writing generic function using method name - scala

Question

I am trying to write the following method:
case class Config2(repoName: String)
def buildAction[A, M, R <: HList]()
(implicit
gen: Generic.Aux[Config2, R],
mod: Modifier.Aux[R, M, A, A, R])
: (A, Config2) => Config2 = {
(arg: A, c: Config2) => {
val rec = mod.apply(gen.to(c), _ => arg)
gen.from(rec)
}
}
When trying to use it with:
buildAction[String, Witness.`'repoName`.T, String :: HList]()
I get an error:
could not find implicit value for parameter gen: shapeless.Generic.Aux[com.advancedtelematic.tuf.cli.Cli.Config2,shapeless.::[String,shapeless.HList]]
[error] val _ = buildAction[String, Witness.`'repoName`.T, String :: HList]()
am I missing some import here?
Second question is, can I somehow rewrite this signature so I don't don't have to specify all the types? In practive the Config2 type takes a long list of fields so it's not pratical to write this all the time
Update:
I simplified this to the following:
val CGen = LabelledGeneric[Config]
def buildAction[A, M]()
(implicit mod: Modifier.Aux[CGen.Repr, M, A, A, CGen.Repr])
: (A, Config) => Config = {
(arg: A, c: Config) => {
val rec = mod.apply(CGen.to(c), _ => arg)
CGen.from(rec)
}
}
Which allows me to just write:
buildAction[String, Witness.`'repoName`.T]()
But I still have to specify that Witness. Is the a way I could write buildAction[String]("repoName") and have some method provide the Witness implicitly?
Update: the following works!
val CGen = LabelledGeneric[Config]
def buildAction[A](witness: Witness)
(implicit mod: Modifier.Aux[CGen.Repr, witness.T, A, A, CGen.Repr]):
(A, Config) => Config = {
(arg: A, c: Config) => {
val rec = mod.apply(CGen.to(c), _ => arg)
CGen.from(rec)
}
}
buildAction[RepoName]('repoName)

Answer 1

am I missing some import here?
No, it's probably just that String :: HList must be String :: HNil
Second question is, can I somehow rewrite this signature so I don't don't have to specify all the types?
You can use a trick known as kinda-curried type parameters:
object buildAction {
class PartiallyApplied[A, M] {
def apply[R <: HList]()(implicit ...)
}
def apply[A, M] = new PartiallyApplied[A, M]
}
Used as
buildAction[String, Witness.`'foo`.T]()
Also, since your code mentions field name, you probably want LabelledGeneric in conjunction with ops.record.Updater