[Haskell-cafe] advice: instantiating/duplicating modules

[Arie Peterson]

Nicolas Frisby wrote:

| I wrote a combination reader/writer monad (a la the RWS monad in the
| mtl) and I find myself wanting to use multiple instances of it in the
| same stack of transformers. The functional dependencies prevent this
| from working out. The class is called MonadRW and the transformer is
| called RWT.
|
| I find myself wishing I could import the same module twice, but
| instead of having two names for the same entity, I want the
| definitions proper of the module to be duplicated: I want two separate
| MonadRW classes and two separate RWT transformers. Since the module
| integrates the MonadRW and RWT transformer with the mtl, I would then
| only need to lift the instances of the instantiated MonadRW classes
| through the other RWTs.
|
| I'm rather unfamiliar with Template Haskell, but it sounds like it
| might fit the bill. Although, if I recall correctly, instances and
| type declarations splicing are yet to be implemented in GHC?
|
| Does anyone have any advice how to do this? I'm envisioning some
| preprocessing. I'd like to know if someone has developed a tool for
| this already or if this path has been attempted.

Sounds familiar :-).

I wrote some TH code to get "named" versions of the StateT monad
transformer. Example usage:

> {-# OPTIONS_GHC -fth #-}
> module Test where
>
> import NamedStateT
>
> data Settings = ...
>
> $(namedStateT "Settings" $ Just $ ConT ''Settings)
>
> q :: (MonadSettings m,MonadIO m) => m ()
> q = do
>   settings <- getSettings
>   liftIO $ print settings

Notes:

- The second argument to namedStateT is the type of the state. |Nothing|
keeps it parameterised, like |StateT| itself.

- The big comment shows the code that would be generated by a typical
splicing, although the code may have been updated at some point without
changing the example accordingly.

- Not all instances and helper functions from Control.Monad.State are in
there; |modify| seems to be missing, for instance. If you need any, I can
add them (or you can do it yourself).

- I just saw I also have a NamedReaderT module, but it has never been
used, so it may not work. Yell if you think you can use it.


Kind regards,

Arie

-- 

Familiarity with the Haskell language is assumed, but no prior experience
with warm, fuzzy things is required.
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