sbt/util/collection/TypeFunctions.scala

/* sbt -- Simple Build Tool
 * Copyright 2010 Mark Harrah
 */
package sbt

trait TypeFunctions
{
	type Id[X] = X
	trait Const[A] { type Apply[B] = A }
	trait Compose[A[_], B[_]] { type Apply[T] = A[B[T]] }
	trait P1of2[M[_,_], A] { type Apply[B] = M[A,B]; type Flip[B] = M[B, A] }

	final val left = new (Id ~> P1of2[Left, Nothing]#Flip) { def apply[T](t: T) = Left(t) }
	final val right = new (Id ~> P1of2[Right, Nothing]#Apply) { def apply[T](t: T) = Right(t) }
	final val some = new (Id ~> Some) { def apply[T](t: T) = Some(t) }
}
object TypeFunctions extends TypeFunctions

trait ~>[-A[_], +B[_]]
{ outer =>
	def apply[T](a: A[T]): B[T]
	// directly on ~> because of type inference limitations
	final def ∙[C[_]](g: C ~> A): C ~> B = new (C ~> B) { def apply[T](c: C[T]) = outer.apply(g(c)) }
	final def ∙[C,D](g: C => D)(implicit ev: D <:< A[D]): C => B[D] = i => apply(ev(g(i)) )
	final def fn[T] = (t: A[T]) => apply[T](t)
}
object ~>
{
	import TypeFunctions._
	val Id: Id ~> Id = new (Id ~> Id) { def apply[T](a: T): T = a }
	implicit def tcIdEquals: (Id ~> Id) = Id
}