first Selective instances

Author: richard-gibson

modules/core/arrow-core-extensions/src/main/kotlin/arrow/core/extensions/function0.kt

@@ -1,5 +1,6 @@
 package arrow.core.extensions
 
+import arrow.Kind
 import arrow.core.*
 import arrow.core.extensions.function0.monad.monad
 import arrow.extension
@@ -42,6 +43,12 @@ interface Function0Applicative : Applicative<ForFunction0> {
     Function0.just(a)
 }
 
+@extension
+interface Function0Selective : Selective<ForFunction0>, Function0Applicative {
+  override fun <A, B> Function0Of<Either<A, B>>.select(f: Kind<ForFunction0, (A) -> B>): Kind<ForFunction0, B> =
+    fix().flatMap  { it.fold({l -> just(l).ap(f)}, {r -> just(identity(r))})}
+}
+
 @extension
 interface Function0Monad : Monad<ForFunction0> {
   override fun <A, B> Function0Of<A>.ap(ff: Function0Of<(A) -> B>): Function0<B> =

modules/core/arrow-core-extensions/src/main/kotlin/arrow/core/extensions/id.kt

@@ -1,4 +1,5 @@
 @file:Suppress("UnusedImports")
+
 package arrow.core.extensions
 
 import arrow.Kind
@@ -58,6 +59,12 @@ interface IdApplicative : Applicative<ForId> {
     Id.just(a)
 }
 
+@extension
+interface IdSelective : Selective<ForId>, IdApplicative {
+  override fun <A, B> IdOf<Either<A, B>>.select(f: Kind<ForId, (A) -> B>): Kind<ForId, B> =
+    fix().flatMap { it.fold({ l -> just(l).ap(f) }, { r -> just(identity(r)) }) }
+}
+
 @extension
 interface IdMonad : Monad<ForId> {
   override fun <A, B> IdOf<A>.ap(ff: IdOf<(A) -> B>): Id<B> =

modules/core/arrow-core-extensions/src/main/kotlin/arrow/core/extensions/option.kt

@@ -123,6 +123,12 @@ interface OptionApplicative : Applicative<ForOption> {
     Option.just(a)
 }
 
+@extension
+interface OptionSelective : Selective<ForOption>, OptionApplicative {
+  override fun <A, B> OptionOf<Either<A, B>>.select(f: OptionOf<(A) -> B>): Option<B> =
+    fix().flatMap  { it.fold({l -> just(l).ap(f)}, {r -> just(identity(r))})}
+}
+
 @extension
 interface OptionMonad : Monad<ForOption> {
   override fun <A, B> OptionOf<A>.ap(ff: OptionOf<(A) -> B>): Option<B> =