cons/nil for pair/unit in scheme backend

src/Lib/CompileExp.newt

@@ -146,12 +146,6 @@ compileTerm tm@(App _ _ _) = case funArgs tm of
             if length' qs == length' args
               then pure $ CConstr tag nm.baseName args' qs NilCon
               else apply nm args' arity
-          -- REVIEW Slower for JS, faster for scheme. (2.88 vs 3.01)
-          -- but all of the "JS" diff is code gen?
-          -- Just (DCon tag info qs _) =>
-          --   if length' qs == length' args
-          --     then pure $ CConstr tag nm.baseName args' qs info
-          --     else apply nm args' arity
           _ => apply nm args' arity
   -- REVIEW maybe we want a different constructor for non-Ref applications?
   (t, args) => do

src/Lib/ProcessDecl.newt

@@ -415,11 +415,14 @@ populateConInfo entries =
     -- Boolean
     | Just (a :: b :: Nil) => (setInfo a FalseCon :: setInfo b TrueCon :: Nil)
     | Just entries => entries in
-  let (a :: b :: Nil) = entries | _ => entries in
-  fromMaybe entries $ checkNat entries <|> checkCons entries
-  -- let (Nothing) = checkNat entries | Just entries => entries in
-  -- let (Nothing) = checkNat entries | Just entries => entries in
-  -- entries
+  case entries of
+    a :: Nil => case countFields a of
+      0 => setInfo a NilCon :: Nil
+      2 => setInfo a ConsCon :: Nil
+      _ => entries
+    a :: b :: Nil =>
+      fromMaybe entries $ checkNat entries <|> checkCons entries
+    _ => entries
   where
     countFields : TopEntry → Int
     countFields (MkEntry fc name type def eflags) = go type