[Haskell-cafe] Adding state to a library

[Bas van Dijk]

On 18 December 2011 22:26, Kevin Jardine <kevinjardine 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/html/Control-Monad-Trans-Reader.html#t:ReaderT

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