not found: value λ

[steinybot]

Any idea why I can use * but not λ?

sbt:hedgehog> show Test/libraryDependencies
[info] test / Test / libraryDependencies
[info] 	List(org.scala-lang:scala-library:2.12.11, org.typelevel:kind-projector:0.11.0:plugin->default(compile))
[info] runner / Test / libraryDependencies
[info] 	List(org.scala-lang:scala-library:2.12.11, org.typelevel:kind-projector:0.11.0:plugin->default(compile))
[info] example / Test / libraryDependencies
[info] 	List(org.scala-lang:scala-library:2.12.11, org.typelevel:kind-projector:0.11.0:plugin->default(compile))
[info] sbt-test / Test / libraryDependencies
[info] 	List(org.scala-lang:scala-library:2.12.11, org.typelevel:kind-projector:0.11.0:plugin->default(compile), org.scala-sbt:test-interface:1.0)
[info] core / Test / libraryDependencies
[info] 	List(org.scala-lang:scala-library:2.12.11, org.typelevel:kind-projector:0.11.0:plugin->default(compile))
[info] Test / libraryDependencies
[info] 	List(org.scala-lang:scala-library:2.12.11, org.typelevel:kind-projector:0.11.0:plugin->default(compile))
sbt:hedgehog> Test/compile
[info] Compiling 18 Scala sources to /Users/jason/source/scala-hedgehog/test/target/scala-2.12/test-classes ...
[error] /Users/jason/source/scala-hedgehog/test/src/test/scala/hedgehog/core/TreeTest.scala:8:28: not found: value λ
[error]   private val ToProperty = λ[PF1[Tree, PropertyT]](treeProperty)
[error]                            ^
[error] /Users/jason/source/scala-hedgehog/test/src/test/scala/hedgehog/core/TreeTest.scala:8:52: missing argument list for method treeProperty in object TreeTest
[error] Unapplied methods are only converted to functions when a function type is expected.
[error] You can make this conversion explicit by writing `treeProperty _` or `treeProperty(_)` instead of `treeProperty`.
[error]   private val ToProperty = λ[PF1[Tree, PropertyT]](treeProperty)
[error]                                                    ^
[error] /Users/jason/source/scala-hedgehog/test/src/test/scala/hedgehog/core/TreeTest.scala:8:15: private val ToProperty in object TreeTest is never used
[error]   private val ToProperty = λ[PF1[Tree, PropertyT]](treeProperty)
[error]               ^
[error] /Users/jason/source/scala-hedgehog/test/src/test/scala/hedgehog/core/TreeTest.scala:15:15: private method treeProperty in object TreeTest is never used
[error]   private def treeProperty[A](tree: Tree[A]): PropertyT[A] =
[error]               ^
[error] /Users/jason/source/scala-hedgehog/test/src/test/scala/hedgehog/predef/EitherTest.scala:10:28: not found: value λ
[error]   private val ToProperty = λ[PF1[Either[Nothing, *], PropertyT]](eitherProperty)
[error]                            ^
[error] /Users/jason/source/scala-hedgehog/test/src/test/scala/hedgehog/predef/EitherTest.scala:10:66: missing argument list for method eitherProperty in object EitherTest
[error] Unapplied methods are only converted to functions when a function type is expected.
[error] You can make this conversion explicit by writing `eitherProperty _` or `eitherProperty(_)` instead of `eitherProperty`.
[error]   private val ToProperty = λ[PF1[Either[Nothing, *], PropertyT]](eitherProperty)
[error]                                                                  ^
[error] /Users/jason/source/scala-hedgehog/test/src/test/scala/hedgehog/predef/EitherTest.scala:5:30: Unused import
[error] import hedgehog.predef.Monad._
[error]                              ^
[error] /Users/jason/source/scala-hedgehog/test/src/test/scala/hedgehog/predef/EitherTest.scala:10:15: private val ToProperty in object EitherTest is never used
[error]   private val ToProperty = λ[PF1[Either[Nothing, *], PropertyT]](eitherProperty)
[error]               ^
[error] /Users/jason/source/scala-hedgehog/test/src/test/scala/hedgehog/predef/EitherTest.scala:17:15: private method eitherProperty in object EitherTest is never used
[error]   private def eitherProperty[A](either: Either[Nothing, A]): PropertyT[A] =
[error]               ^
[error] /Users/jason/source/scala-hedgehog/test/src/test/scala/hedgehog/predef/IdTest.scala:10:28: not found: value λ
[error]   private val ToProperty = λ[PF1[Id, PropertyT]](identityProperty)
[error]                            ^
[error] 21 errors found
[error] (test / Test / compileIncremental) Compilation failed
[error] Total time: 1 s, completed 4/04/2020, 9:54:15 am

Someone else has the same problem on StackOverflow https://stackoverflow.com/questions/48767385/kind-compiler-plugin-%CE%BB-not-found

[larsrh]

Do you have a branch where I could try to reproduce the problem?

[steinybot]

Yep. Here is the branch https://github.com/steinybot/scala-hedgehog/tree/future-result. Here is the failed build https://travis-ci.com/github/steinybot/scala-hedgehog/jobs/313853593.

[larsrh]

Thanks, I'll check it out.

[larsrh]

@steinybot OK, the problem wasn't that kind-projector was triggered, but because there are some type errors underneath that produced a misleading error message.

After applying these changes to commit steinybot/scala-hedgehog@5933a2b:

diff --git a/test/src/test/scala/hedgehog/core/TreeTest.scala b/test/src/test/scala/hedgehog/core/TreeTest.scala
index ba95937..c0af20b 100644
--- a/test/src/test/scala/hedgehog/core/TreeTest.scala
+++ b/test/src/test/scala/hedgehog/core/TreeTest.scala
@@ -5,7 +5,7 @@ import hedgehog.runner._
 
 object TreeTest extends Properties {
 
-  private val ToProperty = λ[PF1[Tree, PropertyT]](treeProperty)
+  private val ToProperty = λ[PF1[Tree, PropertyT]](t => treeProperty(t))
 
   override def tests: List[Test] =
     List(
diff --git a/test/src/test/scala/hedgehog/predef/EitherTest.scala b/test/src/test/scala/hedgehog/predef/EitherTest.scala
index 455220a..46d9355 100644
--- a/test/src/test/scala/hedgehog/predef/EitherTest.scala
+++ b/test/src/test/scala/hedgehog/predef/EitherTest.scala
@@ -7,7 +7,7 @@ import hedgehog.runner._
 
 object EitherTest extends Properties {
 
-  private val ToProperty = λ[PF1[Either[Nothing, *], PropertyT]](eitherProperty)
+  private val ToProperty = λ[PF1[Either[Nothing, *], PropertyT]](e => eitherProperty(e))
 
   override def tests: List[Test] =
     List(

Then I get the following errors:

sbt:hedgehog> ~ Test/compile
[info] Compiling 18 Scala sources to /tmp/scala-hedgehog/test/target/scala-2.12/test-classes ...
[error] /tmp/scala-hedgehog/test/src/test/scala/hedgehog/predef/EitherTest.scala:14:42: no type parameters for method propTailRecMIsStackSafe: (f: hedgehog.StackSafeTest.PF1[F,hedgehog.core.PropertyT])(implicit evidence$1: hedgehog.predef.Monad[F])hedgehog.Property exist so that it can be applied to arguments (hedgehog.StackSafeTest.PF1[[β$0$]scala.util.Either[Nothing,β$0$],hedgehog.core.PropertyT])
[error]  --- because ---
[error] argument expression's type is not compatible with formal parameter type;
[error]  found   : hedgehog.StackSafeTest.PF1[[β$0$]scala.util.Either[Nothing,β$0$],hedgehog.core.PropertyT]
[error]  required: hedgehog.StackSafeTest.PF1[?F,hedgehog.core.PropertyT]
[error]       property("tailRecM is stack safe", propTailRecMIsStackSafe(ToProperty))
[error]                                          ^
[error] /tmp/scala-hedgehog/test/src/test/scala/hedgehog/predef/EitherTest.scala:14:66: type mismatch;
[error]  found   : hedgehog.StackSafeTest.PF1[[β$0$]scala.util.Either[Nothing,β$0$],hedgehog.core.PropertyT]
[error]  required: hedgehog.StackSafeTest.PF1[F,hedgehog.core.PropertyT]
[error]       property("tailRecM is stack safe", propTailRecMIsStackSafe(ToProperty))
[error]                                                                  ^
[error] /tmp/scala-hedgehog/test/src/test/scala/hedgehog/predef/EitherTest.scala:5:30: Unused import
[error] import hedgehog.predef.Monad._
[error]                              ^
[error] /tmp/scala-hedgehog/test/src/test/scala/hedgehog/predef/EitherTest.scala:10:15: private val ToProperty in object EitherTest is never used
[error]   private val ToProperty = λ[PF1[Either[Nothing, *], PropertyT]](e => eitherProperty(e))
[error]               ^

I'm unsure how to proceed from there since I'm not familiar with hedgehog, but I hope that's sufficient for you to debug the problem further. The other occurrences of λ can be modified accordingly, which either makes the error go away completely or turn up a similar type error as above.

[steinybot]

Ahh awesome. Thank you very much @larsrh for taking the time to look into this!

Surprising that the eta-expansion isn't working as expected and that the end result is an error about not being able to find λ.

I made the same changes as you, explicitly using a function literal, and I didn't have any compilation errors.

[larsrh]

Great to hear! Can we close this issue, then?

[steinybot]

Well my particular problem is solved but the error is still confusing. If there is no way to improve it then yeah sure, just close this.

[larsrh]

Good point. I'll leave it open.

[non]

Coming late to this, I think the issue here is that Kind-Projector triggers based on the shape of the trees it works with. It's possible that this code had an unsupported shape. When I get a chance I'll try to create a test case that reliably triggers the issue on valid code.