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Tupled Function

With functions bounded to arities up to 22 it was possible to generalize some operation on all function types using overloading. Now that we have functions and tuples generalized to arities above 22 overloading is not an option anymore. The type class TupleFunction provides a way to abstract directly over a function of any arity converting it to an equivalent function that receives all arguments in a single tuple.

/** Type class relating a `FunctionN[..., R]` with an equivalent tupled function `Function1[TupleN[...], R]`
 *
 *  @tparam F a function type
 *  @tparam G a tupled function type (function of arity 1 receiving a tuple as argument)
 */
@implicitNotFound("${F} cannot be tupled as ${G}")
sealed trait TupledFunction[F, G] {
  def tupled(f: F): G
  def untupled(g: G): F
}

The compiler will synthesize an instance of TupledFunction[F, G] if:

F is a function type of arity N
G is a function with a single tuple argument of size N and its types are equal to the arguments of F
The return type of F is equal to the return type of G
F and G are the same sort of function (both are (...) => R or both are (...) ?=> R)
If only one of F or G is instantiated the second one is inferred.

Examples

TupledFunction can be used to generalize the Function1.tupled, ... Function22.tupled methods to functions of any arities. The following defines tupled as extension method (full example).

/** Creates a tupled version of this function: instead of N arguments,
 *  it accepts a single [[scala.Tuple]] with N elements as argument.
 *
 *  @tparam F the function type
 *  @tparam Args the tuple type with the same types as the function arguments of F
 *  @tparam R the return type of F
 */
extension [F, Args <: Tuple, R](f: F)
  def tupled(using tf: TupledFunction[F, Args => R]): Args => R = tf.tupled(f)

TupledFunction can be used to generalize the Function.untupled to a function of any arities (full example)

/** Creates an untupled version of this function: instead of a single argument of type [[scala.Tuple]] with N elements,
 *  it accepts N arguments.
 *
 *  This is a generalization of [[scala.Function.untupled]] that work on functions of any arity
 *
 *  @tparam F the function type
 *  @tparam Args the tuple type with the same types as the function arguments of F
 *  @tparam R the return type of F
 */
extension [F, Args <: Tuple, R](f: Args => R)
  def untupled(using tf: TupledFunction[F, Args => R]): F = tf.untupled(f)

TupledFunction can also be used to generalize the Tuple1.compose and Tuple1.andThen methods to compose functions of larger arities and with functions that return tuples.

/** Composes two instances of TupledFunction into a new TupledFunction, with this function applied last.
 *
 *  @tparam F a function type
 *  @tparam G a function type
 *  @tparam FArgs the tuple type with the same types as the function arguments of F and return type of G
 *  @tparam GArgs the tuple type with the same types as the function arguments of G
 *  @tparam R the return type of F
 */
extension [F, G, FArgs <: Tuple, GArgs <: Tuple, R](f: F)
  def compose(g: G)(using tg: TupledFunction[G, GArgs => FArgs], tf: TupledFunction[F, FArgs => R]): GArgs => R = {
  (x: GArgs) => tf.tupled(f)(tg.tupled(g)(x))
}