test: Enforce tests for Prism (#259)

src/optical/prism.test.ts

@@ -2,7 +2,8 @@ import { assertEquals } from "../../deps.ts";
 import { opticCat } from "../optical.ts";
 import { ifSome, none, type Option, some } from "../option.ts";
 import { key } from "./lens.ts";
-import { only } from "./prism.ts";
+import { filter } from "./prism.ts";
+import { only, unreachable } from "./prism.ts";
 
 Deno.test("optional", () => {
     const obj = {
@@ -26,7 +27,32 @@ Deno.test("optional", () => {
     );
 });
 
+Deno.test("unreachable", () => {
+    assertEquals(opticCat(4).feed(unreachable()).get(), none());
+});
+
 Deno.test("only", () => {
     assertEquals(opticCat(4).feed(only(4)).get(), some(4));
     assertEquals(opticCat(4).feed(only(5)).get(), none());
+    assertEquals(opticCat(4).feed(only(4)).set(6), 6);
+    assertEquals(opticCat(4).feed(only(5)).set(6), 4);
+});
+
+Deno.test("filter", () => {
+    assertEquals(
+        opticCat(6).feed(filter((x: number) => x % 2 === 0)).get(),
+        some(6),
+    );
+    assertEquals(
+        opticCat(7).feed(filter((x: number) => x % 2 === 0)).get(),
+        none(),
+    );
+    assertEquals(
+        opticCat(6).feed(filter((x: number) => x % 2 === 0)).set(8),
+        8,
+    );
+    assertEquals(
+        opticCat(7).feed(filter((x: number) => x % 2 === 0)).set(8),
+        7,
+    );
 });

src/optical/prism.ts

@@ -47,7 +47,7 @@ export const newPrismSimple =
         newPrism(upcast)((s) => okOr(s)(downcast(s)));
 
 /**
- * @returns The optic which matches nothing. Getting a value through this will throw an error.
+ * @returns The optic which matches nothing. Setting a value through this will throw an error (but will be probably a type error).
  */
 export const unreachable = <S, A>(): Optic<S, S, A, never> =>
     newPrism<never, S>(absurd)<S, A>(err);