如何寫A :: B :: C => D給定A :: B :: C和（A，B，C）=> D？

Question

假設我有一個HList類型A :: B :: C和函數(A, B, C) => D。如何用最簡單，最習慣的方式編寫函數A :: B :: C => D？

Answer 1

有方法toProduct在syntax.std.function：

import shapeless._ 
import syntax.std.function._ 
def testf(d: Double, l: Long, i: Int): String = s"$d, $l, $i" 

(testf _).toProduct 
// Double :: Long :: Int :: HNil => String = <function1> 

見Feature overview: shapeless 2.0.0/Facilities for abstracting over arity。