A monad library in TypeScript
Use monads in your code for functional programming joy.
Catling is influenced by the Scala standard library and Cats
Install using yarn or npm
yarn:
yarn add catling
npm:
npm install catling --save
An option is a great way to represent a value which may or may not exist. An option is either a Some or a None.
import { Option } from 'catling'
const name: Option<string> = getParameter(data, 'name')
const upper = name
.map(trim)
.filter(n => n.length !== 0)
.map(n => n.toUpperCase())
console.log(upper) // either "Some("JIMMY")" or "None"Applies the function given to the value if it's a Some, otherwise return None.
const amount = Option(10).map(x => x * 2)
// Some(20)Applies the function that maps the value in the Option to a new Option.
const age = Option(people[0])
.flatMap(person => Option(person.age))
// Some(20) or NoneReturns the Option if the predicate returns true, otherwise return a None.
const age1 = Option(16).filter(a => a > 18) // None
const age2 = Option(21).filter(a => a > 18) // Some(21)Applies the first function if the Option is a None, else applies the second function.
const name = Option('Jimmy').fold(
() => 'NONAME',
n => n.toUpperCase()
)
// 'JIMMY'Returns the value of the Option if it's a Some, otherwise return undefined
const name = Option('Jimmy').get()
// 'Jimmy'Returns the value of the Option if it's a Some, otherwise return the alternative.
const name = Option(undefined).getOrElse('Bob')
// 'Bob'Returns an array containing the inner value if it's a Some, or an empty array if it's None.
None().toArray() // []
Some('Bob').toArray() // ['Bob']Returns a List containing the inner value if it's a Some, or an empty List if it's None.
None().toList() // List()
Some('Bob').toList() // List('Bob')An either represents a value consisting of one of two possible types. It's typically used to represent the result of something that may fail. Eithers are right-biased.
import { Either, Left, Right } from 'catling'
function divide(divisor: number, n: number): Either<string, number> {
if (divisor === 0) {
return Left('Cannot divide by zero')
} else {
return Right(n / divisor)
}
}
const myNum1 = divide(5, 10)
.map(n => n + 20)
console.log(myNum1) // "Right(22)"
const myNum2 = divide(0, 10)
.map(n => n + 20)
console.log(myNum2) // "Left("Cannot divide by zero")"Returns an array containing the inner value if it's a Right, or an empty array if it's Left.
Left('fail').toArray() // []
Right('success').toArray() // ['success']Returns a List containing the inner value if it's a Right, or an empty List if it's Left.
Left('fail').toList() // List()
Right('success').toList() // List('success')An immutable list behaves much like the native array, expect it cannot be mutated.
import { List } from 'catling'
const result = List(1, 2, 4, 5)
.map(double)
.filter(greaterThan3)
.fold(0, add)
console.log(result) // 22A writer is a context that carries with it some sort of log with its computation.
import { Writer, List } from 'catling'
const myWriter = Writer(List('initial value'), 10)
.flatMap(val => Writer(List('adding 5'), val + 5))
.flatMap(val => Writer(List('doubling'), val * 2))
console.log(myWriter) // Writer(List(initial value, adding 5, doubling), 30)The log part of the writer must be a semigroup according to the fantasy-land spec, meaning it must have a concat method. This is used to combine the logs from the source writer.