optimize chain's get.

.travis.yml

@@ -2,15 +2,13 @@ language: scala
 
 sudo: required
 
-dist: trusty
-
 group: edge
 
 git:
   depth: 9999
 
 jdk:
-  - oraclejdk8
+  - openjdk8
 
 scala_version_211: &scala_version_211 2.11.12
 scala_version_212: &scala_version_212 2.12.9

core/src/main/scala/cats/data/Chain.scala

@@ -512,6 +512,23 @@ sealed abstract class Chain[+A] {
     else false
 
   override def hashCode: Int = hash(Hash.fromUniversalHashCode[A])
+
+  final def get(idx: Long): Option[A] =
+    if (idx < 0) None
+    else {
+      var result: Option[A] = None
+      var i = 0L
+      foreachUntil { a =>
+        if (idx == i) {
+          result = Some(a)
+          true
+        } else {
+          i += 1
+          false
+        }
+      }
+      result
+    }
 }
 
 object Chain extends ChainInstances {
@@ -724,6 +741,8 @@ sealed abstract private[data] class ChainInstances extends ChainInstances1 {
         go(f(a) :: Nil)
         acc
       }
+
+      override def get[A](fa: Chain[A])(idx: Long): Option[A] = fa.get(idx)
     }
 
   implicit def catsDataShowForChain[A](implicit A: Show[A]): Show[Chain[A]] =

core/src/main/scala/cats/data/NonEmptyChain.scala

@@ -448,6 +448,9 @@ sealed abstract private[data] class NonEmptyChainInstances extends NonEmptyChain
         Eval.defer(fa.reduceRightTo(a => Eval.now(f(a))) { (a, b) =>
           Eval.defer(g(a, b))
         })
+
+      override def get[A](fa: NonEmptyChain[A])(idx: Long): Option[A] =
+        if (idx == 0) Some(fa.head) else fa.tail.get(idx - 1)
     }
 
   implicit def catsDataOrderForNonEmptyChain[A: Order]: Order[NonEmptyChain[A]] =

tests/src/test/scala/cats/tests/ChainSuite.scala

@@ -254,4 +254,10 @@ class ChainSuite extends CatsSuite {
     }
   }
 
+  test("Chain#get is consistent with List#lift") {
+    forAll { (x: Chain[Int], idx: Int) =>
+      x.get(idx) should ===(x.toList.lift(idx))
+    }
+  }
+
 }