Bring to latest Scala and SBT versions, fix compile errors

.gitignore

@@ -19,3 +19,6 @@ project/plugins/project/
 # Emacs Ensime specific
 .ensime
 .ensime_cache
+
+# IntelliJ
+.idea

build.sbt

@@ -32,9 +32,9 @@ homepage := Some(url("https://github.com/iht/fpinscala/"))
 
 startYear := Some(2014)
 
-scalaVersion := "2.12.6"
+scalaVersion := "2.13.1"
 
-libraryDependencies += "org.specs2" %% "specs2-core" % "4.2.0" % "test"
+libraryDependencies += "org.specs2" %% "specs2-core" % "4.7.1" % "test"
 
 scalacOptions in Test ++= Seq("-Yrangepos")
 

project/build.properties

@@ -1 +1 @@
-sbt.version=1.1.6
+sbt.version=1.3.2

project/build.sbt

@@ -1,5 +1,5 @@
 addSbtPlugin("org.scoverage" % "sbt-scoverage" % "1.5.1")
 
-addSbtPlugin("org.scoverage" % "sbt-coveralls" % "1.2.5")
+addSbtPlugin("org.scoverage" % "sbt-coveralls" % "1.2.7")
 
-addSbtPlugin("org.wartremover" % "sbt-wartremover" % "2.2.1")
+addSbtPlugin("org.wartremover" % "sbt-wartremover" % "2.4.2")

src/main/scala/chap03/ex26.scala

@@ -19,7 +19,7 @@
  * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
  * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
  * SOFTWARE.
- */  
+ */
 
 // ---------------------
 // Code for example 3.26
@@ -30,10 +30,20 @@ package chap03
 import adt._
 
 object Ex26 {
-
+
+  // Naive version
+  // Causes stack overflow with very deep trees
+  def maximumNaive[A: Numeric](t: Tree[A]): A = {
+    val maxOp: (A, A) => A = implicitly[Numeric[A]].max _
+    t match {
+      case Leaf(x) => x
+      case Branch(l, r) => maxOp(maximum(l), maximum(r))
+    }
+  }
+
   // Tail recursive version
   def maximum[A: Numeric](t: Tree[A]): A = {
-    val maxOp = implicitly[Numeric[A]].max _  
+    val maxOp: (A, A) => A = implicitly[Numeric[A]].max _
 
     // This should be the smallest possible value for
     // Numeric type A
@@ -46,22 +56,12 @@ object Ex26 {
     @annotation.tailrec
     def loop(t1: List[Tree[A]], m: A): A = {
       t1 match {
-	case Nil => m
-	case Leaf(x) :: lt => loop(lt, maxOp(m,x))
-	case Branch(l,r) :: lt =>  loop(l :: r :: lt, m)
+        case Nil => m
+        case Leaf(x) :: lt => loop(lt, maxOp(m, x))
+        case Branch(l, r) :: lt => loop(l :: r :: lt, m)
       }
     }
 
     loop(List(t), zero)
   }
-
-  // Naive version
-  // Causes stack overflow with very deep trees
-  def maximumNaive[A: Numeric](t: Tree[A]): A = {
-    val maxOp = implicitly[Numeric[A]].max _
-    t match {
-      case Leaf(x) => x
-      case Branch(l, r) => maxOp(maximum(l), maximum(r))
-    }
-  }
 }

src/main/scala/chap03/ex29.scala

@@ -62,7 +62,7 @@ object Ex29 {
   }
 
   def maximum[A: Numeric](t: Tree[A]): A = {
-    val max = implicitly[Numeric[A]].max _
+    val max: (A,A) => A = implicitly[Numeric[A]].max _
     val minVal = implicitly[Numeric[A]].fromInt(Int.MinValue)
 
     fold(t, minVal)((m,y) => max(m,y))