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 wrote:

On Fri, Jul 19, 2013 at 5:19 AM, Jose A. Lopes 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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