From f0b89933f97c73c14484643c14438d23e1723334 Mon Sep 17 00:00:00 2001
From: Stewart Stewart <stewinsalot@gmail.com>
Date: Sun, 18 Dec 2016 16:22:14 -0500
Subject: add SameId for typechecked conversion between Ids

---
 src/main/scala/xyz/driver/core/core.scala | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

(limited to 'src/main/scala/xyz')

diff --git a/src/main/scala/xyz/driver/core/core.scala b/src/main/scala/xyz/driver/core/core.scala
index 8ae9122..ce6be34 100644
--- a/src/main/scala/xyz/driver/core/core.scala
+++ b/src/main/scala/xyz/driver/core/core.scala
@@ -1,6 +1,7 @@
 package xyz.driver
 
 import scalaz.Equal
+import scala.annotation.implicitNotFound
 
 package object core {
 
@@ -24,6 +25,24 @@ package object core {
 
 package core {
 
+  @implicitNotFound("No evidence that ${A} has the same Id as ${B}")
+  sealed trait SameId[A, B] {
+    def convert(id: Id[A]): Id[B] = Id[B](id.value)
+  }
+
+  object SameId extends LowPrioritySameIdImplicits {
+    def apply[A, B] = new SameId[A, B] {}
+
+    implicit def reflexive[A]: A ~ A                        = SameId[A, A]
+    implicit def symmetric[A, B](implicit ab: A ~ B): B ~ A = SameId[B, A]
+  }
+
+  trait LowPrioritySameIdImplicits {
+    protected type ~[A, B] = SameId[A, B]
+
+    implicit def transitive[A, B, C](implicit ab: A ~ B, bc: B ~ C): A ~ C = SameId[A, C]
+  }
+
   final case class Id[+Tag](value: String) extends AnyVal {
     @inline def length: Int       = value.length
     override def toString: String = value
@@ -32,6 +51,12 @@ package core {
   object Id {
     implicit def idEqual[T]: Equal[Id[T]]       = Equal.equal[Id[T]](_ == _)
     implicit def idOrdering[T]: Ordering[Id[T]] = Ordering.by[Id[T], String](_.value)
+
+    implicit class InvariantIdOps[Tag](id: Id[Tag]) {
+      def asId[T](implicit eq: SameId[Tag, T]): Id[T] = eq.convert(id)
+    }
+
+    def sameId[A, B] = SameId[A, B]
   }
 
   final case class Name[+Tag](value: String) extends AnyVal {
-- 
cgit v1.2.3