Added contravariant instance for ConfigDecoder

vlovgr · Oct 16, 2021 · dc2fbd5 · dc2fbd5
1 parent 8b92d45
commit dc2fbd5
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Showing 2 changed files with 17 additions and 1 deletion.
diff --git a/modules/core/src/main/scala/ciris/ConfigDecoder.scala b/modules/core/src/main/scala/ciris/ConfigDecoder.scala
@@ -6,7 +6,7 @@
 
 package ciris
 
-import cats.{MonadError, Show}
+import cats.{MonadError, Show, Contravariant}
 import cats.implicits._
 import scala.annotation.tailrec
 import scala.concurrent.duration.{Duration, FiniteDuration}
@@ -369,4 +369,13 @@ object ConfigDecoder {
           go(f(b))
         }
     }
+
+  /**
+    * @group Instances
+    */
+  implicit final def configDecoderContravariant[A]: Contravariant[ConfigDecoder[*, A]] =
+    new Contravariant[ConfigDecoder[*, A]] {
+      override def contramap[I, II](fa: ConfigDecoder[I, A])(f: II => I): ConfigDecoder[II, A] =
+        ConfigDecoder[II].map(f).mapEither(fa.decode)
+    }
 }
diff --git a/modules/core/src/test/scala/ciris/ConfigDecoderSpec.scala b/modules/core/src/test/scala/ciris/ConfigDecoderSpec.scala
@@ -6,6 +6,8 @@ import cats.laws.discipline.MonadErrorTests
 import org.scalacheck.Gen
 import scala.concurrent.duration.{Duration, FiniteDuration}
 import scala.util.Try
+import cats.laws.discipline.ContravariantTests
+import cats.Contravariant
 
 final class ConfigDecoderSpec extends BaseSpec {
   test("ConfigDecoder.as") {
@@ -220,6 +222,11 @@ final class ConfigDecoderSpec extends BaseSpec {
     MonadErrorTests[ConfigDecoder[String, *], ConfigError].monadError[String, String, String]
   )
 
+  checkAll(
+    "ConfigDecoder",
+    ContravariantTests[ConfigDecoder[*, String]].contravariant[String, String, String]
+  )
+
   test("ConfigDecoder.short.success") {
     forAll { (short: Short) =>
       assert(ConfigDecoder[String, Short].decode(None, short.toString) === short.asRight)