diff --git a/core/lexer.py b/core/lexer.py
@@ -11,9 +11,9 @@

 import re

+from asdl import const
 from core import util
 from core.id_kind import Id
-
 from osh import ast_ as ast


@@ -43,7 +43,7 @@ def __init__(self, match_func, line, arena=None):
    self.arena = arena

    self.arena_skip = False  # For MaybeUnreadOne
-    self.last_span_id = 0  # For MaybeUnreadOne
+    self.last_span_id = const.NO_INTEGER  # For MaybeUnreadOne

    self.Reset(line, -1)  # Invalid arena index to start

@@ -88,7 +88,7 @@ def LookAhead(self, lex_mode):
        # would involve interacting with the line reader, and we never need
        # it.  In the OUTER mode, there is an explicit newline token, but
        # ARITH doesn't have it.
-        t = ast.token(Id.Unknown_Tok, '', 0)  # no span ID
+        t = ast.token(Id.Unknown_Tok, '', const.NO_INTEGER)
        return t

      tok_type, end_pos = self.match_func(lex_mode, self.line, pos)
@@ -99,7 +99,7 @@ def LookAhead(self, lex_mode):
        break
      pos = end_pos

-    return ast.token(tok_type, tok_val, 0)  # no span ID
+    return ast.token(tok_type, tok_val, const.NO_INTEGER)

  def AtEnd(self):
    return self.line_pos == len(self.line)
@@ -125,7 +125,7 @@ def Read(self, lex_mode):

    if self.arena is not None:
      if self.arena_skip:
-        assert self.last_span_id != 0
+        assert self.last_span_id != const.NO_INTEGER
        span_id = self.last_span_id
        self.arena_skip = False
      else:
@@ -134,7 +134,7 @@ def Read(self, lex_mode):
    else:
      # Completion parser might not have arena?
      # We should probably get rid of this.
-      span_id = 0
+      span_id = const.NO_INTEGER

    t = ast.token(tok_type, tok_val, span_id)

diff --git a/core/ui.py b/core/ui.py
@@ -12,6 +12,7 @@

 import sys

+from asdl import const
 from core import word


@@ -91,19 +92,18 @@ def MakeStatusLines():

 def PrettyPrintError(parse_error, arena, f):
    #print(parse_error)
-    if parse_error.span_id is not None:
+    if parse_error.span_id != const.NO_INTEGER:
      span_id = parse_error.span_id
    elif parse_error.token:
      span_id = parse_error.token.span_id
    elif parse_error.part:
      span_id = word.LeftMostSpanForPart(parse_error.part)
    elif parse_error.word:
-      # Can be -1
      span_id = word.LeftMostSpanForWord(parse_error.word)
    else:
-      span_id = -1
+      span_id = const.NO_INTEGER  # invalid

-    if span_id == -1:
+    if span_id == const.NO_INTEGER:
      line = '<no position info for token>'
      path = '<unknown>'
      line_num = -1
diff --git a/core/util.py b/core/util.py
@@ -23,8 +23,9 @@
  import inspect
  import types

+from asdl import const

-Buffer = io.BytesIO
+Buffer = io.BytesIO  # used by asdl/format.py


 class _ErrorWithLocation(Exception):
@@ -37,7 +38,7 @@ def __init__(self, msg, *args, **kwargs):
    self.args = args
    # NOTE: We use a kwargs dict because Python 2 doesn't have keyword-only
    # args.
-    self.span_id = kwargs.pop('span_id', None)
+    self.span_id = kwargs.pop('span_id', const.NO_INTEGER)
    self.token = kwargs.pop('token', None)
    self.part = kwargs.pop('part', None)
    self.word = kwargs.pop('word', None)
diff --git a/core/word.py b/core/word.py
@@ -6,6 +6,7 @@
 from osh import ast_ as ast
 from core.id_kind import Id, Kind, LookupKind
 from core import util
+from asdl import const

 p_die = util.p_die

@@ -108,7 +109,7 @@ def LeftMostSpanForPart(part):

  if part.tag == word_part_e.ArrayLiteralPart:
    if not part.words:
-      return -1
+      return const.NO_INTEGER
    else:
      return LeftMostSpanForWord(part.words[0])  # Hm this is a=(1 2 3)

@@ -123,14 +124,14 @@ def LeftMostSpanForPart(part):
    if part.tokens:
      return part.tokens[0].span_id
    else:
-      return -1
+      return const.NO_INTEGER

  elif part.tag == word_part_e.DoubleQuotedPart:
    if part.parts:
      return LeftMostSpanForPart(part.parts[0])
    else:
      # We need the double quote location
-      return -1
+      return const.NO_INTEGER

  elif part.tag == word_part_e.SimpleVarSub:
    return part.token.span_id
@@ -142,7 +143,7 @@ def LeftMostSpanForPart(part):
    return part.spids[0]

  elif part.tag == word_part_e.TildeSubPart:
-    return -1
+    return const.NO_INTEGER

  elif part.tag == word_part_e.ArithSubPart:
    # begin, end
@@ -176,31 +177,31 @@ def _RightMostSpanForPart(part):
    if part.tokens:
      return part.tokens[-1].span_id
    else:
-      return -1
+      return const.NO_INTEGER

  elif part.tag == word_part_e.DoubleQuotedPart:
    if part.parts:
      return LeftMostSpanForPart(part.parts[-1])
    else:
      # We need the double quote location
-      return -1
+      return const.NO_INTEGER

  elif part.tag == word_part_e.SimpleVarSub:
    return part.token.span_id

  elif part.tag == word_part_e.BracedVarSub:
    spid = part.spids[0]
-    assert spid != -1
+    assert spid != const.NO_INTEGER
    return spid

  elif part.tag == word_part_e.CommandSubPart:
    return part.spids[1]

  elif part.tag == word_part_e.TildeSubPart:
-    return -1
+    return const.NO_INTEGER

  elif part.tag == word_part_e.ArithSubPart:
-    return -1
+    return const.NO_INTEGER

  elif part.tag == word_part_e.ExtGlobPart:
    return part.spids[1]
@@ -227,7 +228,7 @@ def LeftMostSpanForWord(w):
  # TODO: Really we should use par
  if w.tag == word_e.CompoundWord:
    if len(w.parts) == 0:
-      return -1
+      return const.NO_INTEGER
    elif len(w.parts) == 1:
      return LeftMostSpanForPart(w.parts[0])
    else:
@@ -245,7 +246,7 @@ def UNUSED_RightMostSpanForWord(w):
  # TODO: Really we should use par
  if w.tag == word_e.CompoundWord:
    if len(w.parts) == 0:
-      return -1
+      return const.NO_INTEGER
    elif len(w.parts) == 1:
      return _RightMostSpanForPart(w.parts[0])
    else:
diff --git a/osh/cmd_parse.py b/osh/cmd_parse.py
@@ -12,6 +12,8 @@

 import sys

+from asdl import const
+
 from core import braces
 from core import word
 from core.id_kind import Id, Kind
@@ -216,7 +218,7 @@ def ParseRedirect(self):
    if first_char.isdigit():
      fd = int(first_char)
    else:
-      fd = -1
+      fd = const.NO_INTEGER

    if self.c_id in (Id.Redir_DLess, Id.Redir_DLessDash):  # here doc
      node = ast.HereDoc()
diff --git a/osh/word_parse_test.py b/osh/word_parse_test.py
@@ -11,6 +11,8 @@

 import unittest

+from asdl import const
+
 from core import alloc
 from core.id_kind import Id
 from core import word
@@ -70,7 +72,7 @@ def _assertSpanForWord(test, code_str):
  print(code_str)
  print(span_id)

-  if span_id != -1:
+  if span_id != const.NO_INTEGER:
    span = arena.GetLineSpan(span_id)
    print(span)