From c24d865eb6c122932e2af0d9052abdb58ead5ab4 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Gustaf=20R=C3=A4ntil=C3=A4?= <g.rantila@gmail.com>
Date: Wed, 8 May 2019 17:30:14 +0200
Subject: [PATCH] feat(types): Added meta functions PromiseOf<T> and
 PromiseElement<T>

---
 README.md    | 25 +++++++++++++++++++++++++
 lib/index.ts | 19 +++++++++++++++++++
 2 files changed, 44 insertions(+)

diff --git a/README.md b/README.md
index bedb21d..f61b514 100644
--- a/README.md
+++ b/README.md
@@ -12,6 +12,10 @@ The functions are standalone and depends on no particular Promise implementation
 
 This library is written in TypeScript but is exposed as ES7 (if imported as `already`) and ES5 (if imported as `already/es5`). Typings are provided too, so any TypeScript project using this library will automatically get full type safety of all the functions.
 
+# Types
+  * [PromiseOf\<P\>](#PromiseOf)
+  * [PromiseElement\<P\>](#PromiseElement)
+
 # Functions
 
   * [delay](#delay)
@@ -33,6 +37,27 @@ This library is written in TypeScript but is exposed as ES7 (if imported as `alr
   * [wrapFunction](#wrapfunction)
   * [funnel](#funnel)
 
+---
+
+# Types
+
+## PromiseOf
+
+`PromiseOf< P >` returns the Promise wrapped value of `P`, unless it's already a promise, where the promise itself is returned instead.
+
+  * For `P` (being `Promise< E >`), it returns `P`
+  * For non-promise `P`, it returns `Promise< P >`
+
+
+## PromiseElement
+
+`PromiseElement< P >` returns the element type of a promise, or the type itself if it isn't wrapped in a promise.
+
+  * For `P` (being `Promise< E >`), it returns `E`
+  * For non-promise `P`, it returns `P`
+
+
+# Functions
 
 ## delay
 
diff --git a/lib/index.ts b/lib/index.ts
index af23639..6f758a3 100644
--- a/lib/index.ts
+++ b/lib/index.ts
@@ -24,6 +24,25 @@ export default {
 };
 
 
+/**
+ * Returns the Promise wrapped value of P, unless it's already a promise, where
+ * the promise itself is returned instead.
+ *
+ * For P being Promise<E>, it returns P
+ * For non-promise P, it returns Promise<P>
+ */
+export type PromiseOf< P > = P extends Promise< infer U > ? P : Promise< P >;
+
+/**
+ * Returns the element type of a promise, or the type itself if it isn't
+ * wrapped in a promise.
+ *
+ * For P being Promise<E>, it returns E
+ * For non-promise P, it returns P
+ */
+export type PromiseElement< P > = P extends Promise< infer U > ? U : P;
+
+
 function toReadonlyArray< T >( arr: ConcatArray< T > ): ReadonlyArray< T >
 {
 	/* istanbul ignore else */