Show constructor to use receiver type to enable direct object propert…

…y access, Show and Eq doc examples (arrow-kt#835)

modules/core/arrow-typeclasses/src/main/kotlin/arrow/typeclasses/Show.kt

@@ -17,12 +17,12 @@ interface Show<in A> {
   companion object {
 
     /**
-     * Construct a [Show] instance from a function `(A) -> String`
+     * Construct a [Show] instance from a function `A.() -> String`
      *
      * @param fshow function that defines a textual representation for type [A].
      * @returns a [Show] instance that is defined by the [fshow] function.
      */
-    inline operator fun <A> invoke(crossinline fshow: (A) -> String): Show<A> = object : Show<A> {
+    inline operator fun <A> invoke(crossinline fshow: A.() -> String): Show<A> = object : Show<A> {
       override fun A.show(): String =
         fshow(this)
     }

modules/docs/arrow-docs/docs/docs/typeclasses/eq/README.md

@@ -68,6 +68,11 @@ Eq.any().run { Some(1).eqv(Option.just(1)) }
 Eq.any().run { Eval.later { 1 }.eqv(Eval.later { 1 }) }
 ```
 
+```kotlin:ank
+// using invoke constructor
+val intEq = Eq<Int> { a, b -> a == b }
+```
+
 See [Deriving and creating custom typeclass]({{ '/docs/patterns/glossary' | relative_url }}) to provide your own `Eq` instances for custom datatypes.
 
 ### Data Types

modules/docs/arrow-docs/docs/docs/typeclasses/show/README.md

@@ -43,6 +43,12 @@ import arrow.typeclasses.*
 Show.any().run { Option.just(1).show() }
 ```
 
+```kotlin:ank
+// using invoke constructor
+class Person(val firstName: String, val lastName: String)
+val personShow = Show<Person> { "Hello $firstName $lastName" }
+```
+
 See [Deriving and creating custom typeclass]({{ '/docs/patterns/glossary' | relative_url }}) to provide your own `Show` instances for custom datatypes.