Simplify error handling.

- Remove unneeded None checks.
- Arithmetic word parsing raises exceptions.

Addresses issue #27 and #103.

core/completion_test.py

@@ -268,6 +268,7 @@ def testCommandSub(self):
    # CommandSubPart
    print(f('echo $('))
    print(f('echo $(ls '))
    print(f('echo $(ls foo'))

    #print(f('echo `'))
    #print(f('echo `ls '))

osh/cmd_parse.py

@@ -1498,7 +1498,7 @@ def ParseWholeFile(self):
    # This calls ParseAndOr(), but I think it should be a loop that calls
    # ParseCommandLine(), like oil.InteractiveLoop.
    node = self.ParseCommandTerm()
    if node is None: return None
    assert node is not None
    assert node is not False

    return node

osh/word_parse.py

@@ -276,8 +276,6 @@ def _ReadSubscript(self):
      self._Peek()
    else:
      anode = self._ReadArithExpr()
      if not anode:
        return None
      op = ast.ArrayIndex(anode)

    if self.token_type != Id.Arith_RBracket:  # Should be looking at ]
@@ -723,15 +721,7 @@ def _ReadCommandSubPart(self, token_type):
    c_parser = parse_lib.MakeParserForCommandSub(self.line_reader, self.lexer)

    node = c_parser.ParseWholeFile()  # `` and $() allowed
    if not node:
      # Example of parse error:
      # echo $(cat |)  OR
      # echo `cat |`
      error_stack = c_parser.Error()
      self.error_stack.extend(error_stack)
      print(self.error_stack)
      self.AddErrorContext('Error parsing commmand list in command sub')
      return None
    assert node is not None

    # Hm this creates its own word parser, which is thrown away?
    #print('X', self.cur_token)
@@ -789,10 +779,6 @@ def _ReadArithSubPart(self):
    # $((echo / foo))  # looks like division

    anode = self._ReadArithExpr()
    if not anode:
      self.AddErrorContext("Error parsing arith sub part")
      return None

    if self.token_type != Id.Arith_RParen:
      p_die('Expected first ) to end arith sub, got %r', self.cur_token.val,
            token=self.cur_token)
@@ -818,10 +804,6 @@ def _ReadArithSub2Part(self):
    left_span_id = self.cur_token.span_id

    anode = self._ReadArithExpr()
    if not anode:
      self.AddErrorContext("Error parsing arith sub part")
      return None

    if self.token_type != Id.Arith_RBracket:
      p_die('Expected ], got %r', self.cur_token.val, token=self.cur_token)

@@ -1040,14 +1022,11 @@ def _ReadCompoundWord(self, eof_type=Id.Undefined_Tok,

  def _ReadArithWord(self):
    """Helper function for ReadArithWord."""
    #assert self.token_type != Id.Undefined_Tok

    self._Peek()
    #print('_ReadArithWord', self.cur_token)

    if self.token_kind == Kind.Unknown:
      self.AddErrorContext("Unknown token in arith context: %s",
          self.cur_token, token=self.cur_token)
      return None, False
      p_die('Unexpected token in arithmetic context', token=self.cur_token)

    elif self.token_kind == Kind.Eof:
      # Just return EOF token
@@ -1079,8 +1058,7 @@ def _ReadArithWord(self):
      return w, False

    else:
      self._BadToken("Unexpected token parsing arith sub: %s", self.cur_token)
      return None, False
      assert False, ("Unexpected token parsing arith sub: %s", self.cur_token)

    raise AssertionError("Shouldn't get here")

@@ -1192,8 +1170,10 @@ def ReadWord(self, lex_mode):
      if not need_more:
        break

    # TODO: Just let it raise
    if not w:  # Assumes AddErrorContext was already called
    # TODO: Change the interface of _ReadArithWord and _ReadWord.
    # Error cases should raise ParseError.
    # Then they should return either the word, or None to try again?
    if not w:
      error_stack = self.Error()
      p_die('ReadWord: %s', error_stack[-1])

test/parse-errors.sh

@@ -125,6 +125,11 @@ arith-expr() {

  # Triggered a crash!
  _error-case '$(( - ; ))'

  # NOTE: This is confusing, should point to ` for command context?
  _error-case '$(( ` ))'

  _error-case '$(( $ ))'
}

bool-expr() {