the tricky part then is to add support for other types.

another approach to existentially package type classes with the data type!

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.adam@gmail.com> wrote:

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