 2: get the first item in list // first :: [A] -> A const first = a => a[0] first([42]) // 42 first([]) // undefined //or even worse: first([[42]])[0] // 42 first([])[0] // Uncaught TypeError: Cannot read property '0' of undefined

// example 3: repeat function N times // times :: Number -> (Number -> Number) -> Number const times = n => fn => n && (fn(n), times(n - 1)(fn)) times(3)(console.log) // 3 // 2 // 1 times(-1)(console.log) // RangeError: Maximum call stack size exceeded ```

Partial functions are dangerous as they need to be treated with great caution. You might get an unexpected (wrong) result or run into runtime errors. Sometimes a partial function might not return at all. Being aware of and treating all these edge cases accordingly can become very tedious. Fortunately a partial function can be converted to a regular (or total) one. We can provide default values or use guards to deal with inputs for which the (previously) partial function is undefined. Utilizing the

Optiontype, we can yield either

Some(value)or

Nonewhere we would otherwise have behaved unexpectedly: ```js // example 1: sum of the list // we can provide default value so it will always return result // sum :: [Number] -> Number const sum = arr => arr.reduce((a, b) => a + b, 0) sum([1, 2, 3]) // 6 sum([]) // 0

// example 2: get the first item in list // change result to Option // first :: [A] -> Option A const first = a => a.length ? Some(a[0]) : None() first([42]).map(a => console.log(a)) // 42 first([]).map(a => console.log(a)) // console.log won't execute at all //our previous worst case first([[42]]).map(a => console.log(a[0])) // 42 first([]).map(a => console.log(a[0])) // won't execte, so we won't have error here // more of that, you will know by function return type (Option) // that you should use

.mapmethod to access the data and you will never forget // to check your input because such check become built-in into the function

// example 3: repeat function N times // we should make function always terminate by changing conditions: // times :: Number -> (Number -> Number) -> Number const times = n => fn => n > 0 && (fn(n), times(n - 1)(fn)) times(3)(console.log) // 3 // 2 // 1 times(-1)(console.log) // won't execute anything ``` Making your partial functions total ones, these kinds of runtime errors can be prevented. Always returning a value will also make for code that is both easier to maintain as well as to reason about.

- mori
- Immutable
- Immer
- Ramda
- ramda-adjunct
- Folktale
- monet.js
- lodash
- Underscore.js
- Lazy.js
- maryamyriameliamurphies.js
- Haskell in ES6
- Sanctuary
- Crocks
- Fluture
- fp-ts

**P.S:** This repo is successful due to the wonderful contributions!