PEP 827: Add typescript-style "utility types" to Examples section

peps/pep-0827.rst

@@ -1193,6 +1193,57 @@ until one of them is empty.
     )
 
 
+.. _pep827-ts-utils:
+
+TypeScript-style "Utility Types"
+--------------------------------
+
+TypeScript defines a number of `utility types
+<https://www.typescriptlang.org/docs/handbook/utility-types.html>`__
+for performing common type operations.
+
+We present implementations of a selection of them::
+
+    # Pick<T, Keys>
+    # Constructs a type by picking the set of properties Keys from T.
+    type Pick[T, Keys] = typing.NewProtocol[
+        *[
+            p
+            for p in typing.Iter[typing.Members[T]]
+            if typing.IsAssignable[p.name, Keys]
+        ]
+    ]
+
+    # Omit<T, Keys>
+    # Constructs a type by picking all properties from T and then removing Keys.
+    type Omit[T, Keys] = typing.NewProtocol[
+        *[
+            p
+            for p in typing.Iter[typing.Members[T]]
+            if not typing.IsAssignable[p.name, Keys]
+        ]
+    ]
+
+    # Partial<T>
+    # Constructs a type with all properties of T set to optional (T | None).
+    type Partial[T] = typing.NewProtocol[
+        *[
+            typing.Member[p.name, p.type | None, p.quals]
+            for p in typing.Iter[typing.Attrs[T]]
+        ]
+    ]
+
+    # PartialTD<T>
+    # Like Partial, but for TypedDicts: wraps all fields in NotRequired
+    # rather than making them T | None.
+    type PartialTD[T] = typing.NewProtocol[
+        *[
+            typing.Member[p.name, NotRequired[p.type], p.quals]
+            for p in typing.Iter[typing.Attrs[T]]
+        ]
+    ]
+
+
 Rationale
 =========