Data.Typeable

[Simon Peyton-Jones]

Folks

Ralf Laemmel and I have been making good progress with "scrap your
boilerplate".  (In fact there's a new paper at
http://research.microsoft.com/~simonpj/papers/hmap/gmap2.ps; we'd love
your comments.)

Section 7 describes how to extend the Typeable story to higher kinds, in
a manner that I know George Russell has already tried.  It's rather
beautiful, aside from the need to define a family of classes Typeable1,
Typeable2, etc.  

But we've turned up a bad shortcoming in the Data.Typeable library.  The
present construction signature is like this:

	mkAppTy  :: TyCon -> [TypeRep] -> TypeRep

	funTc :: TyCon
	mkFunTy  :: TypeRep -> TypeRep -> TypeRep

	-- | Applies a type to a function type.  Returns: @'Just' u@ if
the
	-- first argument represents a function of type @t -> u@ and the
	-- second argument represents a function of type @t at . 
	applyTy :: TypeRep -> TypeRep -> Maybe TypeRep

What is missing is a function that takes two TypeReps and simply applies
one to the other.  So if one TypeRep is (ST T), and the other is Int, we
want to build (ST T Int).

The obvious name for this is 'applyTy' or 'mkAppTy', but they are both
taken.  Indeed, 'applyTy' is a bad name, because it only applies to
*function* types.

I'd like to suggest some re-naming

	mkTyConApp :: TyCon -> [TypeRep] -> TypeRep	
		-- renaming of mkAppTy
	mkAppTy :: TypeRep -> TypeRep -> TypeRep
		-- new function

	mkFunTy 	-- no change
	funResultTy :: TypeRep -> TypeRep -> Maybe TypeRep
		-- ranming of applyTy

I'm pretty sure about the first three. The last is the only "take this
TypeRep apart" function that is currently offered, and it seems pretty
specialised.  Perhaps a more generic function would be
	splitTyConApp :: TypeRep -> (TyCon, [TypeRep])

(Remember, TypeReps only represent monotypes, so they are always a
TyConApp.)

Then we could build funResultTy from splitTyConApp.


Is everyone happy with this?   The main thing that seems unclear is what
'funResultTy' should be called. I guess we could just about stick with
'applyTy'.  Opinions?

Simon