Add doctests for Semigroup, Group, and Monoid

[DennisVDB]

PR for part of #2479.

Adds doctests for the Semigroup, Group, and Monoid typeclasses.

[DennisVDB]

sbt validate is failing with:

error] /Users/dennis/Documents/Programming/scala/cats/kernel/.jvm/target/scala-2.12/src_managed/test/cats/kernel/GroupDoctest.scala:4:19: object scalacheck is not a member of package org
[error] import _root_.org.scalacheck.Prop._
[error]                   ^
[error] /Users/dennis/Documents/Programming/scala/cats/kernel/.jvm/target/scala-2.12/src_managed/test/cats/kernel/GroupDoctest.scala:7:24: object scalacheck is not a member of package org
[error]     extends _root_.org.scalacheck.Properties("Group.scala") {
[error]                        ^
[error] /Users/dennis/Documents/Programming/scala/cats/kernel/.jvm/target/scala-2.12/src_managed/test/cats/kernel/GroupDoctest.scala:7:46: no arguments allowed for nullary constructor Object: ()Object
[error]     extends _root_.org.scalacheck.Properties("Group.scala") {
[error]                                              ^
[error] /Users/dennis/Documents/Programming/scala/cats/kernel/.jvm/target/scala-2.12/src_managed/test/cats/kernel/GroupDoctest.scala:15:3: not found: value include
[error]   include(new _root_.org.scalacheck.Properties("Group.scala:10: inverse") {
...

[kailuowang]

probably need to add this setting to the kernel settings

.settings(libraryDependencies += "org.scalacheck" %%% "scalacheck" % scalaCheckVersion(scalaVersion.value) % "test")

[LukaJCB]

Don't you mean combineAll here?

[DennisVDB]

@kailuowang it doesn't seem to fix the problem.

Locally I'm using the updated build.sbt from this PR #2382 as I couldn't import the project in IntelliJ. But that shouldn't influence the fact that travis fails.

[kailuowang]

I might missed something but I don't see build.sbt updated

[DennisVDB]

You didn't miss it.

I now pushed the updated build.sbt with the fix you suggested but it still fails the same way as before. It seems that the code generated for the doctest doesn't compile.

The generated code: /cats/kernel/.jvm/target/scala-2.12/src_managed/test/cats/kernel/*Doctest.scala

Travis failure: https://travis-ci.org/typelevel/cats/jobs/432625136
Local failure: https://gist.github.com/DennisVDB/1fa934962f2f192af80731b1336adb1d

[larsrh]

You have to change the imports to be more specific and use the objects where the instances are defined, like so:

$ git diff
diff --git a/kernel/src/main/scala/cats/kernel/Group.scala b/kernel/src/main/scala/cats/kernel/Group.scala
index c792245e..31d01914 100644
--- a/kernel/src/main/scala/cats/kernel/Group.scala
+++ b/kernel/src/main/scala/cats/kernel/Group.scala
@@ -14,7 +14,7 @@ trait Group[@sp(Int, Long, Float, Double) A] extends Any with Monoid[A] {
    *
    * Example:
    * {{{
-   * scala> import cats.implicits._
+   * scala> import cats.kernel.instances.int._
    *
    * scala> Group[Int].inverse(5)
    * res0: Int = -5
@@ -29,7 +29,7 @@ trait Group[@sp(Int, Long, Float, Double) A] extends Any with Monoid[A] {
    *
    * Example:
    * {{{
-   * scala> import cats.implicits._
+   * scala> import cats.kernel.instances.int._
    *
    * scala> Group[Int].remove(5, 2)
    * res0: Int = 3

This is a bit annoying, but I don't see any way around it, short of defining a cats.kernel.implicits._ import object.

[larsrh]

The problem is that cats.implicits is only defined in cats-core, not in cats-kernel, so importing it here won't work.

[larsrh]

Pending successful build, approval from me.

[codecov-io]

Codecov Report

Merging #2523 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2523      +/-   ##
==========================================
- Coverage   95.35%   95.34%   -0.02%     
==========================================
  Files         358      358              
  Lines        6530     6535       +5     
  Branches      282      279       -3     
==========================================
+ Hits         6227     6231       +4     
- Misses        303      304       +1

Impacted Files	Coverage Δ
kernel/src/main/scala/cats/kernel/Semigroup.scala	68.75% <ø> (ø)	⬆️
kernel/src/main/scala/cats/kernel/Group.scala	71.42% <ø> (ø)	⬆️
kernel/src/main/scala/cats/kernel/Monoid.scala	100% <ø> (ø)	⬆️
core/src/main/scala/cats/syntax/eq.scala	0% <0%> (ø)	⬆️
core/src/main/scala/cats/data/Validated.scala	100% <0%> (ø)	⬆️
core/src/main/scala/cats/data/Ior.scala	98.5% <0%> (+0.02%)	⬆️
core/src/main/scala/cats/data/NonEmptyChain.scala	98.98% <0%> (+1.01%)	⬆️

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