add Tuple.mapFst / Tuple.mapSnd

prelude/__internal__/Tuple.mad

@@ -4,9 +4,20 @@ export fst = (tuple) => where(tuple) {
     a
 }
 
-
 snd :: #[a, b] -> b
 export snd = (tuple) => where(tuple) {
   #[_, b] =>
     b
 }
+
+mapFst :: (a -> b) -> #[a, c] -> #[b, c]
+export mapFst = (fn, tuple) => where(tuple) {
+  #[a, b] =>
+    #[fn(a), b]
+}
+
+mapSnd :: (b -> c) -> #[a, b] -> #[a, c]
+export mapSnd = (fn, tuple) => where(tuple) {
+  #[a, b] =>
+    #[a, fn(b)]
+}

prelude/__internal__/Tuple.spec.mad

@@ -0,0 +1,10 @@
+import { assertEquals, test } from "Test"
+
+import { fst, mapFst, mapSnd, snd } from "./Tuple"
+
+
+
+test("fst", () => assertEquals(fst(#["a", 200]), "a"))
+test("snd", () => assertEquals(snd(#["a", 200]), 200))
+test("mapFst", () => assertEquals(mapFst((x) => x / 2, #[200, "a"]), #[100, "a"]))
+test("mapSnd", () => assertEquals(mapSnd((x) => x / 2, #["a", 200]), #["a", 100]))