Re: [Haskell-cafe] Building pattern and trying monads

28 May 2012


      Actually, I think the backtracking property here stems more from the
MonadPlus StateT instance than from the properties of Maybe.
(mplus a b runs a and b by passing explicitely the same state to them).

2012/5/28 Roman Cheplyaka 
...
...
...
observe $ flip runStateT 10 $ (put 0 >> mzero) <|> modify (+3)
  ((),13)
If the only thing you need is backtracking, using LogicT might be a
* Yves Parès  [2012-05-28 11:28:22+0200]
little
...
overkill, using Maybe in the bottom of you monad stack suits just fine:
case flip runStateT 10 $ (put 0 >> mzero) <|> modify (+3) of
    Just x -> ....
    Nothing -> ....
Indeed, I didn't realise that Maybe may be (no pun intended) sufficient
here!
--
Roman I. Cheplyaka :: http://ro-che.info/