Add way to refer to Predef symbol in Elab. by declaring it in Prelude

TODO We have not yet properly implemented declarations:
their implementations currently create new symbols.

hkmc2/shared/src/main/scala/hkmc2/semantics/Elaborator.scala

@@ -64,14 +64,20 @@ object Elaborator:
     def getBuiltin(nme: Str): Opt[Ctx.Elem] =
       parent.filter(_.parent.nonEmpty).fold(env.get(nme))(_.getBuiltin(nme))
     object Builtins:
-      private def assumeBuiltinCls(nme: Str): ClassSymbol =
+      private def assumeBuiltin(nme: Str): Symbol =
         getBuiltin(nme)
-          .getOrElse(throw new NoSuchElementException(s"builtin $nme ${env.keySet} $parent"))
+          .getOrElse(throw new NoSuchElementException(s"builtin $nme not in ${parent.map(_.env.keySet)}"))
           .symbol.getOrElse(throw new NoSuchElementException(s"builtin symbol $nme"))
-          .asCls.getOrElse(throw new NoSuchElementException(s"builtin class symbol $nme"))
+      private def assumeBuiltinCls(nme: Str): ClassSymbol =
+        assumeBuiltin(nme).asCls.getOrElse(throw new NoSuchElementException(
+          s"builtin class symbol $nme"))
+      private def assumeBuiltinMod(nme: Str): ModuleSymbol =
+        assumeBuiltin(nme).asMod.getOrElse(throw new NoSuchElementException(
+          s"builtin module symbol $nme"))
       val Int = assumeBuiltinCls("Int")
       val Num = assumeBuiltinCls("Num")
       val Str = assumeBuiltinCls("Str")
+      val Predef = assumeBuiltinMod("Predef")
 
   object Ctx:
     abstract class Elem:

hkmc2/shared/src/test/mlscript-compile/Predef.mjs

@@ -1,6 +1,11 @@
 const Predef$class = class Predef {
   constructor() {
-
+    this.Test = class Test {
+      constructor() {
+        this.y = 1;
+      }
+      toString() { return "Test"; }
+    };
   }
   id(x) {
     return x;
@@ -15,9 +20,14 @@ const Predef$class = class Predef {
   pipe(x2, f) {
     return f(x2);
   } 
-  call(receiver, f1) {
-    return (arg) => {
-      return f1.call(receiver, arg);
+  apply(receiver, f1) {
+    return (...args) => {
+      return f1(receiver, ...args);
+    };
+  } 
+  call(receiver1, f2) {
+    return (...args) => {
+      return f2.call(receiver1, ...args);
     };
   } 
   print(x3) {

hkmc2/shared/src/test/mlscript-compile/Predef.mls

@@ -7,7 +7,9 @@ fun not(x) = x is false
 
 fun (|>) pipe(x, f) = f(x)
 
-fun (|>.) call(receiver, f)(arg) = f.call(receiver, arg)
+fun (.) apply(receiver, f)(...args) = f(receiver, ...args)
+
+fun (|>.) call(receiver, f)(...args) = f.call(receiver, ...args)
 
 fun print(x) =
   console.log(String(x))
@@ -29,3 +31,6 @@ fun checkArgs(functionName, expected, isUB, got) =
     //   + " arguments but got " + got)
   else ()
 
+class Test with
+  val y = 1
+

hkmc2/shared/src/test/mlscript/basics/PredefTest.mls

@@ -0,0 +1,17 @@
+:js
+
+
+Test === Predef.Test
+//│ = true
+
+Predef.Test === globalThis.Predef.Test
+//│ = true
+
+(new Predef.Test).y
+//│ = 1
+
+:re
+(new Predef.Test).x
+//│ ═══[RUNTIME ERROR] Error: Access to required field 'x' yielded 'undefined'
+
+

hkmc2/shared/src/test/mlscript/decls/Prelude.mls

@@ -20,6 +20,11 @@ declare val process
 declare val fs
 
 
+declare module Predef with
+  class Test with
+    val x = 0
+
+
 // TODO: rm
 
 declare fun log: (Any) -> ()