Haskell typeclass hierarchy issue

Question

I have a scenario that involves type classes and I'm not quite sure how to go about solving it.

I have

class Event a where
     timestamp :: a -> UTCTime
     rawData :: a -> ByteString

class Something a where
    something :: a -> SomethingElse

In my code, I want to create an object that implements both Event and Something. However, in certain cases, the function something is going to need the return from a call to rawData to construct the SomethingElse object. I was wondering if there was to structure these type classes to be able to build a function like

convert :: (Event a, Event b, Something b) => a -> b

being able to call convert x :: (Instance of something) in order to convert, a bit like how binary get is used.

I realize that this is a rather vague description, but please let me know if I can add anything else.

Thanks

I'm not entirely clear what you're asking, but in order for convert to be able to return a value of any Event & Something type, you need a way to construct a value of any Event & Something type while knowing only that the type belongs to those two classes. You could give one of the classes a method like unraw :: ByteString -> a or, if such a method would require utilizing features of both classes for some reason, make a new class that inherits both Event & Something and has the desired constructor. — jwodder
– jwodder, Commented Jun 30, 2011 at 0:58
you have to give some context to SomethingElse, way not just ByteString? — wuxb
– wuxb, Commented Jun 30, 2011 at 1:20
Pretty much SomethingElse is just a deserialized object where the bytestring is the serialized version — Charles Durham
– Charles Durham, Commented Jun 30, 2011 at 2:51

Ankur · Accepted Answer · 2011-06-30 09:28:15Z

2

In type class Something you need to make sure that the type a has implemented type class Event, hence the definition of Something becomes:

class Event a => Something a where
     something :: a -> SomethingElse

answered Jun 30, 2011 at 9:28

Ankur

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2 Comments

Charles Durham Over a year ago

How would I abstract it out if Something doesn't always need an event. Lets say only in certain cases a Something is constructed by an event. How would I organize that hierarchy? I'm not that familiar with standard Object oriented practices.

Ankur Over a year ago

In that case you can remove the (Event a) restriction from Something type class and put this restriction on a new method inside Something class.. somethingFromEvent ... this method will create something from event and the parameter to this method must have a instance of Event class.

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