Re: [Haskell-cafe] Declaring Functors for Type constrained data types

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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