Ord overview

The Ord type class represents types which support comparisons with a total order.

Instances should satisfy the laws of total orderings:

  1. Reflexivity: S.compare(a, a) <= 0
  2. Antisymmetry: if S.compare(a, b) <= 0 and S.compare(b, a) <= 0 then a <-> b
  3. Transitivity: if S.compare(a, b) <= 0 and S.compare(b, c) <= 0 then S.compare(a, c) <= 0

Added in v2.0.0

Table of contents

constructors

fromCompare

Signature

export declare const fromCompare: <A>(compare: (first: A, second: A) => Ordering) => Ord<A>

Added in v2.0.0

defaults

equalsDefault

Signature

export declare const equalsDefault: <A>(compare: (first: A, second: A) => Ordering) => (x: A, y: A) => boolean

Added in v2.10.0

instances

Contravariant

Signature

export declare const Contravariant: Contravariant1<'Ord'>

Added in v2.7.0

getMonoid

Returns a Monoid such that:

  • its concat(ord1, ord2) operation will order first by ord1, and then by ord2
  • its empty value is an Ord that always considers compared elements equal

Signature

export declare const getMonoid: <A = never>() => Monoid<Ord<A>>

Example

import { sort } from 'fp-ts/Array'
import { contramap, reverse, getMonoid } from 'fp-ts/Ord'
import * as S from 'fp-ts/string'
import * as B from 'fp-ts/boolean'
import { pipe } from 'fp-ts/function'
import { concatAll } from 'fp-ts/Monoid'
import * as N from 'fp-ts/number'

interface User {
  readonly id: number
  readonly name: string
  readonly age: number
  readonly rememberMe: boolean
}

const byName = pipe(
  S.Ord,
  contramap((p: User) => p.name)
)

const byAge = pipe(
  N.Ord,
  contramap((p: User) => p.age)
)

const byRememberMe = pipe(
  B.Ord,
  contramap((p: User) => p.rememberMe)
)

const M = getMonoid<User>()

const users: Array<User> = [
  { id: 1, name: 'Guido', age: 47, rememberMe: false },
  { id: 2, name: 'Guido', age: 46, rememberMe: true },
  { id: 3, name: 'Giulio', age: 44, rememberMe: false },
  { id: 4, name: 'Giulio', age: 44, rememberMe: true },
]

// sort by name, then by age, then by `rememberMe`
const O1 = concatAll(M)([byName, byAge, byRememberMe])
assert.deepStrictEqual(sort(O1)(users), [
  { id: 3, name: 'Giulio', age: 44, rememberMe: false },
  { id: 4, name: 'Giulio', age: 44, rememberMe: true },
  { id: 2, name: 'Guido', age: 46, rememberMe: true },
  { id: 1, name: 'Guido', age: 47, rememberMe: false },
])

// now `rememberMe = true` first, then by name, then by age
const O2 = concatAll(M)([reverse(byRememberMe), byName, byAge])
assert.deepStrictEqual(sort(O2)(users), [
  { id: 4, name: 'Giulio', age: 44, rememberMe: true },
  { id: 2, name: 'Guido', age: 46, rememberMe: true },
  { id: 3, name: 'Giulio', age: 44, rememberMe: false },
  { id: 1, name: 'Guido', age: 47, rememberMe: false },
])

Added in v2.4.0

getSemigroup

A typical use case for the Semigroup instance of Ord is merging two or more orderings.

For example the following snippet builds an Ord for a type User which sorts by created date descending, and then lastName

Signature

export declare const getSemigroup: <A = never>() => Semigroup<Ord<A>>

Example

import * as D from 'fp-ts/Date'
import { pipe } from 'fp-ts/function'
import { contramap, getSemigroup, Ord, reverse } from 'fp-ts/Ord'
import * as RA from 'fp-ts/ReadonlyArray'
import * as S from 'fp-ts/string'

interface User {
  readonly id: string
  readonly lastName: string
  readonly created: Date
}

const ordByLastName: Ord<User> = pipe(
  S.Ord,
  contramap((user) => user.lastName)
)

const ordByCreated: Ord<User> = pipe(
  D.Ord,
  contramap((user) => user.created)
)

const ordUserByCreatedDescThenLastName = getSemigroup<User>().concat(reverse(ordByCreated), ordByLastName)

assert.deepStrictEqual(
  RA.sort(ordUserByCreatedDescThenLastName)([
    { id: 'c', lastName: 'd', created: new Date(1973, 10, 30) },
    { id: 'a', lastName: 'b', created: new Date(1973, 10, 30) },
    { id: 'e', lastName: 'f', created: new Date(1980, 10, 30) },
  ]),
  [
    { id: 'e', lastName: 'f', created: new Date(1980, 10, 30) },
    { id: 'a', lastName: 'b', created: new Date(1973, 10, 30) },
    { id: 'c', lastName: 'd', created: new Date(1973, 10, 30) },
  ]
)

Added in v2.0.0

model

Ord (interface)

Signature

export interface Ord<A> extends Eq<A> {
  readonly compare: (first: A, second: A) => Ordering
}

Added in v2.0.0

type lambdas

URI

Signature

export declare const URI: 'Ord'

Added in v2.0.0

URI (type alias)

Signature

export type URI = typeof URI

Added in v2.0.0

utils

between

Test whether a value is between a minimum and a maximum (inclusive)

Signature

export declare const between: <A>(O: Ord<A>) => (low: A, hi: A) => (a: A) => boolean

Added in v2.0.0

clamp

Clamp a value between a minimum and a maximum

Signature

export declare const clamp: <A>(O: Ord<A>) => (low: A, hi: A) => (a: A) => A

Added in v2.0.0

contramap

A typical use case for contramap would be like, given some User type, to construct an Ord<User>.

We can do so with a function from User -> X where X is some value that we know how to compare for ordering (meaning we have an Ord<X>)

For example, given the following User type, there are lots of possible choices for X, but let’s say we want to sort a list of users by lastName.

If we have a way of comparing lastNames for ordering (ordLastName: Ord<string>) and we know how to go from User -> string, using contramap we can do this

Signature

export declare const contramap: <A, B>(f: (b: B) => A) => (fa: Ord<A>) => Ord<B>

Example

import { pipe } from 'fp-ts/function'
import { contramap, Ord } from 'fp-ts/Ord'
import * as RA from 'fp-ts/ReadonlyArray'
import * as S from 'fp-ts/string'

interface User {
  readonly firstName: string
  readonly lastName: string
}

const ordLastName: Ord<string> = S.Ord

const ordByLastName: Ord<User> = pipe(
  ordLastName,
  contramap((user) => user.lastName)
)

assert.deepStrictEqual(
  RA.sort(ordByLastName)([
    { firstName: 'a', lastName: 'd' },
    { firstName: 'c', lastName: 'b' },
  ]),
  [
    { firstName: 'c', lastName: 'b' },
    { firstName: 'a', lastName: 'd' },
  ]
)

Added in v2.0.0

equals

Signature

export declare const equals: <A>(O: Ord<A>) => (second: A) => (first: A) => boolean

Added in v2.11.0

geq

Test whether one value is non-strictly greater than another

Signature

export declare const geq: <A>(O: Ord<A>) => (first: A, second: A) => boolean

Added in v2.0.0

gt

Test whether one value is strictly greater than another

Signature

export declare const gt: <A>(O: Ord<A>) => (first: A, second: A) => boolean

Added in v2.0.0

leq

Test whether one value is non-strictly less than another

Signature

export declare const leq: <A>(O: Ord<A>) => (first: A, second: A) => boolean

Added in v2.0.0

lt

Test whether one value is strictly less than another

Signature

export declare const lt: <A>(O: Ord<A>) => (first: A, second: A) => boolean

Added in v2.0.0

max

Take the maximum of two values. If they are considered equal, the first argument is chosen

Signature

export declare const max: <A>(O: Ord<A>) => (first: A, second: A) => A

Added in v2.0.0

min

Take the minimum of two values. If they are considered equal, the first argument is chosen

Signature

export declare const min: <A>(O: Ord<A>) => (first: A, second: A) => A

Added in v2.0.0

reverse

Signature

export declare const reverse: <A>(O: Ord<A>) => Ord<A>

Added in v2.10.0

trivial

Signature

export declare const trivial: Ord<unknown>

Added in v2.11.0

tuple

Given a tuple of Ords returns an Ord for the tuple.

Signature

export declare const tuple: <A extends readonly unknown[]>(...ords: { [K in keyof A]: Ord<A[K]> }) => Ord<Readonly<A>>

Example

import { tuple } from 'fp-ts/Ord'
import * as B from 'fp-ts/boolean'
import * as S from 'fp-ts/string'
import * as N from 'fp-ts/number'

const O = tuple(S.Ord, N.Ord, B.Ord)
assert.strictEqual(O.compare(['a', 1, true], ['b', 2, true]), -1)
assert.strictEqual(O.compare(['a', 1, true], ['a', 2, true]), -1)
assert.strictEqual(O.compare(['a', 1, true], ['a', 1, false]), 1)

Added in v2.10.0

zone of death

getDualOrd

Use reverse instead.

Signature

export declare const getDualOrd: <A>(O: Ord<A>) => Ord<A>

Added in v2.0.0

getTupleOrd

Use tuple instead.

Signature

export declare const getTupleOrd: <T extends readonly Ord<any>[]>(
  ...ords: T
) => Ord<{ [K in keyof T]: T[K] extends Ord<infer A> ? A : never }>

Added in v2.0.0

ordBoolean

Use Ord instead.

Signature

export declare const ordBoolean: Ord<boolean>

Added in v2.0.0

ordDate

Use Ord instead.

Signature

export declare const ordDate: Ord<Date>

Added in v2.0.0

ordNumber

Use Ord instead.

Signature

export declare const ordNumber: Ord<number>

Added in v2.0.0

ordString

Use Ord instead.

Signature

export declare const ordString: Ord<string>

Added in v2.0.0

ord

Use Contravariant instead.

Signature

export declare const ord: Contravariant1<'Ord'>

Added in v2.0.0