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:

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I have a library of functions that all take a config parameter (and usually others) and return results in the IO monad.

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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

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I have found that I can wrap all my functions like so:

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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

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stateF a b c d =    getConfig >>= \cfg -> liftIO $ withLibrary cfg    libraryF a b c d

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notice that I need stateF and libraryF lines, each with n parameters.

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Upgrading my library like this is rather tedious.

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I would prefer to just write something like

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stateF = upgrade libraryF

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but I can find no way to define the function upgrade in Haskell.

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This must be a fairly common problem. Is there a simple solution?

What do you mean by "upgrading"?

Cheers,

Bas

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