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
https://lukepalmer.wordpress.com/2010/01/24/haskell-antipattern-existential-...
.
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
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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