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Consider a case class with a possibly large number of members; to illustrate the case assume two arguments, as in 

case class C(s1: String, s2: String)

and therefore assume an array with size of at least that many arguments,

val a = Array("a1", "a2")

Then 

scala> C(a(0), a(1))
res9: C = c(a1,a2)

However, is there an approach to case class instantiation where there is no need to refer to each element in the array for any (possibly large) number of predefined class members ?

elm
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3 Answers3

7

No, you can't. You cannot guarantee your array size is at least the number of members of your case class.

You can use tuples though.

Suppose you have a mentioned case class and a tuple that looks like this:

val t = ("a1", "a2")

Then you can do:

c.tupled(t)
serejja
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  • Note a solution following all the answers. – elm Mar 07 '14 at 14:08
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    This ought to be the accepted answer. aside from the fact that the question author is the same person who accepted their own answer, not sure why the answer above is the accepted answer and not this one. Apparently the OP was interested in an answer that relies on the Shapeless library, but Shapeless is nowhere mentioned in the OP, it just appears in a tag. What's more, as this answer shows, Shapeless isn't necessary. – doug Dec 22 '16 at 08:03
4

Having gathered bits and pieces from the other answers, a solution that uses Shapeless 2.0.0 is thus as follows,

import shapeless._
import HList._
import syntax.std.traversable._

val a = List("a1", 2)                    // List[Any]
val aa = a.toHList[String::Int::HNil]
val aaa = aa.get.tupled                  // (String, Int)

Then we can instantiate a given case class with

case class C(val s1: String, val i2: Int)
val ins = C.tupled(aaa)

and so 

scala> ins.s1
res10: String = a1

scala> ins.i2
res11: Int = 2

The type signature of toHList is known at compile time as much as the case class members types to be instantiate onto.

elm
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3

To convert a Seq to a tuple see this answer: https://stackoverflow.com/a/14727987/2483228

Once you have a tuple serejja's answer will get you to a c.

Note that convention would have us spell c with a capital C.

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Mark Lister
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