[Haskell-cafe] Adding state to a library

[Kevin Jardine]

By upgrading, I meant move a function from an IO monad with a config
parameter to a state monad without one.

On Dec 18, 10:52 pm, Bas van Dijk <v.dijk.... at gmail.com> wrote:
> On 18 December 2011 22:26, Kevin Jardine <kevinjard... at gmail.com> wrote:
>
> > I have a library of functions that all take a config parameter (and usually
> > others) and return results in the IO monad.
>
> > It is sometimes useful to drop the config parameter by using a state-like
> > monad..
>
> If you're not modifying the configuration, a reader monad transformer
> is probably enough:
>
> http://hackage.haskell.org/packages/archive/transformers/0.2.2.0/doc/...
>
> You probably want to define your own monad transformer for your library:
>
> newtype MyMonad m a = M {unM :: ReaderT Config m a}
>   deriving (Functor, Applicative, Monad, MonadTrans, MonadIO)
>
> getConfig :: MyMonad m Config
> getConfig = M ask
>
> > I have found that I can wrap all my functions like so:
>
> > withLibrary cfg f = f cfg
>
> This can now be defined as:
>
> withLibrary :: Config -> MyMonad m a -> m a
> withLibrary cfg m = runReaderT (unM m) cfg
>
> > stateF a b c d =
> >    getConfig >>= \cfg -> liftIO $ withLibrary cfg
> >    libraryF a b c d
>
> > notice that I need stateF and libraryF lines, each with n parameters.
>
> > Upgrading my library like this is rather tedious.
>
> > I would prefer to just write something like
>
> > stateF = upgrade libraryF
>
> > but I can find no way to define the function upgrade in Haskell.
>
> > This must be a fairly common problem. Is there a simple solution?
>
> What do you mean by "upgrading"?
>
> Cheers,
>
> Bas
>
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