Lazy overview
Whilst the definition of Lazy happens to be the same as IO, they represent different intentions, specifically with respect to Lazy representing a pure thunk.
Thinking in terms of Haskell, Lazy can be considered equivalent to a pure function that takes () (unit) as input.
Added in v0.12.0
Table of contents
0 Types
Lazy (type alias)
Re-exported from fp-ts for convenience.
Signature
export type Lazy<A> = () => A
type Lazy a = () -> a
Added in v0.12.0
1 Typeclass Instances
Applicative
Formal Applicative instance for Lazy to be provided to higher-kinded functions that require it.
Signature
export declare const Applicative: Applicative1<'Lazy'>
Applicative :: Applicative1 "Lazy"
Added in v0.12.0
Apply
Formal Apply instance for Lazy to be provided to higher-kinded functions that require it.
Signature
export declare const Apply: Apply1<'Lazy'>
Apply :: Apply1 "Lazy"
Added in v0.12.0
Chain
Formal Chain instance for Lazy to be provided to higher-kinded functions that require it.
Signature
export declare const Chain: Chain1<'Lazy'>
Chain :: Chain1 "Lazy"
Added in v0.12.0
ChainRec
Formal ChainRec instance for Lazy to be provided to higher-kinded functions that require it.
Signature
export declare const ChainRec: ChainRec1<'Lazy'>
ChainRec :: ChainRec1 "Lazy"
Added in v0.12.0
Functor
Formal Functor instance for Lazy to be provided to higher-kinded functions that require it.
Signature
export declare const Functor: Functor1<'Lazy'>
Functor :: Functor1 "Lazy"
Added in v0.12.0
Monad
Formal Monad instance for Lazy to be provided to higher-kinded functions that require it.
Signature
export declare const Monad: Monad1<'Lazy'>
Monad :: Monad1 "Lazy"
Added in v0.12.0
Pointed
Formal Pointed instance for Lazy to be provided to higher-kinded functions that require it.
Signature
export declare const Pointed: Pointed1<'Lazy'>
Pointed :: Pointed1 "Lazy"
Added in v0.12.0
2 Typeclass Methods
ApT
Identity for Lazy as applied to sequenceT.
Signature
export declare const ApT: Lazy<readonly []>
ApT :: Lazy ([])
Added in v0.12.0
Do
Initiate do notation in the context of Lazy.
Signature
export declare const Do: Lazy<{}>
Do :: Lazy {}
Added in v0.12.0
ap
Apply a function within a Lazy.
Signature
export declare const ap: <A>(fa: Lazy<A>) => <B>(fab: Lazy<(a: A) => B>) => Lazy<B>
ap :: Lazy a -> Lazy (a -> b) -> Lazy b
Added in v0.12.0
apFirst
Sequence actions, discarding the value of the first argument.
Signature
export declare const apFirst: <B>(second: Lazy<B>) => <A>(first: Lazy<A>) => Lazy<A>
apFirst :: Lazy b -> Lazy a -> Lazy a
Added in v0.12.0
apS
Bind the provided value to the specified key in do notation.
Signature
export declare const apS: <N, A, B>(
name: Exclude<N, keyof A>,
fb: Lazy<B>
) => (fa: Lazy<A>) => Lazy<{ readonly [K in N | keyof A]: K extends keyof A ? A[K] : B }>
apS :: (Exclude n (keyof a), Lazy b) -> Lazy a -> Lazy { [k in (n | (keyof a))]: k extends (keyof a) ? a[k] : b }
Added in v0.12.0
apSecond
Sequence actions, discarding the value of the second argument.
Signature
export declare const apSecond: <B>(second: Lazy<B>) => <A>(first: Lazy<A>) => Lazy<B>
apSecond :: Lazy b -> Lazy a -> Lazy b
Added in v0.12.0
bind
Bind the output of the provided function to the specified key in do notation.
Signature
export declare const bind: <N, A, B>(
name: Exclude<N, keyof A>,
f: (a: A) => Lazy<B>
) => (ma: Lazy<A>) => Lazy<{ readonly [K in N | keyof A]: K extends keyof A ? A[K] : B }>
bind :: (Exclude n (keyof a), (a -> Lazy b)) -> Lazy a -> Lazy { [k in (n | (keyof a))]: k extends (keyof a) ? a[k] : b }
Added in v0.12.0
bindTo
Bind the provided value, typically preceding it in a pipeline, to the specified key in do notation.
Signature
export declare const bindTo: <N>(name: N) => <A>(fa: Lazy<A>) => Lazy<{ readonly [K in N]: A }>
bindTo :: n -> Lazy a -> Lazy { [k in n]: a }
Added in v0.12.0
chain
Map and flatten the output of a Lazy.
Signature
export declare const chain: <A, B>(f: (a: A) => Lazy<B>) => (ma: Lazy<A>) => Lazy<B>
chain :: (a -> Lazy b) -> Lazy a -> Lazy b
Added in v0.12.0
chainFirst
Like chain, but discards the new output.
Signature
export declare const chainFirst: <A, _>(f: (a: A) => Lazy<_>) => (first: Lazy<A>) => Lazy<A>
chainFirst :: (a -> Lazy _) -> Lazy a -> Lazy a
Added in v0.12.0
flap
Takes a function in a functorial Lazy context and applies it to an ordinary value.
Signature
export declare const flap: <A>(a: A) => <B>(fab: Lazy<(a: A) => B>) => Lazy<B>
flap :: a -> Lazy (a -> b) -> Lazy b
Added in v0.12.0
flatMap
Alias of chain.
Signature
export declare const flatMap: <A, B>(f: (a: A) => Lazy<B>) => (ma: Lazy<A>) => Lazy<B>
flatMap :: (a -> Lazy b) -> Lazy a -> Lazy b
Added in v0.17.0
flatten
Flatten a nested Lazy.
Signature
export declare const flatten: <A>(mma: Lazy<Lazy<A>>) => Lazy<A>
flatten :: Lazy (Lazy a) -> Lazy a
Added in v0.12.0
let
Assign a variable in do notation.
Signature
export declare const let: <N, A, B>(
name: Exclude<N, keyof A>,
f: (a: A) => B
) => (fa: Lazy<A>) => Lazy<{ readonly [K in N | keyof A]: K extends keyof A ? A[K] : B }>
let :: (Exclude n (keyof a), (a -> b)) -> Lazy a -> Lazy { [k in (n | (keyof a))]: k extends (keyof a) ? a[k] : b }
Added in v0.17.0
map
Map the output of a Lazy.
Signature
export declare const map: <A, B>(f: (x: A) => B) => (fa: Lazy<A>) => Lazy<B>
map :: (a -> b) -> Lazy a -> Lazy b
Added in v0.12.0
of
Raise any value to a Lazy.
Signature
export declare const of: <A>(a: A) => Lazy<A>
of :: a -> Lazy a
Added in v0.12.0
sequenceArray
Equivalent to Array#sequence(Applicative).
Signature
export declare const sequenceArray: <A>(arr: readonly Lazy<A>[]) => Lazy<readonly A[]>
sequenceArray :: Array (Lazy a) -> Lazy (Array a)
Added in v2.9.0
traverseArray
Equivalent to Array#traverse(Applicative).
Signature
export declare const traverseArray: <A, B>(f: (a: A) => Lazy<B>) => (as: readonly A[]) => Lazy<readonly B[]>
traverseArray :: (a -> Lazy b) -> Array a -> Lazy (Array b)
Added in v0.12.0
traverseArrayWithIndex
Equivalent to Array#traverseWithIndex(Applicative).
Signature
export declare const traverseArrayWithIndex: <A, B>(
f: (index: number, a: A) => Lazy<B>
) => (as: readonly A[]) => Lazy<readonly B[]>
traverseArrayWithIndex :: ((number, a) -> Lazy b) -> Array a -> Lazy (Array b)
Added in v0.12.0
traverseReadonlyArrayWithIndex
Equivalent to ReadonlyArray#traverseWithIndex(Applicative).
Signature
export declare const traverseReadonlyArrayWithIndex: <A, B>(
f: (index: number, a: A) => Lazy<B>
) => (as: readonly A[]) => Lazy<readonly B[]>
traverseReadonlyArrayWithIndex :: ((number, a) -> Lazy b) -> Array a -> Lazy (Array b)
Added in v0.12.0
traverseReadonlyNonEmptyArrayWithIndex
Equivalent to ReadonlyNonEmptyArray#traverseWithIndex(Applicative).
Signature
export declare const traverseReadonlyNonEmptyArrayWithIndex: <A, B>(
f: (index: number, a: A) => Lazy<B>
) => (as: ReadonlyNonEmptyArray<A>) => Lazy<ReadonlyNonEmptyArray<B>>
traverseReadonlyNonEmptyArrayWithIndex :: ((number, a) -> Lazy b) -> ReadonlyNonEmptyArray a -> Lazy (ReadonlyNonEmptyArray b)
Added in v0.12.0
3 Functions
execute
Execute a Lazy, returning the value within. Helpful for staying within function application and composition pipelines.
Signature
export declare const execute: <A>(x: Lazy<A>) => A
execute :: Lazy a -> a
Example
import * as Lazy from 'fp-ts-std/Lazy'
assert.strictEqual(Lazy.execute(Lazy.of(5)), 5)
Added in v0.12.0
lazy
A constructor for Lazy values. Given Lazy is a type alias around () => A, this function’s only purpose is to aid in readability and express intentional laziness, as opposed to for example forgetting or opting not to use constant.
Signature
export declare const lazy: <A>(f: () => A) => Lazy<A>
lazy :: (() -> a) -> Lazy a
Example
import { lazy } from 'fp-ts-std/Lazy'
const calc = lazy(() => 'do something expensive here')
Added in v0.13.0
memoize
Memoize a Lazy. Provided the input function is pure, this function is too.
Signature
export declare const memoize: <A>(f: Lazy<A>) => Lazy<A>
memoize :: Lazy a -> Lazy a
Example
import { lazy, memoize } from 'fp-ts-std/Lazy'
const expensive = lazy(() => 42)
const payOnce = memoize(expensive)
assert.strictEqual(payOnce(), payOnce())
Added in v0.14.0
4 Minutiae
URI
Typeclass machinery.
Signature
export declare const URI: 'Lazy'
URI :: "Lazy"
Added in v0.12.0
URI (type alias)
Typeclass machinery.
Signature
export type URI = typeof URI
type URI = typeof URI
Added in v0.12.0