29 May
2012
29 May
'12
2:16 a.m.
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 <roma@ro-che.info>
* Yves Parès <yves.pares@gmail.com> [2012-05-28 11:28:22+0200]
observe $ flip runStateT 10 $ (put 0 >> mzero) <|> modify (+3) ((),13)
If the only thing you need is backtracking, using LogicT might be a 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/