Implicit not resolved on high order function - scala

Question

I've got a function create is going to provide the parameter DSLContext of op: DSLContext => T :
def create[T](op: DSLContext => T)(implicit errorHandler: Throwable => Unit):Option[T] = {
...
op(new DSLContext)
...
}
Then I pass the following function to create:
def query(implicit dsl:DSLContext)
And here is the call
create(query)
I got this compiler error:
could not find implicit value for parameter dsl: org.jooq.DSLContext
[error] create (query)
I can resolve this issue this way : create {implicit dsl => query}
However i wonder why is the compiler unable to resolve the implicit parameter dsl as a given parameter ?

Answer 1

The issue here is that when you pass a method where a function is expected the compiler needs to perform an eta-expansion to convert it.
In other words the method query needs to become a Function1[DSLContext, T].
However, the compiler needs to resolve the implicit parameter in order to be able to apply the eta-expansion. In your case there's no DSLContext in scope, so the compiler fails in doing so.
You can reproduce this behavior with a simpler example, here's a REPL session demonstrating it:
scala> def foo(implicit i: Int) = "hello"
foo: (implicit i: Int)String
scala> foo _
<console>:10: error: could not find implicit value for parameter i: Int
foo _
^
scala> implicit val x = 5
x: Int = 5
scala> foo _
res3: scala.collection.immutable.WrappedString = hello
where _ manually triggers the eta-expansion.
For the record, this is described in the language reference, paragraph 6.26.2.
If the method takes only implicit parameters, implicit arguments are passed following the rules of §7.2.