Should I use typeclasses or not?

Question

I have some difficulties to understand when use and when not use typeclass in my code. I mean create my own, and not use already defined typeclasses, of course. By example (very stupid example), should I do: ``` data Cars = Brakes | Wheels | Engine data Computers = Processor | RAM | HardDrive class Repairable a where is_reparaible :: a -> Bool instance Repairable Cars where is_repairable (Brakes) = True is_repairable (Wheels) = False is_repairable (Engine) = False instance Repairable Computers where is_repairable (Processor) = False is_repairable (RAM) = False is_repairable (HardDrive) = True checkState :: (Reparaible a) => a -> ... checkState a = ... ``` (Obviously, this is an stupid, incomplete example). But this is a lot for a little use, no? Why I shouldn't do something simple and only defining functions without defining new data types and typeclasses (with their instances). This example is too simple, but in facts I often see somethings like that (new data types+typeclasses+insta…

Accepted Answer

Why I shouldn't do something simple and only defining functions without defining new data types and typeclasses (with their instances).

Why indeed? You could just define:

```
checkState :: (a -> Bool) -> (a -> b) -> (a -> b) -> a -> b
checkState is_repairable repairs destroy a
    = if (is_repairable a) then repairs a else destroy a
```

People misuse type classes all the time. It doesn't mean that it's idiomatic.

To answer your more general question, here are some rules of thumb for when to use type classes and when not to use them:

Use type classes if:

There is only one correct behavior per given type

The type class has associated equations (i.e. "laws") that all instances must satisfy

Don't use type classes if:

You are trying to just namespace things. That's what modules and namespaces are for.

A person using your type class cannot reason about how it will behave without looking at the source code of the instances

You find that the extensions you have to turn on are getting out of control

Should I use typeclasses or not?

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