[Haskell-beginners] Monad Transformer Type Signatures

[Brent Yorgey]

On Mon, Feb 14, 2011 at 02:10:13PM -0500, Britt Anderson wrote:
> I am looking at code from a tutorial that has the following type
> signature for a function:
> 
> getScreen :: MonadReader AppConfig m => m Surface
> 
> AppConfig is a data structure that was defined previously. My question
> is how am I to interpret this type signature? Is it any m belonging to
> the MonadReader AppConfig class? 

Yes.  And there will be an instance for MonadReader AppConfig m
exactly when m carries around an implicit "read-only" AppConfig value,
available for querying with the 'ask' and 'asks' functions.

> Since I can't see any instance of the MonadReader
> Class being defined for MonadReader AppConfig I am puzzled.

Typically what you will see is some concrete monad later being defined
in terms of ReaderT AppConfig, which will automatically give it an
appropriate MonadReader instance, allowing users of the monad to query
the AppConfig.

> I would appreciate some clarification or a pointer to a prior
> discussion of this issue. I have tried to look on my own.

I recommend reading Cale Gibbard's article "How to use monad
transformers"... except at the moment I can't seem to find it
anywhere. =(

-Brent