[Haskell-beginners] grouping functions together

[Rein Henrichs]

This seems like a case where you only really need a record, not a typeclass.

On Mon, Jun 8, 2015 at 2:47 PM Dimitri DeFigueiredo <
defigueiredo at ucdavis.edu> wrote:

> Hello!
>
> I am trying to tie together a group of functions that turn an unreliable
> remote network call into a reliable one. For each different network
> request I make, a specific group of these functions should always work
> together, but their type signatures are quite different. My first
> thought was to put them all in a typeclass:
>
> import Control.Monad
>
> class Reliable1 m req attempt ack failure result where
>      getRequests1 :: Monad m =>               m [req]
>      mkAttempt1   :: Monad m => req        -> m (Maybe attempt)
>      action1      :: Monad m => attempt    -> m (Maybe ack)
>      getAcks1     :: Monad m => [attempt]  -> m [ack]
>      mkResult1    :: Monad m => req -> ack -> m (Either failure result)
>      run1         :: Monad m => req        -> m result
>
> That doesn't work because not all functions use all parameters. For
> example, getAcks1 has no idea of what the final 'result' type parameter
> is. This lead me to my second attempt. Defining a 'service' type with
> the sole purpose of tying them all together. Here's my current attempt:
>
> {-# LANGUAGE MultiParamTypeClasses #-}
>
> import Control.Monad
>
> class Reliable m service where
>      getReqs     :: Monad m => service ->               m [req]
>      mkAttempt   :: Monad m => service -> req        -> m (Maybe attempt)
>      action      :: Monad m => service -> attempt    -> m (Maybe ack)
>      getAcks     :: Monad m => service -> [attempt]  -> m [ack]
>      mkResult    :: Monad m => service -> req -> ack -> m (Either
> failure result)
>      run         :: Monad m => service -> req        -> m result
>
> data RemoteCall = RemoteCall
>
> instance Reliable IO RemoteCall where
>      getReqs     = undefined
>      mkAttempt   = undefined
>      action      = undefined
>      getAcks     = undefined
>      mkResult    = undefined
>      run         = undefined
>
> This works, but I have to explicitly pass the 'service' argument in
> every call.
> Can I avoid passing this parameter every time?
> Question, is there a better way to do this?
> I wanted to have a wrapper to make my remote calls reliable.
>
> Thanks,
>
> Dimitri
>
>
>
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