Re: [Haskell-cafe] Adding state to a library

18 Dec 2011

      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  wrote:
...
On 18 December 2011 22:26, Kevin Jardine  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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