add mapError

docs/modules/EitherT.ts.md

@@ -559,22 +559,22 @@ Added in v2.10.0
 ```ts
 export declare function mapLeft<F extends URIS3>(
   F: Functor3<F>
-): <E, G>(f: (e: E) => G) => <R, FE, A>(fea: Kind3<F, R, FE, Either<E, A>>) => Kind3<F, R, FE, Either<G, A>>
+): <E, G>(f: (e: E) => G) => <R, FE, A>(self: Kind3<F, R, FE, Either<E, A>>) => Kind3<F, R, FE, Either<G, A>>
 export declare function mapLeft<F extends URIS3, FE>(
   F: Functor3C<F, FE>
-): <E, G>(f: (e: E) => G) => <R, A>(fea: Kind3<F, R, FE, Either<E, A>>) => Kind3<F, R, FE, Either<G, A>>
+): <E, G>(f: (e: E) => G) => <R, A>(self: Kind3<F, R, FE, Either<E, A>>) => Kind3<F, R, FE, Either<G, A>>
 export declare function mapLeft<F extends URIS2>(
   F: Functor2<F>
-): <E, G>(f: (e: E) => G) => <FE, A>(fea: Kind2<F, FE, Either<E, A>>) => Kind2<F, FE, Either<G, A>>
+): <E, G>(f: (e: E) => G) => <FE, A>(self: Kind2<F, FE, Either<E, A>>) => Kind2<F, FE, Either<G, A>>
 export declare function mapLeft<F extends URIS2, FE>(
   F: Functor2C<F, FE>
-): <E, G>(f: (e: E) => G) => <A>(fea: Kind2<F, FE, Either<E, A>>) => Kind2<F, FE, Either<G, A>>
+): <E, G>(f: (e: E) => G) => <A>(self: Kind2<F, FE, Either<E, A>>) => Kind2<F, FE, Either<G, A>>
 export declare function mapLeft<F extends URIS>(
   F: Functor1<F>
-): <E, G>(f: (e: E) => G) => <A>(fea: Kind<F, Either<E, A>>) => Kind<F, Either<G, A>>
+): <E, G>(f: (e: E) => G) => <A>(self: Kind<F, Either<E, A>>) => Kind<F, Either<G, A>>
 export declare function mapLeft<F>(
   F: Functor<F>
-): <E, G>(f: (e: E) => G) => <A>(fea: HKT<F, Either<E, A>>) => HKT<F, Either<G, A>>
+): <E, G>(f: (e: E) => G) => <A>(self: HKT<F, Either<E, A>>) => HKT<F, Either<G, A>>
 ```
 
 Added in v2.10.0

docs/modules/IOEither.ts.md

@@ -48,7 +48,7 @@ Added in v2.0.0
   - [getApplicativeIOValidation](#getapplicativeiovalidation)
   - [getOrElse](#getorelse)
   - [getOrElseW](#getorelsew)
-  - [mapLeft](#mapleft)
+  - [mapError](#maperror)
   - [orElse](#orelse)
   - [orElseFirstIOK](#orelsefirstiok)
   - [orElseW](#orelsew)
@@ -89,6 +89,7 @@ Added in v2.0.0
   - [chainOptionKW](#chainoptionkw)
   - [chainW](#chainw)
   - [fromOptionK](#fromoptionk)
+  - [mapLeft](#mapleft)
   - [orElseFirst](#orelsefirst)
   - [orElseFirstW](#orelsefirstw)
 - [lifting](#lifting)
@@ -515,17 +516,32 @@ export declare const getOrElseW: <E, B>(onLeft: (e: E) => I.IO<B>) => <A>(ma: IO
 
 Added in v2.6.0
 
-## mapLeft
+## mapError
 
-Map a function over the first type argument of a bifunctor.
+Returns a `IOEither` with its error channel mapped using the specified function.
 
 **Signature**
 
 ```ts
-export declare const mapLeft: <E, G>(f: (e: E) => G) => <A>(fa: IOEither<E, A>) => IOEither<G, A>
+export declare const mapError: {
+  <E, G>(f: (e: E) => G): <A>(self: IOEither<E, A>) => IOEither<G, A>
+  <E, A, G>(self: IOEither<E, A>, f: (e: E) => G): IOEither<G, A>
+}
 ```
 
-Added in v2.0.0
+**Example**
+
+```ts
+import * as IOEither from 'fp-ts/IOEither'
+import * as Either from 'fp-ts/Either'
+
+const f = (s: string) => new Error(s)
+
+assert.deepStrictEqual(IOEither.mapError(IOEither.right(1), f)(), Either.right(1))
+assert.deepStrictEqual(IOEither.mapError(IOEither.left('err'), f)(), Either.left(new Error('err')))
+```
+
+Added in v2.16.0
 
 ## orElse
 
@@ -977,6 +993,18 @@ export declare const fromOptionK: <E>(
 
 Added in v2.10.0
 
+## mapLeft
+
+Alias of `mapError`.
+
+**Signature**
+
+```ts
+export declare const mapLeft: <E, G>(f: (e: E) => G) => <A>(fa: IOEither<E, A>) => IOEither<G, A>
+```
+
+Added in v2.0.0
+
 ## orElseFirst
 
 Alias of `tapError`.

docs/modules/ReaderEither.ts.md

@@ -45,7 +45,7 @@ Added in v2.0.0
   - [getApplicativeReaderValidation](#getapplicativereadervalidation)
   - [getOrElse](#getorelse)
   - [getOrElseW](#getorelsew)
-  - [mapLeft](#mapleft)
+  - [mapError](#maperror)
   - [orElse](#orelse)
   - [orElseW](#orelsew)
   - [orLeft](#orleft)
@@ -79,6 +79,7 @@ Added in v2.0.0
   - [chainOptionKW](#chainoptionkw)
   - [chainW](#chainw)
   - [fromOptionK](#fromoptionk)
+  - [mapLeft](#mapleft)
   - [orElseFirst](#orelsefirst)
   - [orElseFirstW](#orelsefirstw)
 - [lifting](#lifting)
@@ -537,17 +538,32 @@ export declare const getOrElseW: <R2, E, B>(
 
 Added in v2.6.0
 
-## mapLeft
+## mapError
 
-Map a function over the second type argument of a bifunctor.
+Returns a `ReaderEither` with its error channel mapped using the specified function.
 
 **Signature**
 
 ```ts
-export declare const mapLeft: <E, G>(f: (e: E) => G) => <R, A>(fa: ReaderEither<R, E, A>) => ReaderEither<R, G, A>
+export declare const mapError: {
+  <R, E, G>(f: (e: E) => G): <A>(self: ReaderEither<R, E, A>) => ReaderEither<R, G, A>
+  <R, E, A, G>(self: ReaderEither<R, E, A>, f: (e: E) => G): ReaderEither<R, G, A>
+}
 ```
 
-Added in v2.0.0
+**Example**
+
+```ts
+import * as ReaderEither from 'fp-ts/ReaderEither'
+import * as Either from 'fp-ts/Either'
+
+const f = (s: string) => new Error(s)
+
+assert.deepStrictEqual(ReaderEither.mapError(ReaderEither.right(1), f)({}), Either.right(1))
+assert.deepStrictEqual(ReaderEither.mapError(ReaderEither.left('err'), f)({}), Either.left(new Error('err')))
+```
+
+Added in v2.16.0
 
 ## orElse
 
@@ -946,6 +962,18 @@ export declare const fromOptionK: <E>(
 
 Added in v2.10.0
 
+## mapLeft
+
+Alias of `mapError`.
+
+**Signature**
+
+```ts
+export declare const mapLeft: <E, G>(f: (e: E) => G) => <R, A>(fa: ReaderEither<R, E, A>) => ReaderEither<R, G, A>
+```
+
+Added in v2.0.0
+
 ## orElseFirst
 
 Alias of `tapError`.

docs/modules/ReaderTaskEither.ts.md

@@ -61,7 +61,7 @@ Added in v2.0.0
   - [getApplicativeReaderTaskValidation](#getapplicativereadertaskvalidation)
   - [getOrElse](#getorelse)
   - [getOrElseW](#getorelsew)
-  - [mapLeft](#mapleft)
+  - [mapError](#maperror)
   - [orElse](#orelse)
   - [orElseW](#orelsew)
   - [orLeft](#orleft)
@@ -106,6 +106,7 @@ Added in v2.0.0
   - [chainW](#chainw)
   - [fromNullableK](#fromnullablek)
   - [fromOptionK](#fromoptionk)
+  - [mapLeft](#mapleft)
   - [orElseFirst](#orelsefirst)
   - [orElseFirstW](#orelsefirstw)
 - [lifting](#lifting)
@@ -816,19 +817,39 @@ export declare const getOrElseW: <R2, E, B>(
 
 Added in v2.6.0
 
-## mapLeft
+## mapError
 
-Map a function over the second type argument of a bifunctor.
+Returns a `ReaderTaskEither` with its error channel mapped using the specified function.
 
 **Signature**
 
 ```ts
-export declare const mapLeft: <E, G>(
-  f: (e: E) => G
-) => <R, A>(fa: ReaderTaskEither<R, E, A>) => ReaderTaskEither<R, G, A>
+export declare const mapError: {
+  <R, E, G>(f: (e: E) => G): <A>(self: ReaderTaskEither<R, E, A>) => ReaderTaskEither<R, G, A>
+  <R, E, A, G>(self: ReaderTaskEither<R, E, A>, f: (e: E) => G): ReaderTaskEither<R, G, A>
+}
 ```
 
-Added in v2.0.0
+**Example**
+
+```ts
+import * as ReaderTaskEither from 'fp-ts/ReaderTaskEither'
+import * as Either from 'fp-ts/Either'
+
+const f = (s: string) => new Error(s)
+
+async function test() {
+  assert.deepStrictEqual(await ReaderTaskEither.mapError(ReaderTaskEither.right(1), f)({})(), Either.right(1))
+  assert.deepStrictEqual(
+    await ReaderTaskEither.mapError(ReaderTaskEither.left('err'), f)({})(),
+    Either.left(new Error('err'))
+  )
+}
+
+test()
+```
+
+Added in v2.16.0
 
 ## orElse
 
@@ -1368,6 +1389,20 @@ export declare const fromOptionK: <E>(
 
 Added in v2.10.0
 
+## mapLeft
+
+Alias of `mapError`.
+
+**Signature**
+
+```ts
+export declare const mapLeft: <E, G>(
+  f: (e: E) => G
+) => <R, A>(fa: ReaderTaskEither<R, E, A>) => ReaderTaskEither<R, G, A>
+```
+
+Added in v2.0.0
+
 ## orElseFirst
 
 Alias of `tapError`.

docs/modules/TaskEither.ts.md

@@ -56,7 +56,7 @@ Added in v2.0.0
   - [getApplicativeTaskValidation](#getapplicativetaskvalidation)
   - [getOrElse](#getorelse)
   - [getOrElseW](#getorelsew)
-  - [mapLeft](#mapleft)
+  - [mapError](#maperror)
   - [orElse](#orelse)
   - [orElseFirstIOK](#orelsefirstiok)
   - [orElseFirstTaskK](#orelsefirsttaskk)
@@ -105,6 +105,7 @@ Added in v2.0.0
   - [chainW](#chainw)
   - [fromNullableK](#fromnullablek)
   - [fromOptionK](#fromoptionk)
+  - [mapLeft](#mapleft)
   - [orElseFirst](#orelsefirst)
   - [orElseFirstW](#orelsefirstw)
 - [lifting](#lifting)
@@ -722,17 +723,36 @@ export declare const getOrElseW: <E, B>(onLeft: (e: E) => T.Task<B>) => <A>(ma:
 
 Added in v2.6.0
 
-## mapLeft
+## mapError
 
-Map a function over the first type argument of a bifunctor.
+Returns a `TaskEither` with its error channel mapped using the specified function.
 
 **Signature**
 
 ```ts
-export declare const mapLeft: <E, G>(f: (e: E) => G) => <A>(fa: TaskEither<E, A>) => TaskEither<G, A>
+export declare const mapError: {
+  <E, G>(f: (e: E) => G): <A>(self: TaskEither<E, A>) => TaskEither<G, A>
+  <E, A, G>(self: TaskEither<E, A>, f: (e: E) => G): TaskEither<G, A>
+}
 ```
 
-Added in v2.0.0
+**Example**
+
+```ts
+import * as TaskEither from 'fp-ts/TaskEither'
+import * as Either from 'fp-ts/Either'
+
+const f = (s: string) => new Error(s)
+
+async function test() {
+  assert.deepStrictEqual(await TaskEither.mapError(TaskEither.right(1), f)(), Either.right(1))
+  assert.deepStrictEqual(await TaskEither.mapError(TaskEither.left('err'), f)(), Either.left(new Error('err')))
+}
+
+test()
+```
+
+Added in v2.16.0
 
 ## orElse
 
@@ -1362,6 +1382,18 @@ export declare const fromOptionK: <E>(
 
 Added in v2.10.0
 
+## mapLeft
+
+Alias of `mapError`.
+
+**Signature**
+
+```ts
+export declare const mapLeft: <E, G>(f: (e: E) => G) => <A>(fa: TaskEither<E, A>) => TaskEither<G, A>
+```
+
+Added in v2.0.0
+
 ## orElseFirst
 
 Alias of `tapError`.

src/EitherT.ts

@@ -412,26 +412,46 @@ export function bimap<F>(
  */
 export function mapLeft<F extends URIS3>(
   F: Functor3<F>
-): <E, G>(f: (e: E) => G) => <R, FE, A>(fea: Kind3<F, R, FE, Either<E, A>>) => Kind3<F, R, FE, Either<G, A>>
+): <E, G>(f: (e: E) => G) => <R, FE, A>(self: Kind3<F, R, FE, Either<E, A>>) => Kind3<F, R, FE, Either<G, A>>
 export function mapLeft<F extends URIS3, FE>(
   F: Functor3C<F, FE>
-): <E, G>(f: (e: E) => G) => <R, A>(fea: Kind3<F, R, FE, Either<E, A>>) => Kind3<F, R, FE, Either<G, A>>
+): <E, G>(f: (e: E) => G) => <R, A>(self: Kind3<F, R, FE, Either<E, A>>) => Kind3<F, R, FE, Either<G, A>>
 export function mapLeft<F extends URIS2>(
   F: Functor2<F>
-): <E, G>(f: (e: E) => G) => <FE, A>(fea: Kind2<F, FE, Either<E, A>>) => Kind2<F, FE, Either<G, A>>
+): <E, G>(f: (e: E) => G) => <FE, A>(self: Kind2<F, FE, Either<E, A>>) => Kind2<F, FE, Either<G, A>>
 export function mapLeft<F extends URIS2, FE>(
   F: Functor2C<F, FE>
-): <E, G>(f: (e: E) => G) => <A>(fea: Kind2<F, FE, Either<E, A>>) => Kind2<F, FE, Either<G, A>>
+): <E, G>(f: (e: E) => G) => <A>(self: Kind2<F, FE, Either<E, A>>) => Kind2<F, FE, Either<G, A>>
 export function mapLeft<F extends URIS>(
   F: Functor1<F>
-): <E, G>(f: (e: E) => G) => <A>(fea: Kind<F, Either<E, A>>) => Kind<F, Either<G, A>>
+): <E, G>(f: (e: E) => G) => <A>(self: Kind<F, Either<E, A>>) => Kind<F, Either<G, A>>
 export function mapLeft<F>(
   F: Functor<F>
-): <E, G>(f: (e: E) => G) => <A>(fea: HKT<F, Either<E, A>>) => HKT<F, Either<G, A>>
+): <E, G>(f: (e: E) => G) => <A>(self: HKT<F, Either<E, A>>) => HKT<F, Either<G, A>>
 export function mapLeft<F>(
   F: Functor<F>
-): <E, G>(f: (e: E) => G) => <A>(fea: HKT<F, Either<E, A>>) => HKT<F, Either<G, A>> {
-  return (f) => (fea) => F.map(fea, E.mapLeft(f))
+): <E, G>(f: (e: E) => G) => <A>(self: HKT<F, Either<E, A>>) => HKT<F, Either<G, A>> {
+  const mapErrorF = mapError(F)
+  return (f) => (self) => mapErrorF(self, f)
+}
+
+/** @internal */
+export function mapError<F extends URIS2>(
+  F: Functor2<F>
+): <R, E, A, G>(self: Kind2<F, R, Either<E, A>>, f: (e: E) => G) => Kind2<F, R, Either<G, A>>
+/** @internal */
+export function mapError<F extends URIS>(
+  F: Functor1<F>
+): <E, A, G>(self: Kind<F, Either<E, A>>, f: (e: E) => G) => Kind<F, Either<G, A>>
+/** @internal */
+export function mapError<F>(
+  F: Functor<F>
+): <E, A, G>(self: HKT<F, Either<E, A>>, f: (e: E) => G) => HKT<F, Either<G, A>>
+/** @internal */
+export function mapError<F>(
+  F: Functor<F>
+): <E, A, G>(self: HKT<F, Either<E, A>>, f: (e: E) => G) => HKT<F, Either<G, A>> {
+  return (self, f) => F.map(self, E.mapLeft(f))
 }
 
 /**