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Use updated cats-infographic, v2.0 #4907

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Fix some markdown style issues for typeclasses.md
ShapelessCat Sep 15, 2026
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Update cats-infographic
ShapelessCat Sep 15, 2026
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44 changes: 21 additions & 23 deletions docs/typeclasses.md
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# Type Classes

Type classes are a powerful tool used in functional programming to enable ad-hoc polymorphism, more commonly
known as overloading. Where many object-oriented languages leverage subtyping for polymorphic code, functional
programming tends towards a combination of parametric polymorphism (think type parameters, like Java generics)
and ad-hoc polymorphism.

## Example: collapsing a list

The following code snippets show code that sums a list of integers, concatenates a list of strings, and unions a list
of sets.

Expand Down Expand Up @@ -42,6 +44,7 @@ def combineAll[A](list: List[A], m: Monoid[A]): A = list.foldRight(m.empty)(m.co
```

## Type classes vs. subtyping

The definition above takes an actual monoid argument instead of doing the usual object-oriented practice of using
subtype constraints.

Expand Down Expand Up @@ -156,6 +159,7 @@ combineAll(List(Paired(1, "hello"), Paired(2, " "), Paired(3, "world")))
```

## A note on syntax

In many cases, including the `combineAll` function above, the implicit arguments can be written with syntactic sugar.

```scala mdoc:compile-only
Expand Down Expand Up @@ -227,36 +231,30 @@ You can find out more about law testing [here](typeclasses/lawtesting.md).

## Type classes in Cats

<img src="https://cdn.rawgit.com/tpolecat/cats-infographic/master/cats.svg" alt="infographic" style="width: 100%;"/>
From [cats-infographic by @tpolecat](https://github.com/tpolecat/cats-infographic).

<img src="https://raw.githubusercontent.com/ShapelessCat/cats-infographic/main/cats-infographic.svg" alt="infographic" style="width: 100%;"/>
From [cats-infographic by @tpolecat, updated by @ShapelessCat](https://github.com/ShapelessCat/cats-infographic).

## Incomplete type class instances in cats

Originally from [@hobwekiva](https://gist.github.com/hobwekiva/d63508ddb6a728015ace53cb70a1fd5d)


| Type | Functor | Apply | Applicative | Monad | MonoidK | ApplicativeError | MonadError | CoflatMap | Comonad | Bimonad |
| --------------- |:-------:|:-----------------:|:-----------:|:-----:|:-------:|:-----------------:|:----------:|:---------:|:-------:|:-------:|
| `Id[A]` | ✔ | ✔ | ✔ | ✔ | ✗ | ✗ | ✗ | ✔ | ✔ |✔ |
| `Eval[A]` | ✔ | ✔ | ✔ | ✔ | ✗ | ✗ | ✗ | ✔ | ✔ |✔ |
| `Option[A]` | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✗ |✗ |
| `Const[K, A]` | ✔ | ✔ (`K:Monoid`) | ✔ | ✗ | ✗ | ✗ | ✗ | ✗ | ✗ |✗ |
| `Either[E, A]` | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✗ | ✗ |✗ |
| `List[A]` | ✔ | ✔ | ✔ | ✔ | ✔ | ✗ | ✗ | ✔ | ✗ |✗ |
| `NonEmptyList[A]` | ✔ | ✔ | ✔ | ✔ | ✗ | ✗ | ✗ | ✔ | ✔ |✔ |
| `Stream[A]` | ✔ | ✔ | ✔ | ✔ | ✔ | ✗ | ✗ | ✔ | ✗ |✗ |
| `Map[K, A]` | ✔ | ✔ | ✗ | ✗ | ✔ | ✗ | ✗ | ✗ | ✗ |✗ |
| `Validated[E, A]` | ✔ | ✔ (`E: Semigroup`)| ✔ | ✗ | ✗ | ✔ (`E: Semigroup`)| ✗ | ✗ | ✗ |✗ |
| `Reader[E, A]` | ✔ | ✔ | ✔ | ✔ | ✗ | ✗ | ✗ | ✗ | ✗ |✗ |
| `Writer[E, A]` | ✔ | ✔ (`E:Monoid`) | ✔ | ✔ | ✗ | ✗ | ✗ | ✔ | ✗ |✗ |



| Type | Functor | Apply | Applicative | Monad | MonoidK | ApplicativeError | MonadError | CoflatMap | Comonad | Bimonad |
| ----------------- |:-------:|:-----------------:|:-----------:|:-----:|:-------:|:-----------------:|:----------:|:---------:|:-------:|:-------:|
| `Id[A]` | ✔ | ✔ | ✔ | ✔ | ✗ | ✗ | ✗ | ✔ | ✔ |✔ |
| `Eval[A]` | ✔ | ✔ | ✔ | ✔ | ✗ | ✗ | ✗ | ✔ | ✔ |✔ |
| `Option[A]` | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✗ |✗ |
| `Const[K, A]` | ✔ | ✔ (`K:Monoid`) | ✔ | ✗ | ✗ | ✗ | ✗ | ✗ | ✗ |✗ |
| `Either[E, A]` | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✗ | ✗ |✗ |
| `List[A]` | ✔ | ✔ | ✔ | ✔ | ✔ | ✗ | ✗ | ✔ | ✗ |✗ |
| `NonEmptyList[A]` | ✔ | ✔ | ✔ | ✔ | ✗ | ✗ | ✗ | ✔ | ✔ |✔ |
| `Stream[A]` | ✔ | ✔ | ✔ | ✔ | ✔ | ✗ | ✗ | ✔ | ✗ |✗ |
| `Map[K, A]` | ✔ | ✔ | ✗ | ✗ | ✔ | ✗ | ✗ | ✗ | ✗ |✗ |
| `Validated[E, A]` | ✔ | ✔ (`E: Semigroup`)| ✔ | ✗ | ✗ | ✔ (`E: Semigroup`)| ✗ | ✗ | ✗ |✗ |
| `Reader[E, A]` | ✔ | ✔ | ✔ | ✔ | ✗ | ✗ | ✗ | ✗ | ✗ |✗ |
| `Writer[E, A]` | ✔ | ✔ (`E:Monoid`) | ✔ | ✔ | ✗ | ✗ | ✗ | ✔ | ✗ |✗ |

## Further reading
* [Returning the "Current" Type in Scala][fbounds]


* [Returning the "Current" Type in Scala][fbounds]

[fbounds]: http://tpolecat.github.io/2015/04/29/f-bounds.html "Returning the "Current" Type in Scala"
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