help with rewriting simple tactic

Stream: metaprogramming / tactics

[Joe Palermo]

How can I rewrite the following tactic in such a way that the return value is just 'bool' i.e. not 'tactic bool'?

meta def can_parse (parser : string -> tactic pexpr) (input : string) : tactic bool :=
  (parser input) >> return tt <|> return ff