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

[Clinton Mead]

I've done a bit of work on a library that allows you to `fmap` say over
sets, (i.e. fmap :: Ord a, Ord b => (a -> b) -> Set a -> Set b) which is
more powerful than Monofunctor as it doesn't require the types to be the
same.

And it will involve importing a new version of "Functor" and "fmap",
however it should be largely backwards compatible.

However, it still needs a little bit of work to be in a state ready to be
uploaded to Hackage (and even then, it probably needs even more work) but
I'll give you a buzz when its up, hopefully in the next week or so.

On Tue, Feb 28, 2017 at 2:01 PM, Guru Devanla <gurudev.devanla at gmail.com>
wrote:

> 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 <will.yager at gmail.com> 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 <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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>
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