This problem I was solving is more of a re-implementation of some code I had in Python. In Python, I had a very class-based structure of this design. By class based structure, I mean we have class-level methods for each class (where each class is a type of Task), and all of them implement the same set of methods. This classes are never instantiated, but all methods are invoked on the class.

WIth that perspective, the record patter @Patrick recommended directly maps to that design. The record data type here becomes a abstract-representation of my class(abstract base class in OO terms) and each task provides its methods. I see that relationship. Is that the approach I should be aiming for?

Secondly, while choosing the type-class approach, I imagined all these required class methods to be an interface and therefore an interface could directly map to a type class in Haskell. Is that thinking usually an anti-pattern.

The existential antipattern. link was very useful and made me re-asses my inclination to defining type classes and embedded types right away.

On Tue, Feb 28, 2017 at 9:55 AM, Patrick Chilton <chpatrick@gmail.com> wrote:

You could also consider representing tasks like this instead of using a typeclass:

data Task = Task
{ process :: m ()
, canRun :: m Bool
}

The Taskable + existential GADT example seems like it could be an example of the existential antipattern.

If your GADT really does have a as a type parameter, it would be more idiomatic to check for the typeclass when you use it:

doStuffWithTasks :: Taskable a => Task a -> ...

But then what's the point of the Task datatype?

On Tue, Feb 28, 2017 at 1:48 AM, Guru Devanla <gurudev.devanla@gmail.com> 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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