Eq overview

The Eq type class represents types which support decidable equality.

Instances must satisfy the following laws:

  1. Reflexivity: E.equals(a, a) === true
  2. Symmetry: E.equals(a, b) === E.equals(b, a)
  3. Transitivity: if E.equals(a, b) === true and E.equals(b, c) === true, then E.equals(a, c) === true

Added in v2.0.0

Table of contents

constructors

fromEquals

Signature

export declare const fromEquals: <A>(equals: (x: A, y: A) => boolean) => Eq<A>

Added in v2.0.0

instances

Contravariant

Signature

export declare const Contravariant: Contravariant1<'Eq'>

Added in v2.7.0

eqStrict

Signature

export declare const eqStrict: Eq<unknown>

Added in v2.5.0

getMonoid

Signature

export declare const getMonoid: <A>() => Monoid<Eq<A>>

Added in v2.6.0

getSemigroup

Signature

export declare const getSemigroup: <A>() => Semigroup<Eq<A>>

Added in v2.10.0

model

Eq (interface)

Signature

export interface Eq<A> {
  readonly equals: (x: A, y: A) => boolean
}

Added in v2.0.0

type lambdas

URI

Signature

export declare const URI: 'Eq'

Added in v2.0.0

URI (type alias)

Signature

export type URI = typeof URI

Added in v2.0.0

utils

contramap

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

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

For example, given the following User type, we want to construct an Eq<User> that just looks at the key field for each user (since it’s known to be unique).

If we have a way of comparing UUIDs for equality (eqUUID: Eq<UUID>) and we know how to go from User -> UUID, using contramap we can do this

Signature

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

Example

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

type UUID = string

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

const eqUUID: Eq<UUID> = S.Eq

const eqUserByKey: Eq<User> = pipe(
  eqUUID,
  contramap((user) => user.key)
)

assert.deepStrictEqual(
  eqUserByKey.equals({ key: 'k1', firstName: 'a1', lastName: 'b1' }, { key: 'k2', firstName: 'a1', lastName: 'b1' }),
  false
)
assert.deepStrictEqual(
  eqUserByKey.equals({ key: 'k1', firstName: 'a1', lastName: 'b1' }, { key: 'k1', firstName: 'a2', lastName: 'b1' }),
  true
)

Added in v2.0.0

struct

Signature

export declare const struct: <A>(eqs: { [K in keyof A]: Eq<A[K]> }) => Eq<{ readonly [K in keyof A]: A[K] }>

Added in v2.10.0

tuple

Given a tuple of Eqs returns a Eq for the tuple

Signature

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

Example

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

const E = tuple(S.Eq, N.Eq, B.Eq)
assert.strictEqual(E.equals(['a', 1, true], ['a', 1, true]), true)
assert.strictEqual(E.equals(['a', 1, true], ['b', 1, true]), false)
assert.strictEqual(E.equals(['a', 1, true], ['a', 2, true]), false)
assert.strictEqual(E.equals(['a', 1, true], ['a', 1, false]), false)

Added in v2.10.0

zone of death

eqBoolean

Use Eq instead.

Signature

export declare const eqBoolean: Eq<boolean>

Added in v2.0.0

eqDate

Use Eq instead.

Signature

export declare const eqDate: Eq<Date>

Added in v2.0.0

eqNumber

Use Eq instead.

Signature

export declare const eqNumber: Eq<number>

Added in v2.0.0

eqString

Use Eq instead.

Signature

export declare const eqString: Eq<string>

Added in v2.0.0

eq

This instance is deprecated, use small, specific instances instead. For example if a function needs a Contravariant instance, pass E.Contravariant instead of E.eq (where E is from import E from 'fp-ts/Eq')

Signature

export declare const eq: Contravariant1<'Eq'>

Added in v2.0.0

getStructEq

Use struct instead.

Signature

export declare const getStructEq: <O extends Readonly<Record<string, any>>>(eqs: { [K in keyof O]: Eq<O[K]> }) => Eq<O>

Added in v2.0.0

getTupleEq

Use tuple instead.

Signature

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

Added in v2.0.0

strictEqual

Use eqStrict instead

Signature

export declare const strictEqual: <A>(a: A, b: A) => boolean

Added in v2.0.0