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

[Simon Peyton-Jones]

Misha

I'm happy to tell you that your wish has been granted. I was able to implement the change you wanted (you called it deferred instance declarations) as part of an overhaul of the type-class machinery in GHC.

I'll send out a more detailed message about this to ghc-users.

Simon

| -----Original Message-----
| From: glasgow-haskell-users-bounces at haskell.org [mailto:glasgow-haskell-users-bounces at haskell.org]
| On Behalf Of Misha Aizatulin
| Sent: 27 September 2006 21:08
| To: GHC Users Mailing List
| Subject: Re: [Haskell] Deferred instance declarations (serialization of existential boxes)
|
| 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
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