patch applied (ghc): Three improvements to Template Haskell (fixes #3467)

[Isaac Dupree]

Simon Peyton-Jones wrote:
> | >   Declaration-level splices with no "$"
> | >   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> | >   This change simply allows you to omit the "$(...)" wrapper for
> | >   declaration-level TH splices.  An expression all by itself is
> | >   not legal, so we now treat it as a TH splice.  Thus you can now
> | >   say
> | >   	data T = T1 | T2
> | >    	deriveMyStuff ''T
> | 
> | Nice.
> | 
> | There has been talk int the cast about allowing things like
> |     class T, type T
> | etc in the module export list. Perhaps you should also be able to say
> | 
> |     data T = T1 | T2
> |     deriveMyStuff (type T)
> | 
> | rather than using the somewhat odd '' syntax.
> 
> Interesting idea.  You're making use of the *keyword* 'type', although it's unexpected in an expression context.
> 
> Trouble is, we lack a keyword for 'value'!  I suppose we could invent one.  Thus
> 
>    module Foo(  type T(..), class C,  value f, value g  )
> and
>    deriveMyStuff [type T, value f]
> 
> I wonder if that would have merit?  What do other TH afficionados think?

well, sometime there was a suggestion for (constructor T) syntax in 
export-lists.  Which seems to overlap, sort of, with the "value" idea -- 
(value T)?  Although it almost seems like *that* ought to export just 
the constructor-function, not the deconstructor (pattern match) ...

-Isaac