✨ Add String#charAt function

string/char_at.ts

@@ -0,0 +1,22 @@
+// Copyright 2021-present the Core-fn authors. All rights reserved. MIT license.
+import { curry } from "../deps.ts";
+import { charAt as _charAt } from "../uncurry/string/char_at.ts";
+
+/**
+ * Returns the character at the specified index.
+ * @param pos - The zero-based index of the desired character
+ * @param val - Any `String`
+ * @returns The result of `val.charAt(pos)`
+ *
+ * @example
+ * ```ts
+ * charAt(0, 'hello') // 'h'
+ * charAt(4, 'hello') // 'o'
+ * charAt(100, 'hello') // ''
+ * ```
+ *
+ * @beta
+ */
+const charAt = curry(_charAt);
+
+export { charAt };

string/char_at_test.ts

@@ -0,0 +1,9 @@
+// Copyright 2021-present the Core-fn authors. All rights reserved. MIT license.
+import { charAt } from "./char_at.ts";
+import { assertEqualsType } from "../dev_deps.ts";
+
+Deno.test("charAt", () => {
+  assertEqualsType<(val: string) => string>(charAt(0));
+  assertEqualsType<string>(charAt(0)(""));
+  assertEqualsType<string>(charAt(0, ""));
+});

string/mod.ts

@@ -11,3 +11,4 @@ export { repeat } from "./repeat.ts";
 export { trimEnd } from "./trim_end.ts";
 export { trimStart } from "./trim_start.ts";
 export { match } from "./match.ts";
+export { charAt } from "./char_at.ts";

uncurry/string/char_at.ts

@@ -0,0 +1,41 @@
+// Copyright 2021-present the Core-fn authors. All rights reserved. MIT license.
+
+type IsNegative<T extends number | bigint> = `${T}` extends `-${infer _}` ? true
+  : false;
+type String2Array<T extends string> = T extends "" ? []
+  : T extends `${infer F}${infer R}` ? [F, ...String2Array<R>]
+  : string[];
+
+/**
+ * @internal
+ */
+type CharAt<T extends number, U extends string> = IsNegative<T> extends true
+  ? ""
+  : String2Array<U>[T] extends infer R ? R extends undefined ? "" : R
+  : string;
+
+/**
+ * Returns the character at the specified index.
+ * @param pos - The zero-based index of the desired character
+ * @param val - Any `String`
+ * @returns The result of `val.charAt(pos)`
+ *
+ * @remarks
+ * Due to the TypeScript recursion limit, the maximum number of valid characters is `23`. If you want to adapt a `string` longer than that, need to cast it to a `string` type.
+ *
+ * @example
+ * ```ts
+ * charAt(0, 'hello') // 'h'
+ * charAt(4, 'hello') // 'o'
+ * charAt(100, 'hello') // ''
+ * ```
+ *
+ * @beta
+ */
+const charAt = <T extends number, U extends string>(
+  pos: T,
+  val: U,
+): CharAt<T, U> => val.charAt(pos) as CharAt<T, U>;
+
+export { charAt };
+export type { CharAt };

uncurry/string/char_at_test.ts

@@ -0,0 +1,40 @@
+// Copyright 2021-present the Core-fn authors. All rights reserved. MIT license.
+import { assertEquals, assertEqualsType } from "../../dev_deps.ts";
+import { CharAt, charAt } from "./char_at.ts";
+
+Deno.test("charAt", () => {
+  const table: [number, string, string][] = [
+    [0, "", ""],
+    [0, "abc", "a"],
+    [1, "abc", "b"],
+    [2, "abc", "c"],
+    [3, "abc", ""],
+    [100, "abc", ""],
+    [-1, "abc", ""],
+    [-100, "abc", ""],
+    [-0, "abc", "a"],
+    [-0, "", ""],
+  ];
+  table.forEach(([pos, val, expected]) => {
+    assertEquals(
+      charAt(pos, val),
+      expected,
+      `charAt(${pos}, ${val}) -> ${expected}`,
+    );
+  });
+
+  assertEqualsType<"", CharAt<0, "">>();
+  assertEqualsType<"", CharAt<1, "">>();
+  assertEqualsType<"", CharAt<2, "">>();
+  assertEqualsType<"a", CharAt<0, "a">>();
+  assertEqualsType<"", CharAt<1, "a">>();
+  assertEqualsType<"a", CharAt<0, "ab">>();
+  assertEqualsType<"b", CharAt<1, "ab">>();
+  assertEqualsType<"", CharAt<2, "ab">>();
+  assertEqualsType<"", CharAt<-1, "">>();
+  assertEqualsType<"", CharAt<-1, "a">>();
+  assertEqualsType<"", CharAt<-1, "ab">>();
+  assertEqualsType<"", CharAt<-100, "ab">>();
+  assertEqualsType<"", CharAt<100, "ab">>();
+  assertEqualsType<"a", CharAt<-0, "ab">>();
+});

uncurry/string/mod.ts

@@ -4,3 +4,4 @@ export { startsWith } from "./starts_with.ts";
 export { endsWith } from "./ends_with.ts";
 export { repeat } from "./repeat.ts";
 export { match } from "./match.ts";
+export { charAt } from "./char_at.ts";