feat: flatMapState

src/StateReaderTaskEither.ts

@@ -575,6 +575,23 @@ export const flatMapReaderTaskEither: {
   ): StateReaderTaskEither<S, R1 & R2, E1 | E2, B> => flatMap(self, (a: A) => fromReaderTaskEitherK(f)<S>(a))
 )
 
+/**
+ * @category sequencing
+ * @since 2.16.0
+ */
+export const flatMapState: {
+  <S, A, B>(f: (a: A) => State<S, B>): <R, E>(
+    self: StateReaderTaskEither<S, R, E, A>
+  ) => StateReaderTaskEither<S, R, E, B>
+  <S, R, E, A, B>(self: StateReaderTaskEither<S, R, E, A>, f: (a: A) => State<S, B>): StateReaderTaskEither<S, R, E, B>
+} = /*#__PURE__*/ dual(
+  2,
+  <S, R, E, A, B>(
+    self: StateReaderTaskEither<S, R, E, A>,
+    f: (a: A) => State<S, B>
+  ): StateReaderTaskEither<S, R, E, B> => flatMap(self, fromStateK(f))
+)
+
 /**
  * Less strict version of [`flatten`](#flatten).
  *
@@ -812,7 +829,9 @@ export const fromStateK: <A extends ReadonlyArray<unknown>, S, B>(
 ) => <R = unknown, E = never>(...a: A) => StateReaderTaskEither<S, R, E, B> = /*#__PURE__*/ fromStateK_(FromState)
 
 /**
- * @category sequencing
+ * Alias of `flatMapState`.
+ *
+ * @category legacy
  * @since 2.11.0
  */
 export const chainStateK: <A, S, B>(

test/StateReaderTaskEither.ts

@@ -11,7 +11,7 @@ import * as RE from '../src/ReaderEither'
 import * as RTE from '../src/ReaderTaskEither'
 import * as RA from '../src/ReadonlyArray'
 import { ReadonlyNonEmptyArray } from '../src/ReadonlyNonEmptyArray'
-import { State } from '../src/State'
+import { of as stateOf, State } from '../src/State'
 import * as _ from '../src/StateReaderTaskEither'
 import * as S from '../src/string'
 import * as T from '../src/Task'
@@ -638,4 +638,14 @@ describe.concurrent('StateReaderTaskEither', () => {
       E.of(2)
     )
   })
+
+  it('flatMapState', async () => {
+    U.deepStrictEqual(
+      await pipe(
+        _.flatMapState(_.of(1), () => stateOf(2)),
+        _.evaluate(state)
+      )(undefined)(),
+      E.of(2)
+    )
+  })
 })