diff --git a/docs/modules/Array.ts.md b/docs/modules/Array.ts.md
index 34225bebd..f8321bf68 100644
--- a/docs/modules/Array.ts.md
+++ b/docs/modules/Array.ts.md
@@ -895,7 +895,7 @@ Same as `reduce` but it carries over the intermediate steps
 **Signature**
 
 ```ts
-export declare const scanLeft: <A, B>(b: B, f: (b: B, a: A) => B) => (as: A[]) => B[]
+export declare const scanLeft: <A, B>(b: B, f: (b: B, a: A) => B) => (as: A[]) => NonEmptyArray<B>
 ```
 
 **Example**
@@ -915,7 +915,7 @@ Fold an array from the right, keeping all intermediate results instead of only t
 **Signature**
 
 ```ts
-export declare const scanRight: <A, B>(b: B, f: (a: A, b: B) => B) => (as: A[]) => B[]
+export declare const scanRight: <A, B>(b: B, f: (a: A, b: B) => B) => (as: A[]) => NonEmptyArray<B>
 ```
 
 **Example**
diff --git a/docs/modules/ReadonlyArray.ts.md b/docs/modules/ReadonlyArray.ts.md
index fba6db291..c257f2be6 100644
--- a/docs/modules/ReadonlyArray.ts.md
+++ b/docs/modules/ReadonlyArray.ts.md
@@ -884,7 +884,7 @@ Same as `reduce` but it carries over the intermediate steps
 **Signature**
 
 ```ts
-export declare function scanLeft<A, B>(b: B, f: (b: B, a: A) => B): (as: ReadonlyArray<A>) => ReadonlyArray<B>
+export declare function scanLeft<A, B>(b: B, f: (b: B, a: A) => B): (as: ReadonlyArray<A>) => ReadonlyNonEmptyArray<B>
 ```
 
 **Example**
@@ -904,7 +904,7 @@ Fold an array from the right, keeping all intermediate results instead of only t
 **Signature**
 
 ```ts
-export declare function scanRight<A, B>(b: B, f: (a: A, b: B) => B): (as: ReadonlyArray<A>) => ReadonlyArray<B>
+export declare function scanRight<A, B>(b: B, f: (a: A, b: B) => B): (as: ReadonlyArray<A>) => ReadonlyNonEmptyArray<B>
 ```
 
 **Example**
diff --git a/src/Array.ts b/src/Array.ts
index 75d20121f..7d8953b78 100644
--- a/src/Array.ts
+++ b/src/Array.ts
@@ -189,7 +189,7 @@ export const foldRight: <A, B>(
  * @category combinators
  * @since 2.0.0
  */
-export const scanLeft: <A, B>(b: B, f: (b: B, a: A) => B) => (as: Array<A>) => Array<B> = RA.scanLeft as any
+export const scanLeft: <A, B>(b: B, f: (b: B, a: A) => B) => (as: Array<A>) => NonEmptyArray<B> = RA.scanLeft as any
 
 /**
  * Fold an array from the right, keeping all intermediate results instead of only the final result
@@ -202,7 +202,7 @@ export const scanLeft: <A, B>(b: B, f: (b: B, a: A) => B) => (as: Array<A>) => A
  * @category combinators
  * @since 2.0.0
  */
-export const scanRight: <A, B>(b: B, f: (a: A, b: B) => B) => (as: Array<A>) => Array<B> = RA.scanRight as any
+export const scanRight: <A, B>(b: B, f: (a: A, b: B) => B) => (as: Array<A>) => NonEmptyArray<B> = RA.scanRight as any
 
 /**
  * Test whether an array is empty
diff --git a/src/ReadonlyArray.ts b/src/ReadonlyArray.ts
index 2afcf0182..7160bba40 100644
--- a/src/ReadonlyArray.ts
+++ b/src/ReadonlyArray.ts
@@ -25,6 +25,7 @@ import { Monad1 } from './Monad'
 import { Monoid } from './Monoid'
 import * as O from './Option'
 import { fromCompare, getMonoid as getOrdMonoid, Ord, ordNumber } from './Ord'
+import { NonEmptyArray } from './NonEmptyArray'
 import { ReadonlyNonEmptyArray } from './ReadonlyNonEmptyArray'
 import { Show } from './Show'
 import { PipeableTraverse1, Traversable1 } from './Traversable'
@@ -299,11 +300,10 @@ export function foldRight<A, B>(
  * @category combinators
  * @since 2.5.0
  */
-export function scanLeft<A, B>(b: B, f: (b: B, a: A) => B): (as: ReadonlyArray<A>) => ReadonlyArray<B> {
+export function scanLeft<A, B>(b: B, f: (b: B, a: A) => B): (as: ReadonlyArray<A>) => ReadonlyNonEmptyArray<B> {
   return (as) => {
     const l = as.length
-    // tslint:disable-next-line: readonly-array
-    const r: Array<B> = new Array(l + 1)
+    const r = new Array(l + 1) as NonEmptyArray<B>
     r[0] = b
     for (let i = 0; i < l; i++) {
       r[i + 1] = f(r[i], as[i])
@@ -323,11 +323,10 @@ export function scanLeft<A, B>(b: B, f: (b: B, a: A) => B): (as: ReadonlyArray<A
  * @category combinators
  * @since 2.5.0
  */
-export function scanRight<A, B>(b: B, f: (a: A, b: B) => B): (as: ReadonlyArray<A>) => ReadonlyArray<B> {
+export function scanRight<A, B>(b: B, f: (a: A, b: B) => B): (as: ReadonlyArray<A>) => ReadonlyNonEmptyArray<B> {
   return (as) => {
     const l = as.length
-    // tslint:disable-next-line: readonly-array
-    const r: Array<B> = new Array(l + 1)
+    const r = new Array(l + 1) as NonEmptyArray<B>
     r[l] = b
     for (let i = l - 1; i >= 0; i--) {
       r[i] = f(as[i], r[i + 1])