version 2.5

CHANGELOG.md

@@ -14,6 +14,33 @@
 **Note**: Gaps between patch versions are faulty/broken releases. **Note**: A feature tagged as Experimental is in a
 high state of flux, you're at risk of it changing without notice.
 
+# 2.5.0
+
+- **New Feature**
+  - add overloadings to `zip`, `zipWith` and `unzip`, closes #1109 (@gcanti)
+  - add `eqStrict`, closes #965 (@gcanti)
+  - `Eq`
+    - add `eqStrict`, closes #965 (@gcanti)
+  - `NonEmptyArray`
+    - add `fold` (@vicrac)
+  - `Semigroup`
+    - add `getIntercalateSemigroup` (@gcanti)
+  - `Set`
+    - add `toggle` (@ryota-ka)
+  - `TaskEither`
+    - add `tryCatchK` (@DenisFrezzato)
+  - `ReaderTaskEither`
+    - add missing `leftReaderTask`, `rightReaderTask` functions (@gcanti)
+  - `StateReaderTaskEither`
+    - add missing `Bifunctor`, `Alt` instances (@gcanti)
+- **Experimental**
+  - add `ReadonlyArray` module (@gcanti)
+  - add `ReadonlyNonEmptyArray` module (@gcanti)
+  - add `ReadonlySet` module (@gcanti)
+  - add `ReadonlyMap` module (@gcanti)
+  - add `ReadonlyRecord` module (@gcanti)
+  - add `ReadonlyTuple` module (@gcanti)
+
 # 2.4.4
 
 - **Polish**

docs/getting-started/Functor.md

@@ -0,0 +1,219 @@
+---
+title: Functor
+parent: Getting started
+nav_order: 6
+---
+
+# Getting started with fp-ts: Functor
+
+In the [last post](./Category.md) about categories I presented the _TS_ category (the TypeScript category) and the central problem with function composition
+
+> How can we compose two generic functions `f: (a: A) => B` and `g: (c: C) => D`?
+
+Why finding solutions to this problem is so important?
+
+Because if categories can be used to model programmming languages, morphisms (i.e. functions in _TS_) can be used to model **programs**.
+
+Therefore solving the problem also means to find **how to compose programs in a general way**. And _this_ is pretty interesting for a developer, isn't it?
+
+## Functions as programs
+
+We call **pure program** a function with the following signature
+
+```scala
+(a: A) => B
+```
+
+Such a signature models a program which accepts an input of type `A` and yields a result of type `B`, without any effect.
+
+We call **effectful program** a function with the following signature
+
+```scala
+(a: A) => F<B>
+```
+
+Such a signature models a program which accepts an input of type `A` and yields a result of type `B`, along with an **effect** `F`, where `F` is some type constructor.
+
+Recall that a [type constructor](https://en.wikipedia.org/wiki/Type_constructor) is an `n`-ary type operator taking as argument zero or more types, and returning another type.
+
+**Example**
+
+Given the concrete type `string`, the `Array` type constructor returns the concrete type `Array<string>`
+
+Here we are interested in `n`-ary type constructors with `n >= 1`, for example
+
+| Type constructor | Effect (interpretation)         |
+| ---------------- | ------------------------------- |
+| `Array<A>`       | a non deterministic computation |
+| `Option<A>`      | a computation that may fail     |
+| `Task<A>`        | an asynchronous computation     |
+
+Now back to our main problem
+
+> How can we compose two generic functions `f: (a: A) => B` and `g: (c: C) => D`?
+
+Since the general problem is intractable, we need to put some _constraint_ on `B` and `C`.
+
+We already know that if `B = C` then the solution is the usual function composition
+
+```ts
+function compose<A, B, C>(g: (b: B) => C, f: (a: A) => B): (a: A) => C {
+  return a => g(f(a))
+}
+```
+
+What about the other cases?
+
+## In which the constraint `B = F<C>` leads to functors
+
+Let's consider the following constraint: `B = F<C>` for some type constructor `F`, or in other words (and after some renaming)
+
+- `f: (a: A) => F<B>` is an effectful program
+- `g: (b: B) => C` is a pure program
+
+In order to compose `f` with `g` we could find a way to **lift** `g` from a function `(b: B) => C` to a function `(fb: F<B>) => F<C>` so that we can use the usual function composition (the output type of `f` would be the same as the input type of the lifted function)
+
+So we turned the original problem into another one: can we find such a `lift` function?
+
+Let's see some examples
+
+**Example** (`F = Array`)
+
+```ts
+function lift<B, C>(g: (b: B) => C): (fb: Array<B>) => Array<C> {
+  return fb => fb.map(g)
+}
+```
+
+**Example** (`F = Option`)
+
+```ts
+import { Option, isNone, none, some } from 'fp-ts/lib/Option'
+
+function lift<B, C>(g: (b: B) => C): (fb: Option<B>) => Option<C> {
+  return fb => (isNone(fb) ? none : some(g(fb.value)))
+}
+```
+
+**Example** (`F = Task`)
+
+```ts
+import { Task } from 'fp-ts/lib/Task'
+
+function lift<B, C>(g: (b: B) => C): (fb: Task<B>) => Task<C> {
+  return fb => () => fb().then(g)
+}
+```
+
+All those `lift` functions almost look the same. It's not a coincidence, there's a functional pattern under the hood.
+
+Indeed all those type constructors (and many others) admit a **functor instance**.
+
+## Functors
+
+Functors are **mappings between categories** that preserve the categorical structure, i.e. that preserve identity morphisms and composition.
+
+Since categories are constituted of two things (objects and morphisms) a functor is constituted of two things as well:
+
+- a **mapping between objects** that associates to each object `X` in _C_ an object in _D_
+- a **mapping between morphisms** that associates to each morphism in _C_ a morphism in _D_
+
+where _C_ and _D_ are two categories (aka two programming languages).
+
+<img src="./images/Functor.jpg" width="300" alt="functor" />
+<center>(source: [functor on ncatlab.org](https://ncatlab.org/nlab/show/functor))</center>
+
+Even if a mapping between two different programming languages is an intriguing idea, we are more interested in a mapping where _C_ and _D_ coincide (with _TS_). In this case we talk about **endofunctors** ("endo" means "within", "inside").
+
+From now on when I write "functor" I actually mean an endofunctor in _TS_.
+
+### Definition
+
+A functor is a pair `(F, lift)` where
+
+- `F` is a `n`-ary type constructor (`n >= 1`) which maps each type `X` to the type `F<X>` (**mapping between objects**)
+- `lift` is a function with the following signature
+
+```ts
+lift: <A, B>(f: (a: A) => B) => ((fa: F<A>) => F<B>)
+```
+
+which maps each function `f: (a: A) => B` to a function `lift(f): (fa: F<A>) => F<B>` (**mapping between morphisms**).
+
+The following properties must hold
+
+- `lift(identity`<sub>X</sub>`)` = `identity`<sub>F(X)</sub> (**identities map to identities**)
+- `lift(g ∘ f) = lift(g) ∘ lift(f)` (**mapping a composition is the composition of the mappings**)
+
+The `lift` function is also known through a variant called `map`, which is basically `lift` with the arguments rearranged
+
+```ts
+lift: <A, B>(f: (a: A) => B) => ((fa: F<A>) => F<B>)
+map:  <A, B>(fa: F<A>, f: (a: A) => B) => F<B>
+```
+
+Note that `map` can be derived from `lift` (and viceversa).
+
+## Functors in `fp-ts`
+
+How can we define a functor instance in `fp-ts`? Let's see a practical example.
+
+The following declaration defines a model for the response of an API call
+
+```ts
+interface Response<A> {
+  url: string
+  status: number
+  headers: Record<string, string>
+  body: A
+}
+```
+
+Note that the `body` field is parametrized, this makes `Response` a good candidate for a functor instance since `Response` is a `n`-ary type constructors with `n >= 1` (a necessary precondition).
+
+In order to define a functor instance for `Response` we must define a `map` function (along with some [technicalities](../recipes/HKT.md) required by `fp-ts`)
+
+```ts
+// `Response.ts` module
+
+import { Functor1 } from 'fp-ts/lib/Functor'
+
+export const URI = 'Response'
+
+export type URI = typeof URI
+
+declare module 'fp-ts/lib/HKT' {
+  interface URItoKind<A> {
+    readonly Response: Response<A>
+  }
+}
+
+export interface Response<A> {
+  url: string
+  status: number
+  headers: Record<string, string>
+  body: A
+}
+
+function map<A, B>(fa: Response<A>, f: (a: A) => B): Response<B> {
+  return { ...fa, body: f(fa.body) }
+}
+
+// functor instance for `Response`
+export const functorResponse: Functor1<URI> = {
+  URI,
+  map
+}
+```
+
+## Is the general problem solved?
+
+Not at all. Functors allow us to compose an effectful program `f` with a pure program `g`, but `g` must be **unary**, that is it must accept only one argument as input. What if `g` accepts two arguments? Or three?
+
+| Program f | Program g               | Composition   |
+| --------- | ----------------------- | ------------- |
+| pure      | pure                    | `g ∘ f`       |
+| effectful | pure (unary)            | `lift(g) ∘ f` |
+| effectful | pure (`n`-ary, `n > 1`) | ?             |
+
+In order to handle such circumstances we need something more: in the next [post](./Applicative.md) I'll talk about another remarkable abstraction of functional programming: **applicative functors**.

docs/guides/HKT.md

@@ -29,7 +29,7 @@ export type URI = typeof URI
 
 declare module 'fp-ts/lib/HKT' {
   interface URItoKind<A> {
-    Identity: Identity<A>
+    readonly Identity: Identity<A>
   }
 }
 
@@ -68,7 +68,7 @@ export interface URItoKind<A> {}
 
 declare module 'fp-ts/lib/HKT' {
   interface URItoKind<A> {
-    Identity: Identity<A> // maps the key "Identity" to the type `Identity`
+    readonly Identity: Identity<A> // maps the key "Identity" to the type `Identity`
   }
 }
 ```
@@ -95,7 +95,7 @@ export type URI = typeof URI
 
 declare module 'fp-ts/lib/HKT' {
   interface URItoKind2<E, A> {
-    Either: Either<E, A>
+    readonly Either: Either<E, A>
   }
 }
 

docs/guides/purescript.md

@@ -54,12 +54,12 @@ TypeScript
 
 ```ts
 interface Bar {
-  type: 'Bar'
-  value: string
+  readonly type: 'Bar'
+  readonly value: string
 }
 interface Baz {
-  type: 'Baz'
-  value: boolean
+  readonly type: 'Baz'
+  readonly value: boolean
 }
 // type
 type Foo = Bar | Baz
@@ -81,7 +81,7 @@ TypeScript
 ```ts
 declare module 'fp-ts/lib/HKT' {
   interface URItoKind<A> {
-    Option: Option<A>
+    readonly Option: Option<A>
   }
 }
 

docs/modules/Apply.ts.md

@@ -199,37 +199,37 @@ Tuple sequencing, i.e., take a tuple of monadic actions and does them from left-
 export function sequenceT<F extends URIS4>(
   F: Apply4<F>
 ): <S, R, E, T extends Array<Kind4<F, S, R, E, any>>>(
-  ...t: T & { 0: Kind4<F, S, R, E, any> }
+  ...t: T & { readonly 0: Kind4<F, S, R, E, any> }
 ) => Kind4<F, S, R, E, { [K in keyof T]: [T[K]] extends [Kind4<F, S, R, E, infer A>] ? A : never }>
 export function sequenceT<F extends URIS3>(
   F: Apply3<F>
 ): <R, E, T extends Array<Kind3<F, R, E, any>>>(
-  ...t: T & { 0: Kind3<F, R, E, any> }
+  ...t: T & { readonly 0: Kind3<F, R, E, any> }
 ) => Kind3<F, R, E, { [K in keyof T]: [T[K]] extends [Kind3<F, R, E, infer A>] ? A : never }>
 export function sequenceT<F extends URIS3, E>(
   F: Apply3C<F, E>
 ): <R, T extends Array<Kind3<F, R, E, any>>>(
-  ...t: T & { 0: Kind3<F, R, E, any> }
+  ...t: T & { readonly 0: Kind3<F, R, E, any> }
 ) => Kind3<F, R, E, { [K in keyof T]: [T[K]] extends [Kind3<F, R, E, infer A>] ? A : never }>
 export function sequenceT<F extends URIS2>(
   F: Apply2<F>
 ): <E, T extends Array<Kind2<F, E, any>>>(
-  ...t: T & { 0: Kind2<F, E, any> }
+  ...t: T & { readonly 0: Kind2<F, E, any> }
 ) => Kind2<F, E, { [K in keyof T]: [T[K]] extends [Kind2<F, E, infer A>] ? A : never }>
 export function sequenceT<F extends URIS2, E>(
   F: Apply2C<F, E>
 ): <T extends Array<Kind2<F, E, any>>>(
-  ...t: T & { 0: Kind2<F, E, any> }
+  ...t: T & { readonly 0: Kind2<F, E, any> }
 ) => Kind2<F, E, { [K in keyof T]: [T[K]] extends [Kind2<F, E, infer A>] ? A : never }>
 export function sequenceT<F extends URIS>(
   F: Apply1<F>
 ): <T extends Array<Kind<F, any>>>(
-  ...t: T & { 0: Kind<F, any> }
+  ...t: T & { readonly 0: Kind<F, any> }
 ) => Kind<F, { [K in keyof T]: [T[K]] extends [Kind<F, infer A>] ? A : never }>
 export function sequenceT<F>(
   F: Apply<F>
 ): <T extends Array<HKT<F, any>>>(
-  ...t: T & { 0: HKT<F, any> }
+  ...t: T & { readonly 0: HKT<F, any> }
 ) => HKT<F, { [K in keyof T]: [T[K]] extends [HKT<F, infer A>] ? A : never }> { ... }
 ```