Splitting MonadTrav, revisited

[Benedikt Huber]

Aaron Tomb wrote:
> Hello,
>
> A while back, I brought up the fact that MonadTrav really serves  
> several purposes, each of which would ideally be described by a  
> separate class. I advocated creating one class for name generation,  
> one for symbol table handling, and one for the rest of what's  
> currently in MonadTrav.
>   
Hi Aaron,
I think your patch outlined below is a good idea.

I had a look at our conversation about this issue - my point was that 
due to dependencies
we will most probably only use MonadTrav as class context in the 
analysis, but can/should
nevertheless split the MonadTrav class in order to modularize things, 
keeping a common
superclass.
By the way, is there any reason not to have MonadError, too ?

cheers, benedikt
> If these are entirely separate classes, then existing code in the  
> MonadTrav monad would need additional class constraints. However, if  
> MonadTrav is a subclass of the other two, most code will work  
> unchanged. Only instance declarations will need any modification (and  
> the modifications they would need are totally mechanical). The new  
> class structure would be like this:
>
> class (Monad m) => MonadName m where
>      -- | unique name generation
>      genName :: m Name
>
> class (Monad m) => MonadSymtab m where
>      -- symbol table handling
>      -- | return the definition table
>      getDefTable :: m DefTable
>      -- | perform an action modifying the definition table
>      withDefTable :: (DefTable -> (a, DefTable)) -> m a
>
> -- | Traversal monad
> class (MonadName m, MonadSymtab m) => MonadTrav m where
>      -- error handling facilities
>
>      -- | throw an 'Error'
>      throwTravError :: Error e => e -> m a
>      -- | catch an 'Error' (we could implement dynamically-typed catch  
> here)
>      catchTravError :: m a -> (CError -> m a) -> m a
>      -- | remember that an 'Error' occured (without throwing it)
>      recordError    :: Error e => e -> m ()
>      -- | return the list of recorded errors
>      getErrors      :: m [CError]
>
>      -- | handling declarations and definitions
>      handleDecl :: DeclEvent -> m ()
>
> I have a patch that makes this change. It compiles cleanly (and, of  
> course, passes all tests, since it's just a type change).
>
> Are there any objections to me pushing it into the repository?
>
> Aaron
>
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