Providing additional arguments to map on HList

Question

I would like to do something like this:

def run(subjects: List[Subject]) = {
  val configs = compute()
  subjects.map(s => configs.map(c => test(s,c)))
  // or flatMap, I don't really care at this point
}

In my use case, subjects are actually Subject[T] and I need a typesafe version of T in the result. So I have:

def run[L <: HList](subjects: L)(implicit mapper: Mapper[testFun.type, L]) = {
  val configs = compute()
  subjects.map(testFun)
}

However, now I am unable to pass in the configurations to testFun which, according to this post, needs to have a singleton type.

An option would be to do:

val cfgHL = HList.fill(subjects.length)(configs)
(subjects zip cfgHL).map(testFun)

But HList currently does not have a fill operation. Any hints?

HList now has a fill method (not in the official release yet though). — Alex Archambault, Jul 26 '14 at 21:31

score 3 · Accepted Answer · edited May 23 '17 at 12:11

You can use mapConst to accomplish the same thing as fill. If we have the following:

val xs = 1 :: 'a :: 'a' :: HNil

We can write:

scala> xs.zip(xs mapConst "x") == (1, "x") :: ('a, "x") :: ('a', "x") :: HNil
res0: Boolean = true

Note that there are other ways to tackle the problem of partially applying (higher-rank) polymorphic functions and then mapping with them—see for example my answer here. Something like that is likely to be overkill for your use case, though.

Providing additional arguments to map on HList

1 Answers1