CODEWITHSUNDEEP
scalascala-cats
softshipper
softshipper

Reputation: 34099

type mismatch with some

I have following function definition:

def addMonoid[A](items: List[A])(implicit monoid: Monoid[A]) =
  items.foldLeft(monoid.empty)((a, b) => monoid.combine(a, b))

When I try to use it as follow: 

add(List(Some(1), Some(2), Some(3)))

I've got:

Error:(35, 15) type mismatch;
 found   : Some[Int]
 required: Int
add(List(Some(1), Some(2), Some(3)));}
         ^

What is wrong?

Upvotes: 1

Views: 92

Answers (2)

Arnon Rotem-Gal-Oz
Arnon Rotem-Gal-Oz

Reputation: 25929

Also you can use Monoid directly in cats

import cats._
import cats.implicits._
Monoid[Option[Int]].combineAll(List(Some(1), Some(2), Some(3)))

Upvotes: 1

Yuval Itzchakov
Yuval Itzchakov

Reputation: 149598

Since a Monoid[Option] is defined, and not a Monoid[Some], cats isn't finding an implicit in scope. If you use the some syntax sugar cats provides which infers to Option[A], this will work:

def addMonoid[A: Monoid](items: List[A]) =
  items.foldLeft(Monoid[A].empty)((a, b) => Monoid[A].combine(a, b))

addMonoid(List(1.some, 2.some, 3.some)).foreach(println)

Yields

6

Upvotes: 3

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