[Haskell-cafe] Re: Is my code too complicated?

[Ertugrul Soeylemez]

Stephen Tetley <stephen.tetley at gmail.com> wrote:

> On 5 July 2010 10:39, Yves Parès <limestrael at gmail.com> wrote:
>
> > Then what is your alternative? How do you replace monad
> > transformers?
>
> Possibly more a case of doing without rather than replacing them with
> something else, you would amalgamate all the monadic effects you want
> into one monad.
>
> E.g. State and Environment (reader) and partiality (Maybe)
>
> newtype Amalgamated s e a = Amalgamated { getAmalgamated :: e -> s ->
> (Maybe a,st) }
>
> instance Monad (Amalgamated s e) where
>   return a = Amalgamated $ \e s -> return (Just a, st)
>   m >>= k  = Amalgamated $ \e s -- TODO (after the first coffee of the
> morning...)

That's what monad transformers are good for.  Why reinvent the wheel?

  type Amalgamated s e m = MaybeT (StateT s (ReaderT e m))

This is all you need to create your own monad with the specified
functionality.

  testComp :: Amalgamated Int Bool IO ()
  testComp = do
    x <- return (Just 3)
    y <- ask
    z0 <- get
    when y $ sets_ (+1)
    z1 <- get
    inBase $ print (x, y, z0, z1)

However, MaybeT as defined in the 'MaybeT' package will probably not
work here.  But this is not a transformer-related problem, just
compatibility.


Greets,
Ertugrul


-- 
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