Coverage Condition woes

[Simon Peyton-Jones]

| The following code (coming from Vertigo), which compiles OK with
| ghc-6.4.2, is rejected by ghc in HEAD (from this week-end) :
| 
| data Range = Range String Float Float Float
| 
| class AsRange nrs ns | nrs -> ns where
|   asRange :: nrs -> (ns, [Range])
| 
| instance (AsRange rsa nsa, AsRange rsb nsb)
|    => AsRange (rsa, rsb) (nsa, nsb) where
|   asRange (a, b) = ((nsa, nsb), rsa++rsb)
|    where
|     (nsa, rsa) = asRange a
|     (nsb, rsb) = asRange b
| 
| produces:
|     Illegal instance declaration for `AsRange (rsa, rsb) (nsa, nsb)'
|         (the Coverage Condition fails for one of the functional
| dependencies)
|     In the instance declaration for `AsRange (rsa, rsb) (nsa, nsb)'
| 
| Is it normal ?

Yes, this is exactly as expected. (The fact that 6.4.2 allowed it was
really a bug.)  The behaviour is documented here:
	
http://www.haskell.org/ghc/dist/current/docs/users_guide/type-extensions
.html#instance-decls
(in 7.4.4.1).   

You can find the background here
	http://research.microsoft.com/%7Esimonpj/papers/fd%2Dchr/

However, if you add -fallow-undecidable-instances it'll be allowed (see
section 7.4.4.2).

Simon