[Haskell-cafe] Re: State & nested structures

steffen steffen.siering at googlemail.com
Fri Oct 29 12:19:16 EDT 2010

> Horribly enough this one seems to work...
> 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

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.

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...).

On 29 Okt., 17:35, Stephen Tetley <stephen.tet... at gmail.com> wrote:
> 2010/10/29 Dupont Corentin <corentin.dup... at gmail.com>:
> > Also, I can't manage to write the more generic function SB x ->  SA x.
> 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.
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