[Haskell-cafe] Re: State & nested structures

you can find a nice introduction on using monad transformers by developing an interpreter at [1] and a little more detailed one at [2] [1]http://www.grabmueller.de/martin/www/pub/Transformers.pdf [2]http://www.haskell.org/all_about_monads/html/ On 29 Okt., 18:46, Dupont Corentin wrote:

Thank you for your responses. I will look at monad transformers. I already use them I think because actually I use something like StateT Game IO a. You mean I have to implement my own instance?

Oh, can you call me Corentin? This is my name ;)

Cheers, Corentin

On Fri, Oct 29, 2010 at 6:19 PM, steffen wrote:

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Horribly enough this one seems to work...

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mapOnBofA :: SB a -> SA a mapOnBofA mf = get >>= \st@(A {b=temp}) ->                let (ans,temp2) = runState mf temp                in put (st { b=temp2}) >> return ans

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There is nothing horrible about that. You just run a new isolated computation in the State Monad for B and use its results. More or less see same solution as Dupont's.

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@Dupont: telling from your possible use case and your last post with your "MAP"- Problem, these two are very similar. You have a monad and inside your monad you temporarily want to run some computation in another Monad. I think Monad-Transformers are maybe the better option for you (especially your interpreter-Problem was a good use case for StateT/ ErrorT instead of State and some Either inside it...).

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On 29 Okt., 17:35, Stephen Tetley wrote:

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2010/10/29 Dupont Corentin :

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Also, I can't manage to write the more generic function SB x ->  SA x.

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However, I'd have to question why you want both SA and SB as state functional types. Having inner runState's is sometimes good practice (its an instance of the Local Effect pattern identified by Ralf Laemmel and Joost Visser), but if you have it "commonly" I'd suspect you design is somehow contrived and could be simplified. _______________________________________________ Haskell-Cafe mailing list Haskell-C...@haskell.orghttp:// www.haskell.org/mailman/listinfo/haskell-cafe  _______________________________________________ Haskell-Cafe mailing list Haskell-C...@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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