idiom for different implementations of same idea

[Hal Daume III]

Hi,

> In similar situations, especially if there is more than one useful way to
> use the various parts of an algorithm, I used often prefer existentials:
> 
> data Model = forall markup table alignments. Model
> 	{
> 	   prepareData :: Data () -> Data markup,
> 	   initialize  :: Data markup -> ST s table,
> 	   doThingOne  :: Data markup -> table -> ST s alignments,
> 	   doThingTwo  :: Data markup -> alignments -> ST s table,
> 	   getResults  :: Data markup -> table -> alignments -> String
> 	}

I like this a lot!  The only problem is I don't think GHC does.  It
complains:

/nfs/isd/hdaume/projects/MESumm/new/Model.hs:13:
    Can't combine named fields with locally-quantified type variables
    In the declaration of data constructor Model
    In the data type declaration for `Model'

....