Hi,
Consider the following code fragment:
data Colour = Red | Black | Blue deriving (Show, Bounded)
instance Enum Colour where
succ Red = Black
succ Black = Blue
succ Blue = error "succ of maxBound"
fromEnum Red = 1
fromEnum Black = 2
fromEnum Blue = 3
toEnum 1 = Red
toEnum 2 = Black
toEnum 3 = Blue
toEnum n = error ("? "++show n)
main = do print $ [Red ..]
Notice that there is no deriving Enum (which would be an error according
to the Report) and there is no definition for 'enumFrom'.
So, in the first place I'd expect a warning for main for the use of the
arithmetic sequence, something about a missing definition. There isn't
one, and the program compiles. Interestingly, -ddump-deriv doesn't show
anything about deriving instances for Colour, yet, obviously something
gets derived.
When you run it, it dies with an error message from 'toEnum n' (or
non-exhaustive pattern if you leave that one out). Looking at core
reveals the reason for this:
Main.lvl7 :: GHC.Base.Int -> [Main.Colour] -> [Main.Colour]
[GlobalId]
[Arity 2]
Main.lvl7 = \ (x1 :: GHC.Base.Int) (ys :: [Main.Colour]) ->
GHC.Base.: @ Main.Colour (Main.toEnum x1) ys
Main.lvl8 :: [Main.Colour]
[GlobalId]
[]
Main.lvl8 = GHC.Enum.eftIntFB
@ [Main.Colour] Main.lvl7 (GHC.Base.[] @ Main.Colour) 1
2147483647
Main.lvl9 :: [Main.Colour]
[GlobalId]
[]
Main.lvl9 = GHC.Enum.eftIntFB
@ [Main.Colour] Main.lvl7 (GHC.Base.[] @ Main.Colour) 2
2147483647
Main.lvl10 :: [Main.Colour]
[GlobalId]
[]
Main.lvl10 = GHC.Enum.eftIntFB
@ [Main.Colour] Main.lvl7 (GHC.Base.[] @ Main.Colour) 3
2147483647
Main.$dmenumFrom :: Main.Colour -> [Main.Colour]
[GlobalId]
[Arity 1
Str: DmdType S]
Main.$dmenumFrom = \ (w :: Main.Colour) ->
case [Main.Colour] w of wild {
Main.Red -> Main.lvl8; Main.Black -> Main.lvl9;
Main.Blue -> Main.lvl10
}
The max bound which gets used is the one for Int, not for Colour!
Perhaps this isn't a bug, but definitely an undocumented and rather
unexpected 'feature'. This is with ghc-6.4 onwards, and I haven't
checked earlier versions.
Share and enjoy.
Laszlo
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