class declaration in boot file

[Simon Peyton-Jones]

You can't use 'deriving' in hs-boot files, nor instance declarations.

In the HEAD (and hence in 6.6) you can put instance declarations in
though, thus:

	data DT2
	instance Show DT2

Simon

| -----Original Message-----
| From: Gaal Yahas [mailto:gaal at forum2.org]
| Sent: 11 August 2006 21:06
| To: Simon Peyton-Jones
| Cc: GHC Users Mailing List
| Subject: Re: class declaration in boot file
| 
| (I forgot to say, this was with 6.4.2.)
| 
| Another problem I'm having is that when I consume a datatype, its
| derived instances aren't available; and even in an hs-boot file, a
| 'deriving' clause is illegal on a type with no constructors. So I
can't
| compile B in this case:
| 
|     module A where
| 
|     import B
|     data DT1 = X | Y deriving Show
| 
|     module B where
| 
|     import {-# SOURCE #-} A
| 
|     data DT2 = MkDT2 { Q :: DT1 } deriving Show
| 
| Because of missing Show instances for DT1. (In practise, I think it
| might work if I spell out the full declaration of DT1 with the
deriving
| clause, but I don't want the creep: X and Y are really records
entailing
| additional types B need not concern itself with.)
| 
| I'm hoping 6.6 will allow me to say this in the hs-boot file:
| 
|     data DT1 deriving Show
| 
| On Fri, Aug 11, 2006 at 10:59:29AM +0100, Simon Peyton-Jones wrote:
| > Well this is odd.  The manual (for 6.6 anyway) says that class decls
are
| > allowed in hs-boot files, but instance decls are not, whereas the
code
| > seems to say that class decls aren't but instance decls are!
| >
| > I will look into this.  Meanwhile, I'm afraid you just can't put a
class
| > decl in the hs-boot file with the version of the compiler you have.
| >
| > Simon
| >
| > | -----Original Message-----
| > | From: glasgow-haskell-users-bounces at haskell.org
| > [mailto:glasgow-haskell-users-bounces at haskell.org]
| > | On Behalf Of Gaal Yahas
| > | Sent: 09 August 2006 18:25
| > | To: GHC Users Mailing List
| > | Subject: class declaration in boot file
| > |
| > | I'm trying to solve a circularity problem with .hs-boot, but am
| > getting
| > | the error: "Illegal class declaration in hs-boot file".
| > |
| > | The offending declaration is:
| > |
| > | class (Monad m, Functor m, Eq a, Data a, Typeable a) => ICoercible
m a
| > | a -> m
| > |
| > | I've tried with and without the fundeps. Data and Typeable are in
| > scope,
| > | and -fglasgow-exts is set. Any ideas on what else I should be
doing?
| > |
| > | --
| > | Gaal Yahas <gaal at forum2.org>
| > | http://gaal.livejournal.com/
| > | _______________________________________________
| > | Glasgow-haskell-users mailing list
| > | Glasgow-haskell-users at haskell.org
| > | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
| 
| --
| Gaal Yahas <gaal at forum2.org>
| http://gaal.livejournal.com/