Passing a Shapeless Extensible Record to a Function

Question

I am trying to learn Shapeless (using version 2.10.2). I have created a very simple extensible record:

val rec1 = (\"foo\" ->> 42) :: HNil

Answer 1

As of shapeless 2.1.0 there is a new syntax to express record types:

scala> :paste
// Entering paste mode (ctrl-D to finish)

import shapeless._
import shapeless.record._
import shapeless.syntax.singleton._

def fun(x: Record.`"foo" -> Int`.T) = x("foo")

// Exiting paste mode, now interpreting.

import shapeless._
import shapeless.record._
import shapeless.syntax.singleton._
fun: (x: shapeless.::[Int with shapeless.labelled.KeyTag[String("foo"),Int],shapeless.HNil])Int

scala> fun( ("foo" ->> 42) :: HNil )
res2: Int = 42

scala> fun( ("foo" ->> 42) :: ("bar" ->> 43) :: HNil )
<console>:30: error: type mismatch;
 found   : shapeless.::[Int with shapeless.labelled.KeyTag[String("foo"),Int],shapeless.::[Int with shapeless.labelled.KeyTag[String("bar"),Int],shapeless.HNil]]
 required: shapeless.::[Int with shapeless.labelled.KeyTag[String("foo"),Int],shapeless.HNil]
       fun( ("foo" ->> 42) :: ("bar" ->> 43) :: HNil )

But the Selector is probably the best approach for OP's use-case.

Answer 2

Here's something a little more general that I think might answer your question. Suppose we want to write a method that will work on any record with a "foo" key. We can use a combination of a witness and a selector:

import shapeless._, record._, syntax.singleton._

val w = Witness("foo")

def fun[L <: HList](xs: L)(implicit sel: ops.record.Selector[L, w.T]) = xs("foo")

And then:

scala> fun(("foo" ->> 42) :: HNil)
res0: Int = 42

Or:

scala> fun(("bar" ->> 'a) :: ("foo" ->> 42) :: HNil)
res1: Int = 42

If we really wanted to only allow records with no other fields, we could write the following:

def fun(l: Int with KeyTag[w.T, Int] :: HNil) = l("foo")

But that's somewhat at odds with the way records are generally used.

We have to define the witness precisely because Scala 2.10 doesn't provide any way to refer to a singleton type directly—see for example my fork of Alois Cochard's Shona project for some discussion.

I will add as a final disclaimer that I'm only just now getting familiar with Shapeless 2.0 myself, but I don't think even Miles is magical enough to get around this limitation.