What is the point of a Semigroup for Int in cats?

Question

In the cats docs they show an example for Semigroup[Int]:

val list = List(1, 2, 3, 4, 5) val rightwards = List(1, 2, 3).foldRight(0)(_ |+| _)

So the |+| operator is a semigroup combine. What's the point of this extra machinery if good 'ol "+" works just fine here? What does the Semigroup buy you for Int, or likely for any simple type for that matter?

Semigroup is an abstraction, Int + is an implementation. Abstractions (FP type classes or OOP interfaces/traits) are good for decoupling. — Dmytro Mitin
– Dmytro Mitin, Commented Oct 10, 2022 at 1:14
The point of an abstraction is usually using it on an abstract context, is like asking what is the point of Animal if I all I want is a Dog - BTW, in this case you can actually see advantages without needing to go abstract, just use combineAll rather than foldRight or use |+| with two maps whose values are Ints — Luis Miguel Mejía Suárez
– Luis Miguel Mejía Suárez, Commented Oct 10, 2022 at 1:16
In addition to the above comments, basically these are all building blocks of a bigger building, so your question can be expanded to: "What's the point of a Semigroup, Monoid, and so on?", and the answer would be clear. — AminMal
– AminMal, Commented Oct 10, 2022 at 10:04
I like the point with Animal and Dog and to fine grain + is the "operation" on them which could be "kill" or "love" each other. Cats provides the implementation for primitive data types. If have a case class let's say case class SalesCustomer( count:Int, value:Int) you can then create a semigroup to add up the number of all sales and revenue without having to implement the operation of Semigroup(Int,+) in first place. — Sven
– Sven, Commented Apr 18, 2023 at 8:16

Daenyth · Accepted Answer · 2023-04-27 15:11:22Z

Semigroup is an abstraction. The point of it is that you can abstract things.

Instead of

def combineIntList(xs: List[Int]) = xs.foldRight(0)(_ + _) def combineDoubleList(xs: List[Double]) = xs.foldRight(0d)(_ + _) def combineStringList(xs: List[String]) = xs.foldRight("")(_ + _)

You write

def combine[A: Monoid](xs: List[A]) = xs.foldRight(Monoid[A].empty)(_ |+| _)

The free (pay what you want) ebook Scala with Cats talks about these concepts in much more detail

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