Return null by default and remove unnecessary ?? nulls (hkust-taco#248)

* use null instead of undefined
* determine if is mls fun in lowering using the trees associated with blockmembersymbol
* make `?? null` default behavior instead of being related to sanity check

Author: CrescentonC

hkmc2/jvm/src/test/scala/hkmc2/JSBackendDiffMaker.scala

@@ -7,7 +7,7 @@ import utils.*
 
 import semantics.*
 import codegen.*
-import codegen.js.{JSBuilder, JSBuilderArgNumSanityChecks, JSBuilderSelSanityChecks}
+import codegen.js.{JSBuilder, JSBuilderArgNumSanityChecks}
 import document.*
 import codegen.Block
 import codegen.js.Scope
@@ -57,7 +57,6 @@ abstract class JSBackendDiffMaker extends MLsDiffMaker:
       given Elaborator.Ctx = curCtx
       val jsb = new JSBuilder
         with JSBuilderArgNumSanityChecks(noSanityCheck.isUnset)
-        with JSBuilderSelSanityChecks(noSanityCheck.isUnset)
       val le = low.program(blk)
       if showLoweredTree.isSet then
         output(s"Lowered:")

hkmc2/shared/src/main/scala/hkmc2/codegen/Block.scala

@@ -121,7 +121,7 @@ sealed abstract class Result
 // type Local = LocalSymbol
 type Local = Symbol
 
-case class Call(fun: Path, args: Ls[Arg]) extends Result
+case class Call(fun: Path, args: Ls[Arg])(val isMlsFun: Bool) extends Result
 
 case class Instantiate(cls: Path, args: Ls[Path]) extends Result
 

hkmc2/shared/src/main/scala/hkmc2/codegen/Lowering.scala

@@ -68,6 +68,11 @@ class Lowering(using TL, Raise, Elaborator.State):
     case st.Ref(sym) =>
       k(subst(Value.Ref(sym)))
     case st.App(f, arg) =>
+      val isMlsFun = f.symbol.fold(f.isInstanceOf[st.Lam]):
+        case _: sem.BuiltinSymbol => true
+        case sym: sem.BlockMemberSymbol =>
+          sym.trmImplTree.fold(sym.clsTree.isDefined)(_.k is syntax.Fun)
+        case _ => false
       arg match
       case Tup(fs) =>
         val as = fs.map:
@@ -79,7 +84,7 @@ class Lowering(using TL, Raise, Elaborator.State):
         val l = new TempSymbol(S(t))
         subTerm(f): fr =>
           def rec(as: Ls[Bool -> st], asr: Ls[Arg]): Block = as match
-            case Nil => k(Call(fr, asr.reverse))
+            case Nil => k(Call(fr, asr.reverse)(isMlsFun))
             case (spd, a) :: as =>
               subTerm(a): ar =>
                 rec(as, Arg(spd, ar) :: asr)
@@ -348,7 +353,7 @@ trait LoweringSelSanityChecks
         val split = Split.Cons(
             Branch(
               selRes.ref(),
-              Pattern.Lit(syntax.Tree.UnitLit(true)),
+              Pattern.Lit(syntax.Tree.UnitLit(false)),
               Split.Else(
                 Term.Throw(Term.New(SynthSel(State.globalThisSymbol.ref(), Tree.Ident("Error"))(N),
                   Term.Lit(syntax.Tree.StrLit(s"Access to required field '${nme.name}' yielded 'undefined'")) :: Nil)
@@ -375,6 +380,9 @@ trait LoweringTraceLog
     stmts.foldRight(rest):
       case ((sym, res), acc) => Assign(sym, res, acc)
 
+  private def call(fn: Path, args: Ls[Arg]): Call =
+    Call(fn, args)(true)
+  
   extension (k: Block => Block)
     def |>: (b: Block): Block = k(b)
 
@@ -415,26 +423,26 @@ trait LoweringTraceLog
         else Arg(false, Value.Ref(s)) :: Arg(false, Value.Lit(Tree.StrLit(", "))) :: acc
 
     assignStmts(psInspectedSyms.map: (pInspectedSym, pSym) =>
-      pInspectedSym -> Call(inspectFn, Arg(false, Value.Ref(pSym)) :: Nil)
+      pInspectedSym -> call(inspectFn, Arg(false, Value.Ref(pSym)) :: Nil)
     *) |>:
     assignStmts(
-      enterMsgSym -> Call(
+      enterMsgSym -> call(
         strConcatFn,
         Arg(false, Value.Lit(Tree.StrLit(s"CALL ${name.getOrElse("[arrow function]")}("))) :: psSymArgs
       ),
-      TempSymbol(N) -> Call(traceLogFn, Arg(false, Value.Ref(enterMsgSym)) :: Nil),
-      prevIndentLvlSym -> Call(traceLogIndentFn, Nil)
+      TempSymbol(N) -> call(traceLogFn, Arg(false, Value.Ref(enterMsgSym)) :: Nil),
+      prevIndentLvlSym -> call(traceLogIndentFn, Nil)
     ) |>: 
     term(bod)(r =>
     assignStmts(
       resSym -> r,
-      resInspectedSym -> Call(inspectFn, Arg(false, Value.Ref(resSym)) :: Nil),
-      retMsgSym -> Call(
+      resInspectedSym -> call(inspectFn, Arg(false, Value.Ref(resSym)) :: Nil),
+      retMsgSym -> call(
         strConcatFn,
         Arg(false, Value.Lit(Tree.StrLit("=> "))) :: Arg(false, Value.Ref(resInspectedSym)) :: Nil
       ),
-      TempSymbol(N) -> Call(traceLogResetFn, Arg(false, Value.Ref(prevIndentLvlSym)) :: Nil),
-      TempSymbol(N) -> Call(traceLogFn, Arg(false, Value.Ref(retMsgSym)) :: Nil)
+      TempSymbol(N) -> call(traceLogResetFn, Arg(false, Value.Ref(prevIndentLvlSym)) :: Nil),
+      TempSymbol(N) -> call(traceLogFn, Arg(false, Value.Ref(retMsgSym)) :: Nil)
     ) |>:
       Ret(Value.Ref(resSym))
     )

hkmc2/shared/src/main/scala/hkmc2/codegen/js/JSBuilder.scala

@@ -93,11 +93,12 @@ class JSBuilder(using Elaborator.State, Elaborator.Ctx) extends CodeBuilder:
     case Call(Value.Ref(l: BuiltinSymbol), args) =>
       err(msg"Illeal arity for builtin symbol '${l.nme}'")
 
-    case Call(fun, args) =>
+    case c @ Call(fun, args) =>
       val base = fun match
         case _: Value.Lam => doc"(${result(fun)})"
         case _ => result(fun)
-      setupCall(base, args.map(result).mkDocument(", "))
+      val argsDoc = args.map(result).mkDocument(", ")
+      if c.isMlsFun then doc"${base}(${argsDoc})" else doc"${base}(${argsDoc}) ?? null"
     case Value.Lam(ps, bod) => scope.nest givenIn:
       val (params, bodyDoc) = setupFunction(none, ps, bod)
       doc"($params) => { #{  # ${
@@ -346,8 +347,6 @@ class JSBuilder(using Elaborator.State, Elaborator.Ctx) extends CodeBuilder:
     (paramsList, this.body(body))
 
 
-  def setupCall(bases: Document, args: Document)(using Raise, Scope): Document =
-    doc"${bases}(${args})"
 
 object JSBuilder:
   import scala.util.matching.Regex
@@ -461,12 +460,3 @@ trait JSBuilderArgNumSanityChecks
     else
       super.setupFunction(name, params, body)
 
-trait JSBuilderSelSanityChecks
-    (instrument: Bool)(using Elaborator.State)
-    extends JSBuilder:
-  
-  override def setupCall(bases: Document, args: Document)(using Raise, Scope): Document =
-    val basic = super.setupCall(bases, args)
-    if instrument
-    then doc"$basic ?? null"
-    else basic

hkmc2/shared/src/main/scala/hkmc2/syntax/ParseRule.scala

@@ -299,8 +299,8 @@ class ParseRules(using State):
     // modified(`type`),
     singleKw(`true`)(BoolLit(true)),
     singleKw(`false`)(BoolLit(false)),
-    singleKw(`undefined`)(UnitLit(true)),
-    singleKw(`null`)(UnitLit(false)),
+    singleKw(`undefined`)(UnitLit(false)),
+    singleKw(`null`)(UnitLit(true)),
     singleKw(`this`)(Ident("this")),
     standaloneExpr,
   )

hkmc2/shared/src/main/scala/hkmc2/syntax/Tree.scala

@@ -23,7 +23,7 @@ sealed trait Literal extends AutoLocated:
         case c if c.isControl => f"\\u${c.toInt}%04x"
         case c => c.toString
       .mkString("\"", "", "\"")
-    case UnitLit(value) => if value then "undefined" else "null"
+    case UnitLit(value) => if value then "null" else "undefined"
     case BoolLit(value) => value.toString
 
   def describeLit: Str =
@@ -115,7 +115,7 @@ enum Tree extends AutoLocated:
     case IntLit(value) => "integer literal"
     case DecLit(value) => "decimal literal"
     case StrLit(value) => "string literal"
-    case UnitLit(value) => if value then "undefined" else "null"
+    case UnitLit(value) => if value then "null" else "undefined"
     case BoolLit(value) => s"$value literal"
     case Block(stmts) => "block"
     case OpBlock(_) => "operator block"

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

@@ -4,7 +4,7 @@ const Example$class = class Example {
 
   }
   funnySlash(f, arg) {
-    return f(arg);
+    return f(arg) ?? null;
   } 
   inc(x) {
     return x + 1;
@@ -27,5 +27,5 @@ const Example$class = class Example {
   toString() { return "Example"; }
 }; const Example = new Example$class;
 Example.class = Example$class;
-undefined
+null
 export default Example;

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

@@ -44,5 +44,5 @@ const Option$class = class Option {
   toString() { return "Option"; }
 }; const Option = new Option$class;
 Option.class = Option$class;
-undefined
+null
 export default Option;

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

@@ -30,31 +30,31 @@ const Predef$class = class Predef {
           this.indentLvl = tmp;
           return prev;
         } else {
-          return undefined;
+          return null;
         }
       } 
       resetIndent(n) {
         let scrut;
         scrut = this.enabled;
         if (scrut) {
           this.indentLvl = n;
-          return undefined;
+          return null;
         } else {
-          return undefined;
+          return null;
         }
       } 
       log(msg) {
         let scrut, tmp, tmp1, tmp2, tmp3, tmp4;
         scrut = this.enabled;
         if (scrut) {
-          tmp = "| ".repeat(this.indentLvl);
-          tmp1 = "  ".repeat(this.indentLvl);
+          tmp = "| ".repeat(this.indentLvl) ?? null;
+          tmp1 = "  ".repeat(this.indentLvl) ?? null;
           tmp2 = "\n" + tmp1;
-          tmp3 = msg.replaceAll("\n", tmp2);
+          tmp3 = msg.replaceAll("\n", tmp2) ?? null;
           tmp4 = tmp + tmp3;
-          return console.log(tmp4);
+          return console.log(tmp4) ?? null;
         } else {
-          return undefined;
+          return null;
         }
       }
       toString() { return "TraceLogger"; }
@@ -79,39 +79,39 @@ const Predef$class = class Predef {
     }
   } 
   pipe(x2, f) {
-    return f(x2);
+    return f(x2) ?? null;
   } 
   apply(receiver, f1) {
     return (...args) => {
-      return f1(receiver, ...args);
+      return f1(receiver, ...args) ?? null;
     };
   } 
   call(receiver1, f2) {
     return (...args) => {
-      return f2.call(receiver1, ...args);
+      return f2.call(receiver1, ...args) ?? null;
     };
   } 
   print(x3) {
     let tmp;
     tmp = String(x3);
-    return console.log(tmp);
+    return console.log(tmp) ?? null;
   } 
   tupleSlice(xs, i, j) {
     let tmp;
     tmp = xs.length - j;
-    return globalThis.Array.prototype.slice.call(xs, i, tmp);
+    return globalThis.Array.prototype.slice.call(xs, i, tmp) ?? null;
   } 
   tupleGet(xs1, i1) {
-    return globalThis.Array.prototype.at.call(xs1, i1);
+    return globalThis.Array.prototype.at.call(xs1, i1) ?? null;
   } 
   stringStartsWith(string, prefix) {
-    return string.startsWith(prefix);
+    return string.startsWith(prefix) ?? null;
   } 
   stringGet(string1, i2) {
-    return string1.at(i2);
+    return string1.at(i2) ?? null;
   } 
   stringDrop(string2, n) {
-    return string2.slice(n);
+    return string2.slice(n) ?? null;
   } 
   checkArgs(functionName, expected, isUB, got) {
     let scrut, name, scrut1, tmp, tmp1, tmp2, tmp3, tmp4, tmp5, tmp6, tmp7, tmp8, tmp9;
@@ -133,13 +133,13 @@ const Predef$class = class Predef {
       tmp7 = tmp6 + expected;
       tmp8 = tmp7 + " arguments but got ";
       tmp9 = tmp8 + got;
-      throw globalThis.Error(tmp9);
+      throw globalThis.Error(tmp9) ?? null;
     } else {
-      return undefined;
+      return null;
     }
   }
   toString() { return "Predef"; }
 }; const Predef = new Predef$class;
 Predef.class = Predef$class;
-undefined
+null
 export default Predef;

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

@@ -28,5 +28,5 @@ const Stack$class = class Stack {
   toString() { return "Stack"; }
 }; const Stack = new Stack$class;
 Stack.class = Stack$class;
-undefined
+null
 export default Stack;

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

@@ -6,10 +6,10 @@ const Str$class = class Str {
     return a + b;
   } 
   string(value) {
-    return globalThis.String(value);
+    return globalThis.String(value) ?? null;
   }
   toString() { return "Str"; }
 }; const Str = new Str$class;
 Str.class = Str$class;
-undefined
+null
 export default Str;