Re: [Haskell-cafe] Dynamic and equality
You can define:
data EqDyn= forall a.(Typeable a, Eq a)= EqDyn a
instance Eq EqDyn where
(EqDyn x) == (EqDyn y)= typeOf x== typeOf y x== unsafeCoerce y
unsafeCoerce is safe synce the expression assures that types are equal
2013/7/20 adam vogt vogt.a...@gmail.com
On Sat, Jul 20, 2013 at 12:31 AM, Carter Schonwald
carter.schonw...@gmail.com wrote:
the tricky part then is to add support for other types.
another approach to existentially package type classes with the data
type!
eg
data HasEq = forall a . HasEq ( Eq a = a)
or its siblinng
data HasEq a = Haseq (Eq a = a )
note this requires more planning in how you structure your program, but
is a
much more pleasant approach than using dynamic when you can get it to
suite
your application needs.
note its also late, so I've not type checked these examples ;)
Hi Carter,
It doesn't seem like the existential one will work as-is, since ghc
rejects this:
{-# LANGUAGE ExistentialQuantification #-}
data HEQ = forall a. Eq a = HEQ a
usingHEQ :: HEQ - HEQ - Bool
usingHEQ (HEQ a) (HEQ b) = a == b
I think you were hinting at this option which is better than my first
suggestion:
{-# LANGUAGE ExistentialQuantification #-}
import Data.Typeable
data DYN = forall a. Typeable a = DYN (a, DYN - Bool)
mkDyn :: (Eq a, Typeable a) = a - DYN
mkDyn x = DYN (x, \(DYN (y, eq2)) - case cast y of
Just y' - x == y'
_ - False)
mkDyn' :: Typeable a = a - DYN
mkDyn' x = DYN (x, \_ - False)
eqDyn :: DYN - DYN - Bool
eqDyn x@(DYN (_, fx)) y@(DYN (_,fy)) = fx y || fy x
Maybe there's some way to get mkDyn' and mkDyn as the same function,
without having to re-write all of the Eq instances as a 2-parameter
class like http://www.haskell.org/haskellwiki/GHC/AdvancedOverlap.
--
Adam
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Alberto.
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Re: [Haskell-cafe] Dynamic and equality
On Sat, Jul 20, 2013 at 12:31 AM, Carter Schonwald
carter.schonw...@gmail.com wrote:
the tricky part then is to add support for other types.
another approach to existentially package type classes with the data type!
eg
data HasEq = forall a . HasEq ( Eq a = a)
or its siblinng
data HasEq a = Haseq (Eq a = a )
note this requires more planning in how you structure your program, but is a
much more pleasant approach than using dynamic when you can get it to suite
your application needs.
note its also late, so I've not type checked these examples ;)
Hi Carter,
It doesn't seem like the existential one will work as-is, since ghc
rejects this:
{-# LANGUAGE ExistentialQuantification #-}
data HEQ = forall a. Eq a = HEQ a
usingHEQ :: HEQ - HEQ - Bool
usingHEQ (HEQ a) (HEQ b) = a == b
I think you were hinting at this option which is better than my first
suggestion:
{-# LANGUAGE ExistentialQuantification #-}
import Data.Typeable
data DYN = forall a. Typeable a = DYN (a, DYN - Bool)
mkDyn :: (Eq a, Typeable a) = a - DYN
mkDyn x = DYN (x, \(DYN (y, eq2)) - case cast y of
Just y' - x == y'
_ - False)
mkDyn' :: Typeable a = a - DYN
mkDyn' x = DYN (x, \_ - False)
eqDyn :: DYN - DYN - Bool
eqDyn x@(DYN (_, fx)) y@(DYN (_,fy)) = fx y || fy x
Maybe there's some way to get mkDyn' and mkDyn as the same function,
without having to re-write all of the Eq instances as a 2-parameter
class like http://www.haskell.org/haskellwiki/GHC/AdvancedOverlap.
--
Adam
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Re: [Haskell-cafe] Dynamic and equality
On Fri, Jul 19, 2013 at 5:19 AM, Jose A. Lopes jabolo...@google.com wrote:
Hello,
How to define equality for Data.Dynamic ?
Hi Jose,
You could try casting the values to different types that do have an
(==). You can treat the case where you have the types matching, but
didn't list that type beforehand differently.
eqTys a b
| Just a' - fromDynamic a, Just b' - fromDynamic b = a' == (b' :: Int)
| Just a' - fromDynamic a, Just b' - fromDynamic b = a' == (b' :: Integer)
| show a == show b = error equal types, but don't know if there's an (==)!
| otherwise = False
{-
eqTys (toDyn 4) (toDyn 5)
False
eqTys (toDyn 4) (toDyn 4)
True
eqTys (toDyn 4) (toDyn 4.5)
False
eqTys (toDyn 4.5) (toDyn 4.5)
*** Exception: equal types, but don't know if there's an (==)!
-}
--
Adam
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Re: [Haskell-cafe] Dynamic and equality
the tricky part then is to add support for other types.
another approach to existentially package type classes with the data type!
eg
data HasEq = forall a . HasEq ( Eq a = a)
or its siblinng
data HasEq a = Haseq (Eq a = a )
note this requires more planning in how you structure your program, but is
a much more pleasant approach than using dynamic when you can get it to
suite your application needs.
note its also late, so I've not type checked these examples ;)
-Carter
On Fri, Jul 19, 2013 at 12:54 PM, adam vogt vogt.a...@gmail.com wrote:
On Fri, Jul 19, 2013 at 5:19 AM, Jose A. Lopes jabolo...@google.com
wrote:
Hello,
How to define equality for Data.Dynamic ?
Hi Jose,
You could try casting the values to different types that do have an
(==). You can treat the case where you have the types matching, but
didn't list that type beforehand differently.
eqTys a b
| Just a' - fromDynamic a, Just b' - fromDynamic b = a' == (b' ::
Int)
| Just a' - fromDynamic a, Just b' - fromDynamic b = a' == (b' ::
Integer)
| show a == show b = error equal types, but don't know if there's an
(==)!
| otherwise = False
{-
eqTys (toDyn 4) (toDyn 5)
False
eqTys (toDyn 4) (toDyn 4)
True
eqTys (toDyn 4) (toDyn 4.5)
False
eqTys (toDyn 4.5) (toDyn 4.5)
*** Exception: equal types, but don't know if there's an (==)!
-}
--
Adam
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