Hi, As mentioned before there are several ways of doing this. 1. Constrain the functions working with your types keeping the type itself unconstrained. 2. Write/Use a replacement for the Functor class that allows you to do this. This will usually involve associated type synonyms or functional dependencies. In my own work [1] I use associated type synonyms to do this. But there are other libraries that wrote similar replacements for the Functor class [2,3 and probably some more]. So if you want to use these you fill have to swap out the Functor (Applicative and Monad) hierarchy with another one that supports constraints. So this is definitly possible and no limitation of GHC. The standard library just does not allow it. Categorically a standard functor goes from the category Hask to Hask. A restricted or constrained functor can be seen as a functor that goes from a category C (Hask with the constraints) to Hask together with an injective functor that embeds C in a subcategory of Hask (which allows you to use it in Haskell). Best, Jan [1]: http://hackage.haskell.org/package/supermonad-0.1/docs/Control-Supermonad-Co... [2]: https://hackage.haskell.org/package/rmonad-0.8.0.2/docs/Control-RMonad.html#... [3]: http://hackage.haskell.org/package/constrained-monads-0.1.0.0/docs/Control-M... 2017-02-28 12:00 GMT+00:00 :

I've done a bit of work on a library that allows you to `fmap` say over sets, (i.e. fmap :: Ord a, Ord b => (a -> b) -> Set a -> Set b) which is more powerful than Monofunctor as it doesn't require the types to be the same.

And it will involve importing a new version of "Functor" and "fmap", however it should be largely backwards compatible.

However, it still needs a little bit of work to be in a state ready to be uploaded to Hackage (and even then, it probably needs even more work) but I'll give you a buzz when its up, hopefully in the next week or so.

On Tue, Feb 28, 2017 at 2:01 PM, Guru Devanla wrote:

...

Thanks, Will. I think adding the constraint to the functions is a good idea. Plus, it makes the intent clearer at the function level.

On Mon, Feb 27, 2017 at 6:02 PM, Will Yager wrote:

...

1. You could use MonoFunctor (complicated and probably not a good idea here) or just put the Taskable constraint on functions instead of on the Task definition (good and easy).

So it would be

data Task a = Task a deriving Functor

And then put "Taskable a" on functions that require it.

2. You can't do it because it doesn't really make sense. A big part of a functor is that it has to be totally agnostic of what it's parametrized over. Otherwise you could easily violate the functor laws.

Good question though, I used to wonder the same thing.

cheers, Will

On Feb 27, 2017, at 6:48 PM, Guru Devanla wrote:

Hello All,

I am working on a program that will define a bunch of tasks. Each task will have to implement certain methods as part of a type class.

-- task 1 data UpdateAcctsTask = UpdateAccts

-- task 2 data EmailConfig = EmaiConfig {someattrs::String} data SendEmailTask = SendEmailsTask EmailConfig

-- task 3 data GeneralWriterTask a = GeneralWriterTask a

Each of these tasks implement a class, Taskable. The return values are simplified for this example.

class Taskable a where process :: a -> Bool can_run :: a -> Bool

This works fine. I can expand on these tasks and execute them.

Now, I wanted to be able to defined dependencies between these (Taskable's). I decided I could create a data type for this dependency and may be also get a FreeMonad around this structure for further processing using a graph of Tasks. But, before that I wanted to create an wrapper for these Taskables and create a functor for it as follows

The first thing I did was, define a Task, which generalizes over all the above defined (and future Taskables)

data Task a where Task :: (Taskable a) => a -> Task a

instance Functor Task where fmap:: (Taskable a, Taskable b) -> (a -> b) -> Task a -> Task b

---

...

...

THIS DOES NOT WORK fmap f (Task a) = Task $ f a

But, I realized that I cannot define an fmap over a type constraint.

My questions are:

1. Is there any way to do this. I see there is an answer of SO. I wanted to make sure if there were any improvements to this since that answer' was posted. http://stackoverflow.com/questions/17157579/functor-instance -for-a-gadt-with-type-constraint

2. Secondly, I would like to know why this is not possible. Is it a current limitation of GHC or if there is some fundamental category theory concepts that dis-allows such declarations that I need to grok!

Appreciate any help on this. Thank you!

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