Improve the typings of Array.isArray (#2552)

.changeset/slimy-dogs-warn.md

@@ -0,0 +1,5 @@
+---
+"effect": patch
+---
+
+Improve typings of Array.isArray

packages/effect/dtslint/Array.ts

@@ -19,10 +19,27 @@ declare const pimitiveNumber: number
 declare const pimitiveNumerOrString: string | number
 declare const predicateNumbersOrStrings: Predicate.Predicate<number | string>
 
+declare const unknownValue: unknown
+declare const stringOrStringArrayOrUnion: string | Array<string> | null
+
 const symA = Symbol.for("a")
 const symB = Symbol.for("b")
 const symC = Symbol.for("c")
 
+// -------------------------------------------------------------------------------------
+// isArray
+// -------------------------------------------------------------------------------------
+
+if (Array.isArray(unknownValue)) {
+  // $ExpectType unknown[]
+  unknownValue
+}
+
+if (Array.isArray(stringOrStringArrayOrUnion)) {
+  // $ExpectType string[]
+  stringOrStringArrayOrUnion
+}
+
 // -------------------------------------------------------------------------------------
 // isEmptyReadonlyArray
 // -------------------------------------------------------------------------------------

packages/effect/src/Array.ts

@@ -345,7 +345,10 @@ export const scanRight: {
  * @category guards
  * @since 2.0.0
  */
-export const isArray: (self: unknown) => self is Array<unknown> = Array.isArray
+export const isArray: {
+  (self: unknown): self is Array<unknown>
+  <T>(self: T): self is Extract<T, ReadonlyArray<any>>
+} = Array.isArray
 
 /**
  * Determine if an `Array` is empty narrowing down the type to `[]`.