Add typecheck method

Author: Atry

shared/build.sbt

@@ -1 +1,3 @@
 libraryDependencies += "org.scalatest" %%% "scalatest" % "3.0.0" % Test
+
+libraryDependencies += "com.chuusai" %%% "shapeless" % "2.3.2"

shared/src/main/scala/com/thoughtworks/Extractor.scala

@@ -1,5 +1,8 @@
 package com.thoughtworks
 
+import shapeless._
+import shapeless.syntax.typeable._
+
 private[thoughtworks] sealed trait LowLowPriorityExtractor {
 
   implicit final class FunctionToExtractor[-A, +B] private[LowLowPriorityExtractor] (underlying: A => B) {
@@ -101,4 +104,13 @@ object Extractor extends LowPriorityExtractor {
     }
   }
 
+  implicit final class OptionFunctionToTypeableExtractor[-A: Typeable, +B] private[Extractor] (
+      underlying: A => Option[B]) {
+    def typecheck = new Extractor[A, B] {
+      override def unapply(a: A): Option[B] = {
+        a.cast[A].flatMap(underlying)
+      }
+    }
+  }
+
 }

shared/src/test/scala/com/thoughtworks/ExtractorSpec.scala

@@ -73,4 +73,24 @@ class ExtractorSpec extends FreeSpec with Matchers {
     test(2) should be("matched by optional function")
     test(3) should be("Not matched")
   }
+
+  "type checking" in {
+    val pairToString = { pair: (Seq[Int], List[String]) =>
+      (pair._1 ++ pair._2).mkString
+    }
+
+    def test(a: Any) = {
+      a match {
+        case pairToString.extract.typecheck(s) => s"strict $s"
+        case pairToString.extract(s) => s
+        case _ => "Not matched"
+      }
+    }
+
+    test((Nil, List("1"))) should be("strict 1")
+    test((Seq("xxx"), List(3))) should be("xxx3")
+    test(0.5) should be("Not matched")
+
+  }
+
 }