Logs: liberachat/#haskell
| 2021-08-05 20:59:42 | <yahb> | dsal: * -> Constraint |
| 2021-08-05 20:59:48 | <dsal> | % :k Ordering |
| 2021-08-05 20:59:48 | <yahb> | dsal: * |
| 2021-08-05 21:00:28 | <dsal> | % :k Ordering |
| 2021-08-05 21:00:28 | <yahb> | dsal: Type |
| 2021-08-05 21:00:44 | <seydar> | dsal: thank you! How would you describe the relationship between a datatype and a class? |
| 2021-08-05 21:00:50 | <dsal> | (we used to call type `*` for some reason) |
| 2021-08-05 21:01:12 | → | lavaman joins (~lavaman@98.38.249.169) |
| 2021-08-05 21:01:29 | <dsal> | The way I did above, mostly. Classes are used to constrain the possible types. |
| 2021-08-05 21:01:37 | <dsal> | % :t abs |
| 2021-08-05 21:01:37 | <yahb> | dsal: Num a => a -> a |
| 2021-08-05 21:02:13 | <seydar> | I guess I'm struggling in describing what Ordering is |
| 2021-08-05 21:02:28 | <dsal> | % :t Ordering |
| 2021-08-05 21:02:28 | <yahb> | dsal: ; <interactive>:1:1: error: Data constructor not in scope: Ordering |
| 2021-08-05 21:02:32 | <dsal> | % :t k Ordering |
| 2021-08-05 21:02:32 | <yahb> | dsal: ; <interactive>:1:1: error: Variable not in scope: k :: t0 -> t; <interactive>:1:3: error: Data constructor not in scope: Ordering |
| 2021-08-05 21:02:34 | <dsal> | % :k Ordering |
| 2021-08-05 21:02:34 | <yahb> | dsal: Type |
| 2021-08-05 21:02:38 | <dsal> | Ordering is just a type. |
| 2021-08-05 21:02:42 | <dsal> | % :i Ordering |
| 2021-08-05 21:02:42 | <yahb> | dsal: type Ordering :: Type; data Ordering = LT | EQ | GT; -- Defined in `GHC.Types'; instance Eq Ordering -- Defined in `GHC.Classes'; instance Monoid Ordering -- Defined in `GHC.Base'; instance Ord Ordering -- Defined in `GHC.Classes'; instance Semigroup Ordering -- Defined in `GHC.Base'; instance Enum Ordering -- Defined in `GHC.Enum'; instance Show Ordering -- Defined in `GHC.Show'; instance Data Ordering |
| 2021-08-05 21:02:52 | <geekosaur> | Ordering is not directly related to Ord |
| 2021-08-05 21:03:10 | <geekosaur> | althoiugh it has an Ord instance |
| 2021-08-05 21:03:48 | <dsal> | And `Ord` *uses* `Ordering` in `compare` |
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| 2021-08-05 21:03:58 | <geekosaur> | types which have an Ord instance promise to have instance methods one of which happens to return Ordering |
| 2021-08-05 21:04:16 | <dsal> | But there's nothing particularly special about it. It might be slightly confusing that `Ordering`s are orderable themselves. |
| 2021-08-05 21:04:38 | <geekosaur> | but there's nothing in the class which requires a class member to do something like that, it's just how Ord works |
| 2021-08-05 21:04:54 | <dsal> | % LT < GT |
| 2021-08-05 21:04:54 | <yahb> | dsal: True |
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| 2021-08-05 21:05:08 | <dsal> | That's actually a little weird. heh |
| 2021-08-05 21:05:14 | <seydar> | When we say that Int is an instance of the Ord class, we're saying that Int implements the constraints prescribed by Ord |
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| 2021-08-05 21:05:46 | <dsal> | I don't think we say "Int is an instance of the Ord class" -- the language is slightly backwards there. |
| 2021-08-05 21:05:50 | <dsal> | We say "Int has an Ord instance" |
| 2021-08-05 21:06:24 | <dsal> | But otherwise, yeah. |
| 2021-08-05 21:06:52 | <seydar> | Ordering is just an arbitrary constructor that the Ord class uses in the implementation of `compare` because it needs to return something |
| 2021-08-05 21:06:53 | <dsal> | The subtle difference is that "Int" doesn't do anything to "implement" the class. The instance can come from somewhere else altogether. |
| 2021-08-05 21:07:16 | <dsal> | seydar: Yes! Some languages use int there, but that's bad for numerous reasons. |
| 2021-08-05 21:07:18 | <seydar> | Ordering is a datatype that can be use by anyone and anything and just happens to be used by the Ord class |
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| 2021-08-05 21:09:02 | <dsal> | Ordering also has a really useful semigroup instance such that you can append multiple orderings together in a useful way. Int can't exactly do that because what it means to append to Ints is too specific to what you're doing. |
| 2021-08-05 21:09:13 | <dsal> | > EQ <> LT <> GT |
| 2021-08-05 21:09:14 | <lambdabot> | LT |
| 2021-08-05 21:10:31 | <dsal> | e.g., you couldn't do something like this if it returned an int: |
| 2021-08-05 21:10:31 | <dsal> | > (comparing fst <> comparing snd) (0, 'c') (0, 'a') |
| 2021-08-05 21:10:33 | <lambdabot> | GT |
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| 2021-08-05 21:12:35 | <dsal> | But in general, you can think of the class from the consumer point of view. If I want to write a function that does some addition, I don't need to specify something concrete like an Int or whatever, I just need to specify that the type I'm operating on knows how to add to values of itself together. This lets me work with Ints or types I've never heard of. |
| 2021-08-05 21:13:53 | <seydar> | Thank you! |
| 2021-08-05 21:13:59 | <seydar> | This has been super helpful |
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