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

Or even: data Task a = UpdateAcctsTask | SendEmailTask EmailConfig | GeneralWriterTask a process :: Task a -> Bool can_run :: Task a -> Bool In general, start with data types based on the structure of your data and functions that operate on that data. Type classes are not usually the right abstraction for domain specific code, in my experience. Erik On 1 March 2017 at 15:19, Patrick Chilton wrote:

But if you have for example a tree of heterogenous tasks as you said, you can no longer dispatch based on type because the implementation depends on the value of each task in the tree, so you're better off representing it like this:

data Task = Task { doTask :: IO () , taskName :: String }

class Taskable a where toTask :: a -> Task

instance Taskable EmailConfig where toTask (EmailConfig ...) = Task { ... }

type TaskTree = Tree Task -- easy

On Wed, Mar 1, 2017 at 3:11 PM, Guru Devanla wrote:

...

Just to make sure I understand. In my initial example, the following are all types and I wanted the dispatch to happen on this types once they are instances of the Taskable 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

Now, later on I expect users to create more types which are all say, instances of Taskable typeclass. So, here I do not expect to have any dispatch based on value, but based on types.

On Wed, Mar 1, 2017 at 5:24 AM, Patrick Chilton wrote:

...

Is that thinking usually an anti-pattern.

Typeclasses choose between implementations of a method based on static *types* known at compile time. In OOP, which implementation of an interface method you get depends on the run-time object *instance*. They're not really related despite the similarities.

For example, if you have a list of widgets and you want to do something different depending on what their run-time value is, a typeclass would be the wrong thing to use since it dispatches on types, not values. It's possible to make them work a little bit like OOP classes, but then you end up with the classic antipattern.

On Wed, Mar 1, 2017 at 2:16 PM, Guru Devanla wrote:

...

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