[Haskell] Deferred instance declarations (serialization of existential boxes)

[Misha Aizatulin]

Simon Peyton-Jones wrote:
> Concerning your application
>
> |   I am having a box like
> | > data Box = forall a. Cxt a => Box a
> |   and want to write a Read instances for it.
>
> I don’t see how it helps to defer the Read instance.

  I would defer the instance declaration till the point where I know all
types that will ever go into the box (for instance in my Main module).
The Show instance of Box would write the representation of a together
with it's type (this means Cxt should contain at least Show and
Typeable). The Read instance will then contain something like

========================================================
  String aType    <- lexP

  result <- case aType of
    "[Int]" ->
      (readPrec :: ReadPrec [Int]) >>= (return . Box)
    "SomeOtherType" ->
      (readPrec :: ReadPrec SomeOtherType) >>= (return . Box)

    -- <more of such cases>

    _ -> error (aType ++ " cannot be read inside of Box")
 ========================================================

  Not very elegant of course, but it seems to be the only way to Read
existentials.

> But I think the case is stronger for top-level constraints, and I will bear it in mind.  
> If you would like to open a Trac feature request, please do so.

  Ok, I will.

Cheers,
  Misha