unexpected arithmetic sequence and deriving Enum behaviour

[Simon Peyton-Jones]

| one, and the program compiles. Interestingly, -ddump-deriv doesn't
show
| anything about deriving instances for  Colour, yet, obviously
something
| gets derived.

For me (ghc 6.4.2) it says:
==================== Derived instances ====================
InstInfo: {GHC.Show.Show Foo4.Colour}
  { GHC.Show.showList = GHC.Show.showList__ (GHC.Show.showsPrec 0)
    GHC.Show.showsPrec (_) (Foo4.Red) = GHC.Show.showString "Red"
    GHC.Show.showsPrec (_) (Foo4.Black) = GHC.Show.showString "Black"
    GHC.Show.showsPrec (_) (Foo4.Blue) = GHC.Show.showString "Blue" }
InstInfo: {GHC.Enum.Bounded Foo4.Colour}
  { GHC.Enum.maxBound = Foo4.Blue
    GHC.Enum.minBound = Foo4.Red }

Not very pretty, but definitely not nothing.

| When you run it, it dies with an error message from 'toEnum n' (or
| non-exhaustive pattern if you leave that one out).

That's exactly as specified by the Report. If you look here, 
	http://haskell.org/onlinereport/standard-prelude.html
and search for class Enum, you'll se

	class  Enum a  where
	   ..[snip]...
	-- NOTE: these default methods only make sense for types
	--   that map injectively into Int using fromEnum
	--  and toEnum.
	    enumFrom x       =  map toEnum [fromEnum x ..]

and this is precisely the behaviour you get.

I agree it isn't the behaviour your want.  If every Enum was Bounded,
the enumFrom default method could stop at maxBound.  But Enum and
Bounded are separate classes.

Simon