funfix-types
Version:
Sub-package of Funfix defining type classes inspired by Haskell's standard library
401 lines (400 loc) • 14.6 kB
TypeScript
/*!
* Copyright (c) 2017 by The Funfix Project Developers.
* Some rights reserved.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
import { HK, Equiv, Constructor } from "./kinds";
import { Functor, FunctorLaws } from "./functor";
/**
* The `CoflatMap` type class, a weaker version of {@link Comonad},
* exposing `coflatMap`, but not `extract`.
*
* This type class is exposed in addition to `Comonad` because
* there are data types for which we can't implement `extract`, but
* that could still benefit from an `coflatMap` definition.
*
* MUST obey the laws defined in {@link CoflatMapLaws}.
*
* Note that having a `CoflatMap` instance implies that a
* {@link Functor} implementation is also available, which is why
* `CoflatMap` is a subtype of `Functor`.
*
* ## Implementation notes
*
* Even though in TypeScript the Funfix library is using `abstract class` to
* express type classes, when implementing this type class it is recommended
* that you implement it as a mixin using "`implements`", instead of extending
* it directly with "`extends`". See
* [TypeScript: Mixins]{@link https://www.typescriptlang.org/docs/handbook/mixins.html}
* for details and note that we already have `applyMixins` defined.
*
* Implementation example:
*
* ```typescript
* import {
* HK, CoflatMap,
* registerTypeClassInstance,
* applyMixins
* } from "funfix"
*
* // Type alias defined for readability.
* // HK is our encoding for higher-kinded types.
* type BoxK<T> = HK<Box<any>, T>
*
* class Box<T> implements HK<Box<any>, T> {
* constructor(public value: T) {}
*
* // Implements HK<Box<any>, A>, not really needed, but useful in order
* // to avoid type casts. Note these can and should be undefined:
* readonly _funKindF: Box<any>
* readonly _funKindA: T
* }
*
* class BoxCoflatMap implements CoflatMap<Box<any>> {
* map<A, B>(fa: BoxK<A>, f: (a: A) => B): Box<B> {
* const a = (fa as Box<A>).value
* return new Box(f(a))
* }
*
* coflatMap<A, B>(fa: BoxK<A>, ff: (a: BoxK<A>) => B): BoxK<B> {
* return new Box(Success(ff(fa)))
* }
*
* coflatten<A>(fa: BoxK<A>): BoxK<BoxK<A>> {
* return new Box(Success(fa))
* }
* }
*
* // At the moment of writing, this call is not needed, but it is
* // recommended anyway to future-proof the code ;-)
* applyMixins(BoxCoflatMap, [CoflatMap])
*
* // Registering global CoflatMap instance for Box, needed in order
* // for the `coflatMapOf(Box)` calls to work
* registerTypeClassInstance(CoflatMap)(Box, new BoxCoflatMap())
* ```
*
* We are using `implements` in order to support multiple inheritance and to
* avoid inheriting any `static` members. In the Flow definitions (e.g.
* `.js.flow` files) for Funfix these type classes are defined with
* "`interface`", as they are meant to be interfaces that sometimes have
* default implementations and not classes.
*
* ## Credits
*
* This type class is inspired by the equivalent in Haskell's
* standard library and the implementation is inspired by the
* [Typelevel Cats]{@link http://typelevel.org/cats/} project.
*/
export declare abstract class CoflatMap<F> implements Functor<F> {
/**
* `coflatMap` is the dual of `flatMap` on {@link FlatMap}.
*
* It applies a value in a context to a function that takes a
* value in a context and returns a normal value.
*/
abstract coflatMap<A, B>(fa: HK<F, A>, ff: (a: HK<F, A>) => B): HK<F, B>;
/**
* `coflatten` is the dual of `flatten` on {@link FlatMap}.
*
* Whereas `flatten` removes a layer of `F`, coflatten adds a
* layer of `F`.
*/
abstract coflatten<A>(fa: HK<F, A>): HK<F, HK<F, A>>;
/** Inherited from {@link Functor.map}. */
map: <A, B>(fa: HK<F, A>, f: (a: A) => B) => HK<F, B>;
/** @hidden */
static readonly _funTypeId: string;
/** @hidden */
static readonly _funSupertypeIds: string[];
/** @hidden */
static readonly _funErasure: CoflatMap<any>;
}
/**
* Type class laws defined for {@link CoflatMap}.
*
* This is an abstract definition. In order to use it in unit testing,
* the implementor must think of a strategy to evaluate the truthiness
* of the returned `Equiv` values.
*
* Even though in TypeScript the Funfix library is using classes to
* express these laws, when implementing this class it is recommended
* that you implement it as a mixin using `implements`, instead of
* extending it directly with `extends`. See
* [TypeScript: Mixins]{@link https://www.typescriptlang.org/docs/handbook/mixins.html}
* for details and note that we already have `applyMixins` defined.
*
* We are doing this in order to support multiple inheritance and to
* avoid inheriting any `static` members. In the Flow definitions (e.g.
* `.js.flow` files) for Funfix these classes are defined with
* `interface`, as they are meant to be interfaces that sometimes have
* default implementations and not classes.
*/
export declare abstract class CoflatMapLaws<F> implements FunctorLaws<F> {
/**
* The {@link CoflatMap} designated instance for `F`,
* to be tested.
*/
readonly F: CoflatMap<F>;
/**
* ```
* fa.coflatMap(f).coflatMap(g) <-> fa.coflatMap(x => g(x.coflatMap(f)))
* ```
*/
coflatMapAssociativity<A, B, C>(fa: HK<F, A>, f: (a: HK<F, A>) => B, g: (b: HK<F, B>) => C): Equiv<HK<F, C>>;
/**
* ```
* fa.coflatten.coflatten <-> fa.coflatten.map(_.coflatten)
* ```
*/
coflattenThroughMap<A>(fa: HK<F, A>): Equiv<HK<F, HK<F, HK<F, A>>>>;
/**
* ```
* fa.coflatMap(f) <-> fa.coflatten.map(f)
* ```
*/
coflattenCoherence<A, B>(fa: HK<F, A>, f: (a: HK<F, A>) => B): Equiv<HK<F, B>>;
/**
* ```
* fa.coflatten <-> fa.coflatMap(identity)
* ```
*/
coflatMapIdentity<A>(fa: HK<F, A>): Equiv<HK<F, HK<F, A>>>;
/** Mixed-in from {@link FunctorLaws.covariantIdentity}. */
covariantIdentity: <A>(fa: HK<F, A>) => Equiv<HK<F, A>>;
/** Mixed-in from {@link FunctorLaws.covariantComposition}. */
covariantComposition: <A, B, C>(fa: HK<F, A>, f: (a: A) => B, g: (b: B) => C) => Equiv<HK<F, C>>;
}
/**
* Given a {@link Constructor} reference, returns its associated
* {@link CoflatMap} instance if it exists, or throws a `NotImplementedError`
* in case there's no such association.
*
* ```typescript
* import { Option, CoflatMap, coflatMapOf } from "funfix"
*
* const F: CoflatMap<Option<any>> = coflatMapOf(Option)
* ```
*/
export declare const coflatMapOf: <F>(c: Constructor<F>) => CoflatMap<F>;
/**
* Given an {@link CoflatMap} instance, returns the {@link CoflatMapLaws}
* associated with it.
*/
export declare function coflatMapLawsOf<F>(instance: CoflatMap<F>): CoflatMapLaws<F>;
/**
* `Comonad` is the dual of {@link Monad}.
*
* Whereas Monads allow for the composition of effectful functions,
* Comonads allow for composition of functions that extract the
* value from their context.
*
* Example:
*
* ```typescript
* const F = comonadOf(Eval)
*
* F.extract(Eval.of(() => 2)) // 2
* ```
*
* Note that having an `Comonad` instance implies {@link Functor} and
* {@link CoflatMap} implementations are also available, which is why
* `Comonad` is a subtype of `Functor` and `CoflatMap`.
*
* ## Implementation notes
*
* Even though in TypeScript the Funfix library is using `abstract class` to
* express type classes, when implementing this type class it is recommended
* that you implement it as a mixin using "`implements`", instead of extending
* it directly with "`extends`". See
* [TypeScript: Mixins]{@link https://www.typescriptlang.org/docs/handbook/mixins.html}
* for details and note that we already have `applyMixins` defined.
*
* Implementation example:
*
* ```typescript
* import {
* HK, Comonad,
* registerTypeClassInstance,
* applyMixins
* } from "funfix"
*
* // Type alias defined for readability.
* // HK is our encoding for higher-kinded types.
* type BoxK<T> = HK<Box<any>, T>
*
* class Box<T> implements HK<Box<any>, T> {
* constructor(public value: T) {}
*
* // Implements HK<Box<any>, A>, not really needed, but useful in order
* // to avoid type casts. Note they can and should be undefined:
* readonly _funKindF: Box<any>
* readonly _funKindA: T
* }
*
* class BoxComonad implements Comonad<Box<any>> {
* map<A, B>(fa: BoxK<A>, f: (a: A) => B): BoxK<B> {
* const a = (fa as Box<A>).value
* return new Box(f(a))
* }
*
* coflatMap<A, B>(fa: BoxK<A>, ff: (a: BoxK<A>) => B): BoxK<B> {
* return new Box(ff(fa))
* }
*
* coflatten<A>(fa: BoxK<A>): BoxK<BoxK<A>> {
* return new Box(fa)
* }
*
* extract<A>(fa: BoxK<A>): A {
* return (fa as Box<A>).value
* }
* }
*
* // At the moment of writing, this call is not needed, but it is
* // recommended anyway to future-proof the code ;-)
* applyMixins(BoxComonad, [Comonad])
*
* // Registering global Comonad instance for Box, needed in order
* // for the `functorOf(Box)`, `coflatMapOf(Box)` and `comonadOf(Box)`
* // calls to work
* registerTypeClassInstance(Comonad)(Box, new BoxComonad())
* ```
*
* We are using `implements` in order to support multiple inheritance and to
* avoid inheriting any `static` members. In the Flow definitions (e.g.
* `.js.flow` files) for Funfix these type classes are defined with
* "`interface`", as they are meant to be interfaces that sometimes have
* default implementations and not classes.
*
* ## Credits
*
* This type class is inspired by the equivalent in Haskell's
* standard library and the implementation is inspired by the
* [Typelevel Cats]{@link http://typelevel.org/cats/} project.
*/
export declare abstract class Comonad<F> implements CoflatMap<F> {
/**
* `extract` is the dual of `pure` on {@link Monad}
* (via {@link Applicative}) and extracts the value from
* its context.
*
* Example:
*
* ```typescript
* const cm = comonadOf(Eval)
*
* cm.extract(Eval.of(() => 10)) //=> 10
* ```
*/
abstract extract<A>(fa: HK<F, A>): A;
/** Inherited from {@link Functor.map}. */
map: <A, B>(fa: HK<F, A>, f: (a: A) => B) => HK<F, B>;
/** Inherited from {@link CoflatMap.coflatMap}. */
coflatMap: <A, B>(fa: HK<F, A>, ff: (a: HK<F, A>) => B) => HK<F, B>;
/** Inherited from {@link CoflatMap.coflatten}. */
coflatten: <A>(fa: HK<F, A>) => HK<F, HK<F, A>>;
/** @hidden */
static readonly _funTypeId: string;
/** @hidden */
static readonly _funSupertypeIds: string[];
/** @hidden */
static readonly _funErasure: Comonad<any>;
}
/**
* Type class laws defined for {@link Comonad}.
*
* This is an abstract definition. In order to use it in unit testing,
* the implementor must think of a strategy to evaluate the truthiness
* of the returned `Equiv` values.
*
* Even though in TypeScript the Funfix library is using classes to
* express these laws, when implementing this class it is recommended
* that you implement it as a mixin using `implements`, instead of extending
* it directly with `extends`. See
* [TypeScript: Mixins]{@link https://www.typescriptlang.org/docs/handbook/mixins.html}
* for details and note that we already have `applyMixins` defined.
*
* We are doing this in order to support multiple inheritance and to
* avoid inheriting any `static` members. In the Flow definitions (e.g.
* `.js.flow` files) for Funfix these classes are defined with
* `interface`, as they are meant to be interfaces that sometimes have
* default implementations and not classes.
*/
export declare abstract class ComonadLaws<F> implements CoflatMapLaws<F> {
/**
* The {@link Comonad} designated instance for `F`,
* to be tested.
*/
readonly F: Comonad<F>;
/**
* ```
* fa.coflatten.extract <-> fa
* ```
*/
extractCoflattenIdentity<A>(fa: HK<F, A>): Equiv<HK<F, A>>;
/**
* ```
* fa.coflatten.map(_.extract) <-> fa
* ```
*/
mapCoflattenIdentity<A>(fa: HK<F, A>): Equiv<HK<F, A>>;
/**
* ```
* fa.map(f) <-> fa.coflatMap(fa0 => f(fa0.extract))
* ```
*/
mapCoflatMapCoherence<A, B>(fa: HK<F, A>, f: (a: A) => B): Equiv<HK<F, B>>;
/**
* ```
* fa.coflatMap(_.extract) <-> fa
* ```
*/
comonadLeftIdentity<A>(fa: HK<F, A>): Equiv<HK<F, A>>;
/**
* ```
* fa.coflatMap(f).extract <-> f(fa)
* ```
*/
comonadRightIdentity<A, B>(fa: HK<F, A>, f: (a: HK<F, A>) => B): Equiv<B>;
/** Mixed-in from {@link CoflatMapLaws.coflatMapAssociativity}. */
coflatMapAssociativity: <A, B, C>(fa: HK<F, A>, f: (a: HK<F, A>) => B, g: (b: HK<F, B>) => C) => Equiv<HK<F, C>>;
/** Mixed-in from {@link CoflatMapLaws.coflattenThroughMap}. */
coflattenThroughMap: <A>(fa: HK<F, A>) => Equiv<HK<F, HK<F, HK<F, A>>>>;
/** Mixed-in from {@link CoflatMapLaws.coflattenCoherence}. */
coflattenCoherence: <A, B>(fa: HK<F, A>, f: (a: HK<F, A>) => B) => Equiv<HK<F, B>>;
/** Mixed-in from {@link CoflatMapLaws.coflatMapIdentity}. */
coflatMapIdentity: <A>(fa: HK<F, A>) => Equiv<HK<F, HK<F, A>>>;
/** Mixed-in from {@link FunctorLaws.covariantIdentity}. */
covariantIdentity: <A>(fa: HK<F, A>) => Equiv<HK<F, A>>;
/** Mixed-in from {@link FunctorLaws.covariantComposition}. */
covariantComposition: <A, B, C>(fa: HK<F, A>, f: (a: A) => B, g: (b: B) => C) => Equiv<HK<F, C>>;
}
/**
* Given a {@link Constructor} reference, returns its associated
* {@link Comonad} instance if it exists, or throws a `NotImplementedError`
* in case there's no such association.
*
* ```typescript
* import { Option, Comonad, comonadOf } from "funfix"
*
* const F: Comonad<Option<any>> = comonadOf(Option)
* ```
*/
export declare const comonadOf: <F>(c: Constructor<F>) => Comonad<F>;
/**
* Given an {@link Comonad} instance, returns the {@link ComonadLaws}
* associated with it.
*/
export declare function comonadLawsOf<F>(instance: Comonad<F>): ComonadLaws<F>;