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

Will Yager will.yager at gmail.com
Tue Feb 28 02:02:16 UTC 2017

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. 


> On Feb 27, 2017, at 6:48 PM, Guru Devanla <gurudev.devanla at 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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