RE: unexpected arithmetic sequence and deriving Enum behaviour
| 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
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unexpected arithmetic sequence and deriving Enum behaviour
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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Re: unexpected arithmetic sequence and deriving Enum behaviour
Hi Laszlo, Laszlo Nemeth wrote: 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) Why do you say that? Can you give a pointer to the relevant text in the report? I can't see anything wrong here. The minimal complete instance of Enum consists of toEnum and fromEnum, which yours does. 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: [ core snipped ] The max bound which gets used is the one for Int, not for Colour! This is because, as per the report, GHC defines the default method for enumFrom as: enumFrom x = map toEnum [fromEnum x ..] the sequence on the RHS is at type Int, and hence doesn't terminate until it reaches maxBound::Int. Perhaps you expect that, because your type is also an instance of Bounded, that enumFrom should behave differently. However, there can only be one default method for enumFrom. 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. You're right, I don't think it's a bug. Also, if you had derived Enum instead of defining it, you would have got a version that behaved in the expected way. You defined an instance of Enum yourself, and got exactly what you asked for! Cheers, Simon ___ Glasgow-haskell-bugs mailing list Glasgow-haskell-bugs@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
Re: unexpected arithmetic sequence and deriving Enum behaviour
Simon, Laszlo presented some code: Consider the following code fragment: data Colour = Red | Black | Blue deriving (Show, Bounded) instance Enum Colour where ... and then remarked: Notice that there is no deriving Enum (which would be an error according to the Report) You asked: Why do you say that? Can you give a pointer to the relevant text in the report? I can't see anything wrong here. I think he was referring to the fact that it is illegal to both *derive* and *define* an instance of a particular class for a type. Regards, Stefan ___ Glasgow-haskell-bugs mailing list Glasgow-haskell-bugs@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
