branching and very basic merging of slots

structures/list.js

@@ -1,4 +1,4 @@
-import { typedFnType } from "./types.js";
+import { fnType, typedFnType } from "./types.js";
 import { Char, GenericType, Type } from "../primitives/types.js";
 import { Int } from "../primitives/types.js";
 import { makeGeneric } from "../generics/generics.js";
@@ -11,6 +11,7 @@ const emptyListType = makeGeneric(a => lsType(a));
 const get = ls => i => ls.l[i];
 const put = ls => i => elem => ({l: ls.l.with(Number(i), elem)});
 const push = ls => elem => ({l:ls.l.concat([elem])});
+const map = ls => fn => ({ l: ls.l.map(elem => fn(elem)) });
 
 export const String = lsType(Char); // alias
 export const Module = lsType(Typed);
@@ -60,4 +61,14 @@ export const ModuleList = {l:[
           (lsType(a))
         )
     ), GenericType),
+
+  // [a] -> (a -> b) -> [b]
+  ...typedFnType(map, fnType =>
+    makeGeneric((a, b) =>
+      fnType
+        (lsType(a))
+        (fnType
+          (fnType(a)(b))
+          (lsType(b))
+        )), GenericType),
 ]};