add day 17

Author: zyhou

2024/17/index.ts

@@ -0,0 +1,37 @@
+import type { Expect, Equal } from "type-testing";
+
+type Compose = <A, B, C, D>(
+  f: (x: A) => B,
+  g: (x: B) => C,
+  h: (x: C) => D,
+) => (a: A) => D;
+
+const compose: Compose = (f, g, h) => (a) => h(g(f(a)));
+
+type FirstChar<T extends string> = T extends `${infer F}${string}` ? F : never;
+
+const upperCase = <T extends string>(x: T) => x.toUpperCase() as Uppercase<T>;
+const lowerCase = <T extends string>(x: T) => x.toLowerCase() as Lowercase<T>;
+const firstChar = <T extends string>(x: T) => x[0] as FirstChar<T>;
+const firstItem = <T extends unknown[]>(x: T) => x[0] as T[0];
+const makeTuple = <T>(x: T) => [x];
+const makeBox = <T>(value: T) => ({ value });
+
+// ------------------- Test section ---------------------
+
+// eslint-disable-next-line @typescript-eslint/no-non-null-assertion
+const t0 = compose(upperCase, makeTuple, makeBox)("hello!").value[0]!;
+//    ^?
+type t0_actual = typeof t0; // =>
+type t0_expected = "HELLO!"; // =>
+type t0_test = Expect<Equal<t0_actual, t0_expected>>;
+
+const t1 = compose(makeTuple, firstItem, makeBox)("hello!" as const).value;
+type t1_actual = typeof t1; // =>
+type t1_expected = "hello!"; // =>
+type t1_test = Expect<Equal<t1_actual, t1_expected>>;
+
+const t2 = compose(upperCase, firstChar, lowerCase)("hello!");
+type t2_actual = typeof t2; // =>
+type t2_expected = "h"; // =>
+type t2_test = Expect<Equal<t2_actual, t2_expected>>;