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: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]
| 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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