First one is closed: there is a very clear list of all possibilities, kept in one place. Even if it's exported, it's impossible to add anything to the list of tasks without modifying that module.
Second is open; if it's exported, users of your module can add their own tasks.
On the other hand, adding new function that works on all tasks is, in the first case, simple: you can just write it in the same way as your `doTask`. Users can do that without modifying the module. In the second case you have to change your `Task` class if you want to add a function.
08.01.2016, 12:56, "Lian Hung Hon"
Dear haskellers,
What is the difference between writing
data Task = GroceryTask String | LaundryTask Int
doTask :: Task -> IO () doTask (GroceryTask s) = print "Going to " ++ s doTask (LaundryTask n) = print (show n ++ " pieces washed"
and
class Task a where work :: a -> IO ()
data GroceryTask = GroceryTask String data LaundryTask = LaundryTask Int
instance Task GroceryTask where ..
instance Task LaundryTask where ..
doTask :: Task a => a -> IO () doTask = work
They seem to be similar functionality wise, except that one is on the data level and another is on the class level. How should one go about deciding to use data or class? Is there a semantic difference? Which is more appropriate here?
Happy new year, Hon ,
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