Allow encoding primitive Maybe types, like Maybe<Int>.

'asdl/run.sh osh-demo' now write a file.  But it doesn't load.

asdl/encode.py

@@ -205,7 +205,16 @@ def EncodeObj(obj, enc, out):
    # - Float would be inline, etc.
    # - Repeated value: write them all adjacent to each other?

    # INLINE

    is_maybe = False
    if isinstance(desc, asdl.MaybeType):
      is_maybe = True
      desc = desc.desc  # descent

    #
    # Now look at types
    #

    if isinstance(desc, asdl.IntType) or isinstance(desc, asdl.BoolType):
      enc.Int(field_val, this_chunk)

@@ -225,35 +234,18 @@ def EncodeObj(obj, enc, out):
      ref = EncodeArray(field_val, item_desc, enc, out)
      enc.Ref(ref, this_chunk)

    elif isinstance(desc, asdl.MaybeType):
      item_desc = desc.desc
      ok = False
      if isinstance(item_desc, asdl.Sum):
        if not asdl.is_simple(item_desc):
          ok = True
      elif isinstance(item_desc, asdl.Product):
        ok = True

      # TODO: We can catch this case when parsing the schema rather than on
      # encoding.
      if not ok:
        raise AssertionError(
            "Simple types shouldn't be optional: %s %s" % (name, desc))

      if field_val is None:
        enc.Ref(0, this_chunk)
      else:
        ref = EncodeObj(field_val, enc, out)
        enc.Ref(ref, this_chunk)

    elif isinstance(desc, asdl.UserType):
      # Assume Id for now
      enc.Int(field_val.enum_value, this_chunk)

    else:
      # Recursive call for child records.  Write children before parents.
      ref = EncodeObj(field_val, enc, out)
      enc.Ref(ref, this_chunk)
      if is_maybe and field_val is None:
        enc.Ref(0, this_chunk)
      else:
        # Recursive call for CompoundObj children.  Write children before
        # parents.
        ref = EncodeObj(field_val, enc, out)
        enc.Ref(ref, this_chunk)

  # Write the parent record
  this_ref = out.Write(enc.PaddedBlock(this_chunk))