I have a type, defined as follows:
import scalaz._, Scalaz._
case class MyInt(i : Int)
I want to make an instance of Semigroup
. I tried this:
object MyInt {
implicit def myIntSemigroup: Semigroup[MyInt] = new Semigroup[MyInt] {
def append(a: MyInt, b: MyInt) : MyInt = MyInt(a.i + b.i)
}
}
When I run sbt console
, I get this error:
[info] Set current project to hello (in build file:/home/mp/code/scala/examples/semigroup/)
[info] Compiling 1 Scala source to /home/mp/code/scala/examples/semigroup/target/scala-2.10/classes...
[error] /home/mp/code/scala/examples/semigroup/src/main/scala/Main.scala:6: object creation impossible, since method append in trait Semigroup of type (f1: MyInt, f2: => MyInt)MyInt is not defined
[error] implicit def myIntSemigroup: Semigroup[MyInt] = new Semigroup[MyInt] {
[error] ^
[error] one error found
[error] (compile:compile) Compilation failed
[error] Total time: 5 s, completed 25-Jun-2015 08:15:37
How do I make MyInt
a Semigroup
so I can use |+|
on it?
For completeness, here is my build.sbt
file:
name := "hello"
version := "1.0"
scalaVersion := "2.10.5"
libraryDependencies += "org.scalaz" %% "scalaz-core" % "7.1.3"
If you look carefully for the signature of the Scalaz semigroup, the second parameter is defined as by name while in your implementation you are having a different signature. Change your second parameter to
b: => MyInt
and it should compile