/
Reader.ts
650 lines (582 loc) · 16.4 KB
/
Reader.ts
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/**
* The `Reader` monad (also called the Environment monad). Represents a computation, which can read values from a shared environment,
* pass values from function to function, and execute sub-computations in a modified environment.
* Using `Reader` monad for such computations is often clearer and easier than using the `State` monad.
*
* In this example the `Reader` monad provides access to variable bindings. `Bindings` are a map of `number` variables.
* The variable count contains number of variables in the bindings. You can see how to run a `Reader` monad and retrieve
* data from it, how to access the `Reader` data with `ask` and `asks`.
*
* @example
* import { pipe } from 'fp-ts/function'
* import * as O from 'fp-ts/Option'
* import * as R from 'fp-ts/Reader'
* import * as RR from 'fp-ts/ReadonlyRecord'
*
* interface Bindings extends RR.ReadonlyRecord<string, number> {}
*
* // The Reader monad, which implements this complicated check.
* const isCountCorrect: R.Reader<Bindings, boolean> = pipe(
* R.Do,
* R.bind('count', () => R.asks(lookupVar('count'))),
* R.bind('bindings', () => R.ask()),
* R.map(({ count, bindings }) => count === RR.size(bindings))
* )
*
* // The selector function to use with 'asks'.
* // Returns value of the variable with specified name.
* const lookupVar = (name: string) => (bindings: Bindings): number =>
* pipe(
* bindings,
* RR.lookup(name),
* O.getOrElse(() => 0)
* )
*
* const sampleBindings: Bindings = { count: 3, a: 1, b: 2 }
*
* assert.deepStrictEqual(isCountCorrect(sampleBindings), true)
*
* @since 2.0.0
*/
import { Applicative2, getApplicativeMonoid } from './Applicative'
import { apFirst as apFirst_, Apply2, apS as apS_, apSecond as apSecond_, getApplySemigroup } from './Apply'
import { Category2 } from './Category'
import * as chainable from './Chain'
import { Choice2 } from './Choice'
import * as E from './Either'
import { constant, dual, flow, identity, pipe } from './function'
import { bindTo as bindTo_, flap as flap_, Functor2, let as let__ } from './Functor'
import * as _ from './internal'
import { Monad2 } from './Monad'
import { Monoid } from './Monoid'
import { NonEmptyArray } from './NonEmptyArray'
import { Pointed2 } from './Pointed'
import { Profunctor2 } from './Profunctor'
import { ReadonlyNonEmptyArray } from './ReadonlyNonEmptyArray'
import { Semigroup } from './Semigroup'
import { Strong2 } from './Strong'
// -------------------------------------------------------------------------------------
// model
// -------------------------------------------------------------------------------------
/**
* @category model
* @since 2.0.0
*/
export interface Reader<R, A> {
(r: R): A
}
// -------------------------------------------------------------------------------------
// constructors
// -------------------------------------------------------------------------------------
/**
* Reads the current context
*
* @category constructors
* @since 2.0.0
*/
export const ask: <R>() => Reader<R, R> = () => identity
/**
* Projects a value from the global context in a Reader
*
* @category constructors
* @since 2.0.0
*/
export const asks: <R, A>(f: (r: R) => A) => Reader<R, A> = identity
// -------------------------------------------------------------------------------------
// combinators
// -------------------------------------------------------------------------------------
/**
* Changes the value of the local context during the execution of the action `ma` (similar to `Contravariant`'s
* `contramap`).
*
* @example
* import { pipe } from 'fp-ts/function'
* import * as R from 'fp-ts/Reader'
* import * as string from 'fp-ts/string'
*
* const calculateContentLen: R.Reader<string, number> = pipe(
* R.Do,
* R.bind('content', () => R.ask<string>()),
* R.map(({ content }) => string.size(content))
* )
*
* // Calls calculateContentLen after adding a prefix to the Reader content.
* const calculateModifiedContentLen: R.Reader<string, number> = pipe(
* calculateContentLen,
* R.local((s) => 'Prefix ' + s)
* )
*
* const s = '12345'
*
* assert.deepStrictEqual(
* "Modified 's' length: " + calculateModifiedContentLen(s) + '\n' + "Original 's' length: " + calculateContentLen(s),
* "Modified 's' length: 12\nOriginal 's' length: 5"
* )
*
* @since 2.0.0
*/
export const local: <R2, R1>(f: (r2: R2) => R1) => <A>(ma: Reader<R1, A>) => Reader<R2, A> = (f) => (ma) => (r2) =>
ma(f(r2))
/**
* Less strict version of [`asksReader`](#asksreader).
*
* The `W` suffix (short for **W**idening) means that the environment types will be merged.
*
* @category constructors
* @since 2.11.0
*/
export const asksReaderW =
<R1, R2, A>(f: (r1: R1) => Reader<R2, A>): Reader<R1 & R2, A> =>
(r) =>
f(r)(r)
/**
* Effectfully accesses the environment.
*
* @category constructors
* @since 2.11.0
*/
export const asksReader: <R, A>(f: (r: R) => Reader<R, A>) => Reader<R, A> = asksReaderW
/* istanbul ignore next */
const _map: Monad2<URI>['map'] = (fa, f) => pipe(fa, map(f))
/* istanbul ignore next */
const _ap: Monad2<URI>['ap'] = (fab, fa) => pipe(fab, ap(fa))
const _compose: Category2<URI>['compose'] = (bc, ab) => pipe(bc, compose(ab))
const _promap: Profunctor2<URI>['promap'] = (fea, f, g) => pipe(fea, promap(f, g))
/**
* `map` can be used to turn functions `(a: A) => B` into functions `(fa: F<A>) => F<B>` whose argument and return types
* use the type constructor `F` to represent some computational context.
*
* @category mapping
* @since 2.0.0
*/
export const map: <A, B>(f: (a: A) => B) => <R>(fa: Reader<R, A>) => Reader<R, B> = (f) => (fa) => (r) => f(fa(r))
/**
* Less strict version of [`ap`](#ap).
*
* The `W` suffix (short for **W**idening) means that the environment types will be merged.
*
* @since 2.8.0
*/
export const apW: <R2, A>(fa: Reader<R2, A>) => <R1, B>(fab: Reader<R1, (a: A) => B>) => Reader<R1 & R2, B> =
(fa) => (fab) => (r) =>
fab(r)(fa(r))
/**
* @since 2.0.0
*/
export const ap: <R, A>(fa: Reader<R, A>) => <B>(fab: Reader<R, (a: A) => B>) => Reader<R, B> = apW
/**
* @category constructors
* @since 2.0.0
*/
export const of: <R = unknown, A = never>(a: A) => Reader<R, A> = constant
/**
* @category sequencing
* @since 2.14.0
*/
export const flatMap: {
<A, R2, B>(f: (a: A) => Reader<R2, B>): <R1>(ma: Reader<R1, A>) => Reader<R1 & R2, B>
<R1, A, R2, B>(ma: Reader<R1, A>, f: (a: A) => Reader<R2, B>): Reader<R1 & R2, B>
} = /*#__PURE__*/ dual(
2,
<R1, A, R2, B>(ma: Reader<R1, A>, f: (a: A) => Reader<R2, B>): Reader<R1 & R2, B> =>
(r) =>
f(ma(r))(r)
)
/**
* Less strict version of [`flatten`](#flatten).
*
* The `W` suffix (short for **W**idening) means that the environment types will be merged.
*
* @category sequencing
* @since 2.11.0
*/
export const flattenW: <R1, R2, A>(mma: Reader<R1, Reader<R2, A>>) => Reader<R1 & R2, A> =
/*#__PURE__*/ flatMap(identity)
/**
* @category sequencing
* @since 2.0.0
*/
export const flatten: <R, A>(mma: Reader<R, Reader<R, A>>) => Reader<R, A> = flattenW
/**
* @since 2.0.0
*/
export const compose: <A, B>(ab: Reader<A, B>) => <C>(bc: Reader<B, C>) => Reader<A, C> = (ab) => (bc) => flow(ab, bc)
/**
* @since 2.0.0
*/
export const promap: <E, A, D, B>(f: (d: D) => E, g: (a: A) => B) => (fea: Reader<E, A>) => Reader<D, B> =
(f, g) => (fea) => (a) =>
g(fea(f(a)))
/**
* @category constructors
* @since 2.0.0
*/
export const id: Category2<URI>['id'] = () => identity
/**
* @since 2.10.0
*/
export const first: Strong2<URI>['first'] =
(pab) =>
([a, c]) =>
[pab(a), c]
/**
* @since 2.10.0
*/
export const second: Strong2<URI>['second'] =
(pbc) =>
([a, b]) =>
[a, pbc(b)]
/**
* @since 2.10.0
*/
export const left: Choice2<URI>['left'] = (pab) => E.fold((a) => _.left(pab(a)), E.right)
/**
* @since 2.10.0
*/
export const right: Choice2<URI>['right'] = (pbc) => E.fold(E.left, (b) => _.right(pbc(b)))
/**
* @category type lambdas
* @since 2.0.0
*/
export const URI = 'Reader'
/**
* @category type lambdas
* @since 2.0.0
*/
export type URI = typeof URI
declare module './HKT' {
interface URItoKind2<E, A> {
readonly [URI]: Reader<E, A>
}
}
/**
* @category instances
* @since 2.7.0
*/
export const Functor: Functor2<URI> = {
URI,
map: _map
}
/**
* @category mapping
* @since 2.10.0
*/
export const flap = /*#__PURE__*/ flap_(Functor)
/**
* @category instances
* @since 2.10.0
*/
export const Pointed: Pointed2<URI> = {
URI,
of
}
/**
* @category instances
* @since 2.10.0
*/
export const Apply: Apply2<URI> = {
URI,
map: _map,
ap: _ap
}
/**
* Combine two effectful actions, keeping only the result of the first.
*
* @since 2.0.0
*/
export const apFirst = /*#__PURE__*/ apFirst_(Apply)
/**
* Less strict version of [`apFirst`](#apfirst).
*
* The `W` suffix (short for **W**idening) means that the environment types will be merged.
*
* @since 2.12.0
*/
export const apFirstW: <R2, B>(second: Reader<R2, B>) => <R1, A>(first: Reader<R1, A>) => Reader<R1 & R2, A> =
apFirst as any
/**
* Combine two effectful actions, keeping only the result of the second.
*
* @since 2.0.0
*/
export const apSecond = /*#__PURE__*/ apSecond_(Apply)
/**
* Less strict version of [`apSecond`](#apsecond).
*
* The `W` suffix (short for **W**idening) means that the environment types will be merged.
*
* @since 2.12.0
*/
export const apSecondW: <R2, B>(second: Reader<R2, B>) => <R1, A>(first: Reader<R1, A>) => Reader<R1 & R2, B> =
apSecond as any
/**
* @category instances
* @since 2.7.0
*/
export const Applicative: Applicative2<URI> = {
URI,
map: _map,
ap: _ap,
of
}
/**
* @category instances
* @since 2.10.0
*/
export const Chain: chainable.Chain2<URI> = {
URI,
map: _map,
ap: _ap,
chain: flatMap
}
/**
* @category instances
* @since 2.7.0
*/
export const Monad: Monad2<URI> = {
URI,
map: _map,
of,
ap: _ap,
chain: flatMap
}
/**
* Composes computations in sequence, using the return value of one computation to determine the next computation and
* keeping only the result of the first.
*
* @category combinators
* @since 2.15.0
*/
export const tap: {
<R1, A, R2, _>(self: Reader<R1, A>, f: (a: A) => Reader<R2, _>): Reader<R1 & R2, A>
<A, R2, _>(f: (a: A) => Reader<R2, _>): <R1>(self: Reader<R1, A>) => Reader<R2 & R1, A>
} = /*#__PURE__*/ dual(2, chainable.tap(Chain))
/**
* @category instances
* @since 2.7.0
*/
export const Profunctor: Profunctor2<URI> = {
URI,
map: _map,
promap: _promap
}
/**
* @category instances
* @since 2.7.0
*/
export const Category: Category2<URI> = {
URI,
compose: _compose,
id
}
/**
* @category instances
* @since 2.8.3
*/
export const Strong: Strong2<URI> = {
URI,
map: _map,
promap: _promap,
first,
second
}
/**
* @category instances
* @since 2.8.3
*/
export const Choice: Choice2<URI> = {
URI,
map: _map,
promap: _promap,
left,
right
}
// -------------------------------------------------------------------------------------
// do notation
// -------------------------------------------------------------------------------------
/**
* @category do notation
* @since 2.8.0
*/
export const bindTo = /*#__PURE__*/ bindTo_(Functor)
const let_ = /*#__PURE__*/ let__(Functor)
export {
/**
* @category do notation
* @since 2.13.0
*/
let_ as let
}
/**
* @category do notation
* @since 2.8.0
*/
export const bind = /*#__PURE__*/ chainable.bind(Chain)
/**
* The `W` suffix (short for **W**idening) means that the environment types will be merged.
*
* @category do notation
* @since 2.8.0
*/
export const bindW: <N extends string, A, R2, B>(
name: Exclude<N, keyof A>,
f: (a: A) => Reader<R2, B>
) => <R1>(fa: Reader<R1, A>) => Reader<R1 & R2, { readonly [K in keyof A | N]: K extends keyof A ? A[K] : B }> =
bind as any
/**
* @category do notation
* @since 2.9.0
*/
export const Do: Reader<unknown, {}> = /*#__PURE__*/ of(_.emptyRecord)
/**
* @category do notation
* @since 2.8.0
*/
export const apS = /*#__PURE__*/ apS_(Apply)
/**
* Less strict version of [`apS`](#aps).
*
* The `W` suffix (short for **W**idening) means that the environment types will be merged.
*
* @category do notation
* @since 2.8.0
*/
export const apSW: <A, N extends string, R2, B>(
name: Exclude<N, keyof A>,
fb: Reader<R2, B>
) => <R1>(fa: Reader<R1, A>) => Reader<R1 & R2, { readonly [K in keyof A | N]: K extends keyof A ? A[K] : B }> =
apS as any
/**
* @since 2.11.0
*/
export const ApT: Reader<unknown, readonly []> = /*#__PURE__*/ of(_.emptyReadonlyArray)
// -------------------------------------------------------------------------------------
// array utils
// -------------------------------------------------------------------------------------
/**
* Equivalent to `ReadonlyNonEmptyArray#traverseWithIndex(Applicative)`.
*
* @category traversing
* @since 2.11.0
*/
export const traverseReadonlyNonEmptyArrayWithIndex =
<A, R, B>(f: (index: number, a: A) => Reader<R, B>) =>
(as: ReadonlyNonEmptyArray<A>): Reader<R, ReadonlyNonEmptyArray<B>> =>
(r) => {
const out: NonEmptyArray<B> = [f(0, _.head(as))(r)]
for (let i = 1; i < as.length; i++) {
out.push(f(i, as[i])(r))
}
return out
}
/**
* Equivalent to `ReadonlyArray#traverseWithIndex(Applicative)`.
*
* @category traversing
* @since 2.11.0
*/
export const traverseReadonlyArrayWithIndex = <A, R, B>(
f: (index: number, a: A) => Reader<R, B>
): ((as: ReadonlyArray<A>) => Reader<R, ReadonlyArray<B>>) => {
const g = traverseReadonlyNonEmptyArrayWithIndex(f)
return (as) => (_.isNonEmpty(as) ? g(as) : ApT)
}
/**
* Equivalent to `ReadonlyArray#traverseWithIndex(Applicative)`.
*
* @category traversing
* @since 2.9.0
*/
export const traverseArrayWithIndex: <R, A, B>(
f: (index: number, a: A) => Reader<R, B>
) => (as: ReadonlyArray<A>) => Reader<R, ReadonlyArray<B>> = traverseReadonlyArrayWithIndex
/**
* Equivalent to `ReadonlyArray#traverse(Applicative)`.
*
* @category traversing
* @since 2.9.0
*/
export const traverseArray = <R, A, B>(
f: (a: A) => Reader<R, B>
): ((as: ReadonlyArray<A>) => Reader<R, ReadonlyArray<B>>) => traverseReadonlyArrayWithIndex((_, a) => f(a))
/**
* Equivalent to `ReadonlyArray#sequence(Applicative)`.
*
* @category traversing
* @since 2.9.0
*/
export const sequenceArray: <R, A>(arr: ReadonlyArray<Reader<R, A>>) => Reader<R, ReadonlyArray<A>> =
/*#__PURE__*/ traverseArray(identity)
// -------------------------------------------------------------------------------------
// legacy
// -------------------------------------------------------------------------------------
/**
* Alias of `flatMap`.
*
* @category legacy
* @since 2.6.0
*/
export const chainW: <R2, A, B>(f: (a: A) => Reader<R2, B>) => <R1>(ma: Reader<R1, A>) => Reader<R1 & R2, B> = flatMap
/**
* Alias of `flatMap`.
*
* @category legacy
* @since 2.0.0
*/
export const chain: <A, R, B>(f: (a: A) => Reader<R, B>) => (ma: Reader<R, A>) => Reader<R, B> = flatMap
/**
* Alias of `tap`.
*
* @category legacy
* @since 2.0.0
*/
export const chainFirst: <A, R, B>(f: (a: A) => Reader<R, B>) => (first: Reader<R, A>) => Reader<R, A> = tap
/**
* Alias of `tap`.
*
* @category legacy
* @since 2.11.0
*/
export const chainFirstW: <R2, A, B>(f: (a: A) => Reader<R2, B>) => <R1>(ma: Reader<R1, A>) => Reader<R1 & R2, A> = tap
// -------------------------------------------------------------------------------------
// deprecated
// -------------------------------------------------------------------------------------
/**
* This instance is deprecated, use small, specific instances instead.
* For example if a function needs a `Functor` instance, pass `R.Functor` instead of `R.reader`
* (where `R` is from `import R from 'fp-ts/Reader'`)
*
* @category zone of death
* @since 2.0.0
* @deprecated
*/
export const reader: Monad2<URI> & Profunctor2<URI> & Category2<URI> & Strong2<URI> & Choice2<URI> = {
URI,
map: _map,
of,
ap: _ap,
chain: flatMap,
promap: _promap,
compose: _compose,
id,
first,
second,
left,
right
}
/**
* Use [`getApplySemigroup`](./Apply.ts.html#getapplysemigroup) instead.
*
* @category zone of death
* @since 2.0.0
* @deprecated
*/
export const getSemigroup: <R, A>(S: Semigroup<A>) => Semigroup<Reader<R, A>> = /*#__PURE__*/ getApplySemigroup(Apply)
/**
* Use [`getApplicativeMonoid`](./Applicative.ts.html#getapplicativemonoid) instead.
*
* @category zone of death
* @since 2.0.0
* @deprecated
*/
export const getMonoid: <R, A>(M: Monoid<A>) => Monoid<Reader<R, A>> = /*#__PURE__*/ getApplicativeMonoid(Applicative)