Re: class declaration in boot file

11 Aug 2006

      (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@haskell.org
[mailto:glasgow-haskell-users-bounces@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 
| http://gaal.livejournal.com/
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-- 
Gaal Yahas 
http://gaal.livejournal.com/