👍 Add PredicateType<P> to infer T of Predicate<T>

Author: lambdalisue

README.md

@@ -34,7 +34,7 @@ Additionally, `is*Of` (or `is.*Of`) functions return type predicate functions to
 predicate types of `x` more precisely like:
 
 ```typescript
-import { is } from "./mod.ts";
+import { is, PredicateType } from "./mod.ts";
 
 const isArticle = is.ObjectOf({
   title: is.String,
@@ -48,6 +48,8 @@ const isArticle = is.ObjectOf({
   ])),
 });
 
+type Article = PredicateType<typeof isArticle>;
+
 const a: unknown = {
   title: "Awesome article",
   body: "This is an awesome article",

is.ts

@@ -3,6 +3,11 @@
  */
 export type Predicate<T> = (x: unknown) => x is T;
 
+/**
+ * A type predicated by Predicate<T>
+ */
+export type PredicateType<P> = P extends Predicate<infer T> ? T : never;
+
 /**
  * Return `true` if the type of `x` is `string`.
  */

is_test.ts

@@ -31,6 +31,7 @@ import is, {
   isUndefined,
   isUniformTupleOf,
   Predicate,
+  PredicateType,
 } from "./is.ts";
 
 const examples = {
@@ -83,6 +84,30 @@ async function testWithExamples<T>(
   }
 }
 
+Deno.test("PredicateType", () => {
+  const isArticle = is.ObjectOf({
+    title: is.String,
+    body: is.String,
+    refs: is.ArrayOf(is.OneOf([
+      is.String,
+      is.ObjectOf({
+        name: is.String,
+        url: is.String,
+      }),
+    ])),
+  });
+  type _ = AssertTrue<
+    IsExact<
+      PredicateType<typeof isArticle>,
+      {
+        title: string;
+        body: string;
+        refs: (string | { name: string; url: string })[];
+      }
+    >
+  >;
+});
+
 Deno.test("isString", async (t) => {
   await testWithExamples(t, isString, { validExamples: ["string"] });
 });