id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
4222	Template Haskell lets you reify supposedly-abstract data types	simonpj		"Serguey Zefirov writes ([http://www.haskell.org/pipermail/haskell-cafe/2010-July/079802.html on Haskell cafe]) ""Data.Map.Map and Data.Set.Set are exported abstractly, without exposing knowledge about their internal structure.

I cannot directly create my own class instances for them because of that. But I found that I can write Template Haskell code that could do that - those data types could be reified just fine.""  

Good point.  It's not quite clear what a better design should be.  Haskell controls data abstraction by whether or not the constructors of the data type are exported.  But they might be exported by the module that ''defined'' them, but ''not'' to ""clients"" of the data type.  So the data type is abstract to some importers but concrete to others.

So when should TH let you reify the representation of a data type?  Maybe it should let you do so iff
  * The data constructors of the data type are in scope (somehow) at the reification site
So, to take an example:
{{{
module Conc( T(..), Blah(..) ) where
  data T = T1 | T2 Blah
  data Blah = A | B

module Abs( T, Blah ) where
  import Conc

module ReifyA where
  import Abs

  foo = reify ''T

module ReifyC where
  import Conc

  foo = reify ''T
}}}
So the `reify` in `ReifyC.foo` would ""see"" the data constructors of `T`, but not the one in `ReifyA`.

But this approach raises related questions. 

 * What if some, but not all, of `T`'s data constructors are in scope?

 * What if the data constructors are all in scope, but some mention a type that is not in scope?   For example, suppose type `Blah` is not in scope, but you reify `T`?

 * At the moment, when you reify a data type you get its `Dec`. But if `T` is abstract, what `Dec` can we give it?  Just giving it an empty constructor list seems wrong; after all, it might really ''be'' a zero-constructor data type. I suspect we may want a richer data type for `Info` (ie what `reify` returns).

All these require design thinking.  Does anyone want to lead that debate?  Otherwise things will probably stay as they are.

I've labelled this as a feature request, although it is a kind of bug, because of this design component.


"	feature request	new	low	7.6.2	Compiler	6.12.3			sergueyz@…	Unknown/Multiple	Unknown/Multiple	None/Unknown