Re: Automatically derived instances
just try to compile it with
ghc -fallow-overlapping-instances -Wall --make Main.hs
or inseart sth. like this at the first line:
{-# OPTIONS -fglasgow-exts -fffi -fallow-undecidable-instances
-fallow-overlapping-instances #-}
- marc
Am Montag, 29. August 2005 05:25 schrieb Juan Carlos Arevalo Baeza:
> David Menendez wrote:
>
> >Juan Carlos Arevalo Baeza writes:
> >
> >
> >
> >The way Haskell type classes
> >work, the overlap is determined without looking at the context, so "Show
> >a" will overlap with every possible instance for Show, including Show
> >Int, which is predefined.
> >
> >
>
>Ah. :-P Bummer.
>
> >I'm not sure what the official justification for that is, but reason is
> >to avoid situations like this:
> >
> >class A t where a :: t
> >class B t where b :: t
> >class C t where c :: t
> >
> >instance A t => C t where c = a
> >instance B t => C t where c = b
> >
> >instance A Char where a = 'a'
> >instance B Char where b = 'b'
> >
> >What should c :: Char evaluate to?
> >
> >
>
>Right. Ambiguity. But as long as there's no ambiguity, there's no
> reason to be this restrictive. Oh, well.
>
> JCAB
>
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Re: Automatically derived instances
David Menendez wrote: Juan Carlos Arevalo Baeza writes: The way Haskell type classes work, the overlap is determined without looking at the context, so "Show a" will overlap with every possible instance for Show, including Show Int, which is predefined. Ah. :-P Bummer. I'm not sure what the official justification for that is, but reason is to avoid situations like this: class A t where a :: t class B t where b :: t class C t where c :: t instance A t => C t where c = a instance B t => C t where c = b instance A Char where a = 'a' instance B Char where b = 'b' What should c :: Char evaluate to? Right. Ambiguity. But as long as there's no ambiguity, there's no reason to be this restrictive. Oh, well. JCAB ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: Automatically derived instances
Juan Carlos Arevalo Baeza writes:
>(BCC'ing the GHC bugs list)
>
>It seems like there's something very funky going on with GHC (6.4)
> and automatically deriving instances. Consider this code:
>
> ---8<--
> 1: class MyClass a
> 2:
> 3:instance MyClass a => Show a
> 4:
> 5:newtype Type1 = Type1 { unType1 :: Int } deriving (Show)
> 6:
> 7:main = putStrLn $ show $ Type1 4
> ---8<--
Your problem is the instance in line 3. The way Haskell type classes
work, the overlap is determined without looking at the context, so "Show
a" will overlap with every possible instance for Show, including Show
Int, which is predefined.
I'm not sure what the official justification for that is, but reason is
to avoid situations like this:
class A t where a :: t
class B t where b :: t
class C t where c :: t
instance A t => C t where c = a
instance B t => C t where c = b
instance A Char where a = 'a'
instance B Char where b = 'b'
What should c :: Char evaluate to?
--
David Menendez <[EMAIL PROTECTED]>
Automatically derived instances
(BCC'ing the GHC bugs list)
It seems like there's something very funky going on with GHC (6.4)
and automatically deriving instances. Consider this code:
---8<--
1: class MyClass a
2:
3:instance MyClass a => Show a
4:
5:newtype Type1 = Type1 { unType1 :: Int } deriving (Show)
6:
7:main = putStrLn $ show $ Type1 4
---8<--
If you try to compile this, you get:
---8<--
Overlapping instances for Show Int
arising from the 'deriving' clause of a data type declaration at
Test.hs:5:8
Matching instances:
Test.hs:3:0:
instance (MyClass a) => Show a
Imported from GHC.Show: instance Show Int
When deriving the `Show' instance for type `Type1'
---8<--
Weird error. I haven't even declared an instance of MyClass!
If I change line 5 to "deriving (MyClass), I get:
*** Exception: stack overflow
Sounds like a compiler bug to me.
If I comment out line 3 and leave "deriving (Show)", it all works as
expected.
So... what's exactly the purpose of omitting the where-part of an
instance declaration? It is supported, and it is even shown in code in
the Haskell 98 report, but I failed to find any place in the whole
document where it was described. Same thing for GHC's documentation.
My purpose here was to do group classes for deriving like so:
---8<--
class MyNum a
instance MyNum a => Eq a
instance MyNum a => Ord a
instance MyNum a => Show a
instance MyNum a => Enum a
instance MyNum a => Num a
instance MyNum a => Real a
instance MyNum a => Integral a
instance MyNum a => Bits a
instance MyNum a => Storable a
-- Too long?... newtype ID = ID { unID :: WORD } deriving (Eq, Ord,
Show, Enum, Num, Real, Integral, Bits, Storable)
newtype ID = ID { unID :: Int } deriving (MyNum)
---8<--
I guess that's not doable?
JCAB
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