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Effect profile image Michael Arnaldi
Michael Arnaldi for Effect

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Encoding HKTs in TS4.1

There have been many attempts at finding valid HKT encodings in TypeScript. Currently the most used and reliable one is implemented by fp-ts.

In fp-ts all the types are recorded into a number of type-level maps that index URI -> Concrete Type and this map is different for each kind:

export interface URItoKind<A> {}

export interface URItoKind2<E, A> {}

export interface URItoKind3<R, E, A> {}

export interface URItoKind4<S, R, E, A> {}
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Those type-level records are filled progressively by using the module augmentation feature. Let's look at how Either & Option are wired in those records:

export const URI = "Either"

export type URI = typeof URI

declare module "./HKT" {
  interface URItoKind2<E, A> {
    readonly [URI]: Either<E, A>
  }
}
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export const URI = "Option"

export type URI = typeof URI

declare module "./HKT" {
  interface URItoKind<A> {
    readonly [URI]: Option<A>
  }
}
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After augmentation the records will look like:

export interface URItoKind<A> {
    Option: Option<A>
    ...
}

export interface URItoKind2<E, A> {
    Either: Either<E, A>
    ...
}

export interface URItoKind3<R, E, A> {
    ReaderTaskEither: ReaderTaskEither<R, E, A>
    ...
}

export interface URItoKind4<S, R, E, A> {
    StateReaderTaskEither: StateReaderTaskEither<S, R, E, A>
    ...
}
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Accessing those types requires getting the proper record key and filling in the params:

type Result = URItoKind<string, number>["Either"]
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Which corresponds to:

Either<string, number>
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Using this method fp-ts defines:

export type URIS = keyof URItoKind<any>
export type URIS2 = keyof URItoKind2<any, any>
export type URIS3 = keyof URItoKind3<any, any, any>
export type URIS4 = keyof URItoKind4<any, any, any, any>

export type Kind<URI extends URIS, A> = URI extends URIS ? URItoKind<A>[URI] : any
export type Kind2<URI extends URIS2, E, A> = URI extends URIS2
  ? URItoKind2<E, A>[URI]
  : any
export type Kind3<URI extends URIS3, R, E, A> = URI extends URIS3
  ? URItoKind3<R, E, A>[URI]
  : any
export type Kind4<URI extends URIS4, S, R, E, A> = URI extends URIS4
  ? URItoKind4<S, R, E, A>[URI]
  : any
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we can now access:

type Result = Kind2<"Either", string, number>
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With these constructs it’s possible to write generic interfaces that don't specify the URI.

For example, we can write:

export interface Functor1<F extends URIS> {
  readonly URI: F
  readonly map: <A, B>(fa: Kind<F, A>, f: (a: A) => B) => Kind<F, B>
}
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and have:

const functorOption: Functor1<"Option"> = {
    URI: "Option",
    map: ... // map is now correctly typed to work with Option<*>
}
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Clearly, this is not enough to generalise over different kinds. In fp-ts you will find multiple definitions of every typeclass (interface with a generic URI, for this matter) for each of the kind.

export interface Functor1<F extends URIS> {
  readonly URI: F
  readonly map: <A, B>(fa: Kind<F, A>, f: (a: A) => B) => Kind<F, B>
}

export interface Functor2<F extends URIS2> {
  readonly URI: F
  readonly map: <E, A, B>(fa: Kind2<F, E, A>, f: (a: A) => B) => Kind2<F, E, B>
}

export interface Functor2C<F extends URIS2, E> {
  readonly URI: F
  readonly _E: E
  readonly map: <A, B>(fa: Kind2<F, E, A>, f: (a: A) => B) => Kind2<F, E, B>
}

export interface Functor3<F extends URIS3> {
  readonly URI: F
  readonly map: <R, E, A, B>(fa: Kind3<F, R, E, A>, f: (a: A) => B) => Kind3<F, R, E, B>
}

export interface Functor3C<F extends URIS3, E> {
  readonly URI: F
  readonly _E: E
  readonly map: <R, A, B>(fa: Kind3<F, R, E, A>, f: (a: A) => B) => Kind3<F, R, E, B>
}

export interface Functor4<F extends URIS4> {
  readonly URI: F
  readonly map: <S, R, E, A, B>(
    fa: Kind4<F, S, R, E, A>,
    f: (a: A) => B
  ) => Kind4<F, S, R, E, B>
}
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As we can see, in addition to the 4 kinds, we also have *C interfaces that are used to add a constraint to the E parameter. This is used in Validation where E represents the Error channel and we ask Monoid<E> to eventually combine errors together.

Let's now look at how to use those typeclasses. How can we write a function that consumes a generic Functor?

Starting from the base case, we want a generic function addOne that works by mapping a number output by adding one:

function addOne<URI extends URIS>(F: Functor1<URI>) {
  return (fa: Kind<F, number>): Kind<F, number> => F.map(fa, (n) => n + 1)
}
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Calling this function with the appropriate typeclass instance it will yield a specific function for the data-type.

const addOneOption = addOne(functorOption) // (fa: Option<number>) => Option<number>
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We can generalise further and support different kinds via overloading:

function addOne<URI extends URIS4, E>(
  F: Functor4C<URI, E>
): <S, R>(fa: Kind4<F, S, R, E, number>) => Kind4<F, S, R, E, number>
function addOne<URI extends URIS4>(
  F: Functor4<URI>
): <S, R, E>(fa: Kind4<F, S, R, E, number>) => Kind4<F, S, R, E, number>
function addOne<URI extends URIS3, E>(
  F: Functor3C<URI, E>
): <R>(fa: Kind3<F, R, E, number>) => Kind3<F, R, E, number>
function addOne<URI extends URIS3>(
  F: Functor3<URI>
): <R, E>(fa: Kind3<F, R, E, number>) => Kind3<F, R, E, number>
function addOne<URI extends URIS2, E>(
  F: Functor2C<URI, E>
): (fa: Kind2<F, E, number>) => Kind2<F, E, number>
function addOne<URI extends URIS2>(
  F: Functor2<URI>
): <E>(fa: Kind2<F, E, number>) => Kind<F, E, number>
function addOne<URI extends URIS>(
  F: Functor1<URI>
): (fa: Kind<F, number>) => Kind<F, number>
function addOne(F: any) {
  return (fa: any): any => F.map(fa, (n) => n + 1)
}
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The only trouble is defining the very scary base case (any, any, any).

In fp-ts we can use HKT defined as:

export interface HKT<URI, A> {
  readonly _URI: URI
  readonly _A: A
}

export interface HKT2<URI, E, A> extends HKT<URI, A> {
  readonly _E: E
}

export interface HKT3<URI, R, E, A> extends HKT2<URI, E, A> {
  readonly _R: R
}

export interface HKT4<URI, S, R, E, A> extends HKT3<URI, R, E, A> {
  readonly _S: S
}
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Now we can define a specific Functor interface for HKT like:

export interface Functor<F> {
  readonly URI: F
  readonly map: <A, B>(fa: HKT<F, A>, f: (a: A) => B) => HKT<F, B>
}
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and use this to type the base case:

function addOne<URI extends URIS4, E>(
  F: Functor4C<URI, E>
): <S, R>(fa: Kind4<F, S, R, E, number>) => Kind4<F, S, R, E, number>
function addOne<URI extends URIS4>(
  F: Functor4<URI>
): <S, R, E>(fa: Kind4<F, S, R, E, number>) => Kind4<F, S, R, E, number>
function addOne<URI extends URIS3, E>(
  F: Functor3C<URI, E>
): <R>(fa: Kind3<F, R, E, number>) => Kind3<F, R, E, number>
function addOne<URI extends URIS3>(
  F: Functor3<URI>
): <R, E>(fa: Kind3<F, R, E, number>) => Kind3<F, R, E, number>
function addOne<URI extends URIS2, E>(
  F: Functor2C<URI, E>
): (fa: Kind2<F, E, number>) => Kind2<F, E, number>
function addOne<URI extends URIS2>(
  F: Functor2<URI>
): <E>(fa: Kind2<F, E, number>) => Kind<F, E, number>
function addOne<URI extends URIS>(
  F: Functor1<URI>
): (fa: Kind<F, number>) => Kind<F, number>
function addOne<URI>(F: Functor<URI>) {
  return (fa: HKT<URI, number>): HKT<URI, number> => F.map(fa, (n) => n + 1)
}
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Short and practical, isn't it? Let’s write a composition of functors:

export interface FunctorComposition<F, G> {
  readonly map: <A, B>(fa: HKT<F, HKT<G, A>>, f: (a: A) => B) => HKT<F, HKT<G, B>>
}

export interface FunctorCompositionHKT1<F, G extends URIS> {
  readonly map: <A, B>(fa: HKT<F, Kind<G, A>>, f: (a: A) => B) => HKT<F, Kind<G, B>>
}

export interface FunctorCompositionHKT2<F, G extends URIS2> {
  readonly map: <E, A, B>(fa: HKT<F, Kind2<G, E, A>>, f: (a: A) => B) => HKT<F, Kind2<G, E, B>>
}

export interface FunctorCompositionHKT2C<F, G extends URIS2, E> {
  readonly map: <A, B>(fa: HKT<F, Kind2<G, E, A>>, f: (a: A) => B) => HKT<F, Kind2<G, E, B>>
}

export interface FunctorComposition11<F extends URIS, G extends URIS> {
  readonly map: <A, B>(fa: Kind<F, Kind<G, A>>, f: (a: A) => B) => Kind<F, Kind<G, B>>
}

export interface FunctorComposition12<F extends URIS, G extends URIS2> {
  readonly map: <E, A, B>(fa: Kind<F, Kind2<G, E, A>>, f: (a: A) => B) => Kind<F, Kind2<G, E, B>>
}

export interface FunctorComposition12C<F extends URIS, G extends URIS2, E> {
  readonly map: <A, B>(fa: Kind<F, Kind2<G, E, A>>, f: (a: A) => B) => Kind<F, Kind2<G, E, B>>
}

export interface FunctorComposition21<F extends URIS2, G extends URIS> {
  readonly map: <E, A, B>(fa: Kind2<F, E, Kind<G, A>>, f: (a: A) => B) => Kind2<F, E, Kind<G, B>>
}

export interface FunctorComposition2C1<F extends URIS2, G extends URIS, E> {
  readonly map: <A, B>(fa: Kind2<F, E, Kind<G, A>>, f: (a: A) => B) => Kind2<F, E, Kind<G, B>>
}

export interface FunctorComposition22<F extends URIS2, G extends URIS2> {
  readonly map: <FE, GE, A, B>(fa: Kind2<F, FE, Kind2<G, GE, A>>, f: (a: A) => B) => Kind2<F, FE, Kind2<G, GE, B>>
}

export interface FunctorComposition22C<F extends URIS2, G extends URIS2, E> {
  readonly map: <FE, A, B>(fa: Kind2<F, FE, Kind2<G, E, A>>, f: (a: A) => B) => Kind2<F, FE, Kind2<G, E, B>>
}

export interface FunctorComposition23<F extends URIS2, G extends URIS3> {
  readonly map: <FE, R, E, A, B>(fa: Kind2<F, FE, Kind3<G, R, E, A>>, f: (a: A) => B) => Kind2<F, FE, Kind3<G, R, E, B>>
}

export interface FunctorComposition23C<F extends URIS2, G extends URIS3, E> {
  readonly map: <FE, R, A, B>(fa: Kind2<F, FE, Kind3<G, R, E, A>>, f: (a: A) => B) => Kind2<F, FE, Kind3<G, R, E, B>>
}
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And that’s not even complete...

Another limitation is that every single type needs to be independently indexed. This makes typeclass transformers extremely impractical.

Our goal is to have the same features (or a bit more) with significantly less boilerplate.

Let’s now look at a restricted version of the encoding used in @effect-ts/core. While @effect-ts/core allows for as much as 10 type parameters (2 of which are used to encode generic keys, i.e. nominal keys for records, generic keys for maps, integer keys for arrays, etc), let’s limit ourselves to 4 for simplicity (the same number as in fp-ts).

The full code is available at:
https://github.com/Matechs-Garage/matechs-effect/tree/master/packages/core/src/Prelude/HKT

And the typeclasses (inspired by zio-prelude):

https://github.com/Matechs-Garage/matechs-effect/tree/master/packages/core/src/Prelude

The first idea is to shrink the number of type-level records, instead of having multiple records we are only going to have one:

export interface URItoKind<S, R, E, A> {
  XPure: XPure<S, S, R, E, A>
  Effect: Effect<R, E, A>
  Either: Either<E, A>
  Option: Option<A>
}

export type URIS = keyof URItoKind<any, any, any, any>
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We can then temporarily define the Kind to be:

export type Kind<F extends URIS, S, R, E, A> = URItoKind<S, R, E, A>[F]
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This already removes quite a bit of boilerplate. We can then define typeclasses as follows:

export interface Functor<F extends URIS> {
  URI: F
  map: <A, A2>(
    f: (a: A) => A2
  ) => <S, R, E>(fa: Kind<F, S, R, E, A>) => Kind<F, S, R, E, A2>
}
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and instances:

export const functorOption: Functor<"Option"> = {
  URI: "Option",
  map: (f) => (fa) => (fa._tag === "None" ? fa : { _tag: "Some", value: f(fa.value) })
}
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and Bifunctor

export interface Bifunctor<F extends URIS> {
  URI: F
  bimap: <A, A2, E, E2>(
    f: (a: A) => A2,
    g: (a: E) => E2
  ) => <S, R>(fa: Kind<F, S, R, E, A>) => Kind<F, S, R, E2, A2>
}
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and instances:

export const bifunctorEither: Bifunctor<"Either"> = {
  URI: "Either",
  bimap: (f, g) => (fa) =>
    fa._tag === "Left"
      ? { _tag: "Left", left: g(fa.left) }
      : { _tag: "Right", right: f(fa.right) }
}
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Looking better, but how are we going to encode constraints like fixing "E" to a specific value?

The answer is to add a generic C to hold the constraints:

export type Param = "S" | "R" | "E"

export interface Fix<P extends Param, K> {
  Fix: {
    [p in P]: K
  }
}

export type OrFix<P extends Param, C, V> = C extends Fix<P, infer K> ? K : V

export type Kind<F extends URIS, C, S, R, E, A> = URItoKind<
  OrFix<"S", C, S>,
  OrFix<"R", C, R>,
  OrFix<"E", C, E>,
  A
>[F]
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and change our typeclasses to become:

export interface Functor<F extends URIS, C = {}> {
  URI: F
  map: <A, A2>(
    f: (a: A) => A2
  ) => <S, R, E>(fa: Kind<F, C, S, R, E, A>) => Kind<F, C, S, R, E, A2>
}

export interface Bifunctor<F extends URIS, C = {}> {
  URI: F
  bimap: <A, A2, E, E2>(
    f: (a: A) => A2,
    g: (a: OrFix<"E", C, E>) => OrFix<"E", C, E2>
  ) => <S, R>(fa: Kind<F, C, S, R, E, A>) => Kind<F, C, S, R, E2, A2>
}
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the code of the instances didn't change at all, but we can now create a constrained instance in this way:

export const bifunctorStringValidation: Bifunctor<"Either", Fix<"E", string>> = {
  URI: "Either",
  bimap: (f, g) => (fa) =>
    fa._tag === "Left"
      ? { _tag: "Left", left: g(fa.left) }
      : { _tag: "Right", right: f(fa.right) }
}

// <A, A2, E, E2>(f: (a: A) => A2, g: (a: string) => string) => <S, R>(fa: Either<string, A>) => Either<string, A2>
export const bimapValidation = bifunctorStringValidation.bimap
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We have a couple more unused parameters but we get the signature we wanted.

Unfortunately there is no way to remove those phantom types without multiple registries and much more boilerplate, but in the end, who cares?

In a single definition we can now compact multiple kinds and multiple potential constraints, in fact Fix is safe for intersection so we could have written Fix<"E", string> & Fix<"S", number>.

Our addOne function looks much better:

function addOne<URI extends URIS, C>(
  F: Functor<URI, C>
): <S, R, E>(fa: Kind<URI, C, S, R, E, number>) => Kind<URI, C, S, R, E, number> {
  return F.map((n) => n + 1)
}
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We could leave it here and already have a decent save but there is a downside; Errors in the generic implementation tend to become unreadable. The reason being, URIS is a very big union and any type error will basically try every possible combination generating unusable error messages.

We can take inspiration from fp-ts in defining one HKT to write the base implementation and leave the rest to overloads, but we don't really want to define separated typeclasses for HKT so we will add HKT into the registry:

export interface F_<A> {
  _tag: "F_"
  A: A
}

export interface URItoKind<S, R, E, A> {
  F_: F_<A>
  XPure: XPure<S, S, R, E, A>
  Effect: Effect<R, E, A>
  Either: Either<E, A>
  Option: Option<A>
}
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and we can now define the base case in terms of "F_":

function addOne<URI extends URIS, C>(
  F: Functor<URI, C>
): <S, R, E>(fa: Kind<URI, C, S, R, E, number>) => Kind<URI, C, S, R, E, number>
function addOne(F: Functor<"F_">): (fa: F_<number>) => F_<number> {
  return F.map((n) => n + 1)
}
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Any type error in the implementation will now be specific to "F_".
However, there is a problem with this solution of "F_".

If we have a single HKT it's fine but what if we have multiple?

For example in cases like getFunctorComposition:

export function getFunctorComposition<F, G>(F: Functor<F>, G: Functor<G>): FunctorComposition<F, G> {
  return {
    map: (fa, f) => F.map(fa, (ga) => G.map(ga, f))
  }
}
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For that we are going to add multiple "fake" types in the registry taking care of discriminating them using a "_tag" field:

export interface F_<A> {
  _tag: "F_"
  A: A
}
export interface G_<A> {
  _tag: "G_"
  A: A
}
export interface H_<A> {
  _tag: "H_"
  A: A
}

export interface URItoKind<S, R, E, A> {
  F_: F_<A>
  G_: G_<A>
  H_: H_<A>
  XPure: XPure<S, S, R, E, A>
  Effect: Effect<R, E, A>
  Either: Either<E, A>
  Option: Option<A>
}
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In this way we can safely "name" multiple HKTs making sure that they cannot be mixed, with a bit more work we could also extend the logic of Kind to accept another form of non primitive URIs that allow for embedding of custom parameters and with that discriminate HKTs like functor-composition-in-core.

Good enough? Not even close, we are going to attempt transformers.
What if we want a Functor for Either<E, Option<A>>? How can we do that without reindexing?

The idea is to make the URI of Kind composable, we can use a variadic tuple to represent a list of URIS and have Kind recursively build up the type:

export type KindURI = [URIS, ...URIS[]]

export type Kind<F extends KindURI, C, S, R, E, A> = ((...args: F) => any) extends (
  head: infer H,
  ...rest: infer Rest
) => any
  ? H extends URIS
    ? Rest extends KindURI
      ? URItoKind<
          OrFix<"S", C, S>,
          OrFix<"R", C, R>,
          OrFix<"E", C, E>,
          Kind<Rest, C, S, R, E, A>
        >[H]
      : URItoKind<OrFix<"S", C, S>, OrFix<"R", C, R>, OrFix<"E", C, E>, A>[H]
    : never
  : never
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our typeclasses and instances change accordingly:

export interface Functor<F extends KindURI, C = {}> {
  URI: F
  map: <A, A2>(
    f: (a: A) => A2
  ) => <S, R, E>(fa: Kind<F, C, S, R, E, A>) => Kind<F, C, S, R, E, A2>
}

export interface Bifunctor<F extends KindURI, C = {}> {
  URI: F
  bimap: <A, A2, E, E2>(
    f: (a: A) => A2,
    g: (a: OrFix<"E", C, E>) => OrFix<"E", C, E2>
  ) => <S, R>(fa: Kind<F, C, S, R, E, A>) => Kind<F, C, S, R, E2, A2>
}

export const functorOption: Functor<["Option"]> = {
  URI: ["Option"],
  map: (f) => (fa) => (fa._tag === "None" ? fa : { _tag: "Some", value: f(fa.value) })
}

export const bifunctorEither: Bifunctor<["Either"]> = {
  URI: ["Either"],
  bimap: (f, g) => (fa) =>
    fa._tag === "Left"
      ? { _tag: "Left", left: g(fa.left) }
      : { _tag: "Right", right: f(fa.right) }
}

export const bifunctorStringValidation: Bifunctor<["Either"], Fix<"E", string>> = {
  URI: ["Either"],
  bimap: (f, g) => (fa) =>
    fa._tag === "Left"
      ? { _tag: "Left", left: g(fa.left) }
      : { _tag: "Right", right: f(fa.right) }
}

function addOne<URI extends KindURI, C>(
  F: Functor<URI, C>
): <S, R, E>(fa: Kind<URI, C, S, R, E, number>) => Kind<URI, C, S, R, E, number>
function addOne<C>(F: Functor<["F_"], C>): (fa: F_<number>) => F_<number> {
  return F.map((n) => n + 1)
}
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But now we can do:

export const functorEitherOption: Functor<["Either", "Option"], {}> = {
  URI: ["Either", "Option"],
  map: (f) => (fa) =>
    fa._tag === "Left"
      ? { _tag: "Left", left: fa.left }
      : fa.right._tag === "None"
      ? { _tag: "Right", right: { _tag: "None" } }
      : { _tag: "Right", right: { _tag: "Some", value: f(fa.right.value) } }
}
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Without any reindex we have created an instance of a composed type. To prove it works we can use our addOne:

// <S, R, E>(fa: Either<E, Option<number>>) => Either<E, Option<number>>
const addOneEitherOption = addOne(functorEitherOption)
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Apart from the excessive phantom types the signature is what we wanted (like before).

In addition, @effect-ts/core uses the C parameter to encode parameter variance and defines utility types to mix generic parameters which respect the annotated variance.

For that check the code at https://github.com/Matechs-Garage/matechs-effect!

This article code extracted to: https://gist.github.com/mikearnaldi/7388dcf4eda013d806858d945c574fbb

Top comments (12)

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beraliv profile image
beraliv •

Hey Michael!

Great article, I come back to it already 3rd time, thank you for your time!

Just wanted to let you know that currently links to github.com/Matechs-Garage/matechs-... and github.com/Matechs-Garage/matechs-... are not working

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smolattack profile image
smolattack •

Is this the official documentation for effect-ts? Will there be a blog that demonstrates some beginner level CRUD style code? Something like a form that accepts an image on the frontend and a backend API/resolver that persists some data, calls a 3rd party service (e.g., to resize the the uploaded image or send an email, etc...). You know, basic but real-life, used out in the wild type of code.

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mikearnaldi profile image
Michael Arnaldi •

Oh by all means it is not supposed to be the official doc, the doc will be created once we have finished porting the api of ZIO 2.0 (planned in a few months from now). Up until the doc arrives this blog series can be an intro to more advanced users. To address beginner friendlier intros we have just launched the "Effect Time" series youtube.com/playlist?list=PLDf3uQL... where every 2 weeks we cover real-word usage.

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smolattack profile image
smolattack •

Thanks I'll have a look. I hope you can cover the following in future series (or maybe it could go in the documentation):

  • conventions of effect-ts. For example, what do some common words mean like functions that end with with (e.g., succeedWith), why are new and extends keywords used (having played with fp-ts a bit, I thought those were 'javascript, the bad parts'). Also, generators are weird, why use them (I think it has something to do with addressing limitations of typescript but a full explanation would be great)?

  • Something along the lines of 'Build a blog, the effect-ts way'. So a your usual react, graphql, postgres/prisma deal but using effect-ts to do the business logic, validations, data fetching, etc... I think something like that would be a great intro to some FP basics (Either, Maybe, Tasks, opaque types, Codecs, etc...). If you do decide to create this, please avoid a common trope these days of building it the 'wrong way' and then slowly fixing it throughout the series. Imagine if piano lessons were taught that way (if you didn't finish the full course, you'd just walk away with a bunch of hard-to-break bad habits).

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mikearnaldi profile image
Michael Arnaldi •

  • the convention of With is still in discussion and may change in the future, it currently identify a lazy version of a function like T.succeedWith(() => 0) vs T.succeed(0) an alternative that we are considering is instead T.succeed(() => 0) and T.succeedNow(0) the reason being it is more common and usually more correct to use the lazy versions (the eager ones are better in some very specific cases for perf but risky to use).

  • new and extends, in this we do not agree with what "the bad parts" are, as a language TypeScript is not Haskell, it is a language that encompass both objects and functions as first-class only focusing on some restricted components limit the possibilities a lot, namely classes are extremely helpful as they are the only typescript construct (apart from enums) that both define a term symbol and a type symbol, namely a class is both a type and a concrete blueprint of an object, this enables for DSLs such as the one exposed in @effect-ts/core/Case that simulates data classes with hash+equals+copy methods.

  • generators this to be honest is trickery and a good degree of magic, the idea was to simulate a monadic do in a way that is native to the language (like an async-await would do for Promise), it turns out to be incredibly powerful as the combination of the generator function with a specialised adapter allows for lifting of a set of natural transformations into the context allowing you to mix and match types, like having a generator of effects that yields Tag, Either, Option, etc out of the box. There is a performance penalty because generators aren't that fast especially if down-level is applied but they are still as fast as async-await (and effect is faster than promise so overall generator+effect is faster than plain async-await) except for multi-shot effects like Array or Stream or Chunk where we have to defensively copy the generator replaying state reconstruction at every element. One of the commonly used patterns is for example to use generators for service construction that usually requires dependencies and flexibility but only runs once at bootstrap time while still use the classic pipe for concrete implementation of procedures.

  • There is some effort ongoing in making some material of the mentioned kind available and we have started a weekly series on youtube where we discuss all things effect with concrete usage and live coding: youtube.com/playlist?list=PLDf3uQL...

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sobolevn profile image
Nikita Sobolev •

We are working on the same thing, but in python: github.com/dry-python/returns

I would love to hear your feedback!

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mikearnaldi profile image
Michael Arnaldi •

Python... That's unexpected will love to take a look (I would really like to restructure many of the python services I have, definitely see the utility)

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mateiadrielrafael profile image
Matei Adriel •

What about this encoding? github.com/pelotom/hkts

It seems way nicer

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mikearnaldi profile image
Michael Arnaldi • • Edited

The known limitations listed in the README.md are enough, and unfortunately the issue noted as "it might solve it" doesn't actually solve the problem.

On the same idea another attempt was recently made: github.com/TylorS/hkt-ts and it did solve part of the issues but without ending up with less boilerplate, a small discussion on this: github.com/gcanti/fp-ts/issues/1276.

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mateiadrielrafael profile image
Matei Adriel •

Nr 2 was solved by recent ts versions and 3 is a problem with type inference in ts in general (eg: I have similar issues w fp-ts)

Nr 1 can be avoided as long as you don't use the ts equivalent of rank n types (which seems like a fair cost for the simplicity of the implementation)

But fp-ts does it the long way, so what do I know

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mikearnaldi profile image
Michael Arnaldi • • Edited

Nr 1 can be avoided as long as you don't use the ts equivalent of rank n types (which seems like a fair cost for the simplicity of the implementation).

If that is your objective yes, you can probably simplify it further.

In that encoding I still don't find how to make transformers work like:
github.com/Matechs-Garage/matechs-...

Personally I prefer making the low level implementation complex (as it does not target user-land code) and provide more features that works smoothly.

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johngurin profile image
JohnGurin •

Should be

function addOne<URI extends URIS>(F: Functor1<URI>) {
  return (fa: Kind</*F*/URI, number>): Kind</*F*/URI, number> => F.map(fa, (n) => n + 1)
}
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