On 01/08/2016 11:38 AM, Imants Cekusins wrote:
How should one go about deciding to use data or class?
class: class lets specify more than one method. when you define instance yet do not implement all methods, compiler warns. if you try to call class method without an instance for that type, compiler warns.
pattern matching: compiler does not warn if methods do not match every constructor of the data type.
Well, there is -fwarn-incomplete-patterns, which should be included in -Wall, which does exactly this.
one way to decide if not sure, is to pick one way which seems easier to refactor. when more code is written, it usually becomes obvious if this approach does not fit. then refactor.
A few things to keep in mind: If you define a type class and instances for Int and String, and later want to add another case for String, you have to add a newtype, otherwise the compiler can not differentiate. Additionally, adding a constructor with multiple fields gets complicated if you choose the type class solution, here you have to add an instance for a tuple. Also you need FlexibleInstances as soon as you want an instance for String or a Tuple more specific than (a, b) (or introduce newtypes). You can also not process a Task if it is hidden in a class. For example, how do you implement doOnlyShoppingTask? In the end you restrict yourself with a type class about the things you can do with the data. So when is this useful? I would argue, if you are writing a library and want your users to be able to define their own tasks. Otherwise I think abstracting a data type with a type class is not worth the hassle.
Which is more appropriate here? depends on the rest of the code. if this is it, then there is no real difference.
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