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

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 -> .... 2012/5/27 Roman Cheplyaka

* L Corbijn [2012-05-27 14:21:39+0200]

...

The solution I've in mind depends on the stack being pure. When the monad stack is pure a rule can be applied, returning a maybe value (or having a MaybeT wrapper) and when returning Nothing (failed rule) reverting the stack to it's point before applying the rule.

As I'm not quite sure about the design (nor good at software design) I've some questions about this approach. 1. Is there a better approach then using a state monad for building the 'products'?

If you need to interleave building your "products" and analyzing them, State seems a reasonable choice here.

...

2. My solution with saving/reverting monad-stacks seems quite a hassle/hack, so is it a good approach or is there something better?

You can use a backtracking monad here ([] or Logic).

The key thing is to put the backtracking monad in the bottom of your stack (everything above it will be restored on mzero). On the other hand, if you want some "global" effects that should not be restored, you can put corresponding monads below LogicT in the stack.

Example:

...

observe $ flip runStateT 10 $ (put 0 >> mzero) <|> modify (+3) ((),13)

Note that "put 0" had no effect here, because it was followed by mzero.

-- Roman I. Cheplyaka :: http://ro-che.info/

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