Re: https://github.com/Zhuinden/guide-to-kotlin/wiki/1.)-Major-Syntax-Differences#interfaces-with-val-var-and-fun

I would add that you can define a default implementation in the interface

interface MyContract {
  var value: String
  val name: String
  val params: Map<String, String> 
     get() = emptyMap()

  fun doSomething(input1: String, input2: String) {
    // Default Implementation that does nothing
  }
}

Default implementations are great especially when you add an optional feature to an interface later. You don't break the existing implementations if you have a default implementaiton.

And that using properties instead of methods is such a great idea because you can use all the powerful encapsulation features that come with properties.

Like defining the overriding properties in the constructor

class SimpleContract(override var value: String, override val name: String) 
  : MyContract 

Hello @Zhuinden
I have finished reviewing your guide.
Nice job, this will be useful to lots of people! 👍

There is one big thing I rewrote that you probably will want to review: Kotlin properties.
They look simple so people don't pay attention, but I find them one of the most useful and under-hyped aspects of Kotlin.

https://github.com/Zhuinden/guide-to-kotlin/wiki/4.)-The-Tricky-Stuff#kotlin-properties-and-encapsulation
(Not sure if they should stay in the "tricky stuff" page!)

Another concern I have is that by creating, moving or renaming some paragraphs, I have probably broken some of the links in the README.

In the visibility section, it says:

There is NO package visibility in Kotlin

This is actually not true, as explained in the official guide, in the section Visibility modifiers - package.

I was also confused by the explanation about internal. In the wiki it says that internal is not equivalent to package, and it is true, but I'd add that internal is "module" visibility (where modules are IntelliJ modules, Maven projects, gradle source sets, etc).

Re: https://github.com/Zhuinden/guide-to-kotlin/wiki/1.)-Major-Syntax-Differences#object-for-singleton

Proposed good use cases for object:

1. Have an object where you define your constants:

object Network {
    const val API_URL = "http://httpbin.org"
    const val TIMEOUT = 1000 // ms
}

2. Use it as a namespace for pure functions

object Math {
    fun sum(a: Int, b: Int) : Int = a+b
    //.... 
}

I now have writing rights but I prefer to open an issue about this.

Re: https://github.com/Zhuinden/guide-to-kotlin/wiki/2.)-Basic-Kotlin-Features#data-classes

For me data class with zero parameters is simply an object.

object gives you the right toString(), equals() and hashCode().

This happens most frequently in the context of a sealed class

sealed class Cmd

data class Reset(val value: Int) : Cmd()

// ERROR: Data Class must have at least one primary constructor parameter
data class Increment : Cmd()

// BAD: Decrement() != Decrement()
class Decrement : Cmd()

The correct thing to do here is to use an object and JetBrains actually has a quick-fix for it:

sealed class Cmd
data class Reset(val value: Int) : Cmd()
object Increment : Cmd()
object Decrement : Cmd()

And JetBrains actually made a quick-fix for that!

Re: https://github.com/Zhuinden/guide-to-kotlin/wiki/1.)-Major-Syntax-Differences#arrayof-and-arrayofnulls-and-listof-and-mutablelistof-and-linkedmapof

There is a handy collection initializer that is not very well-known where you create a list with a size of elements and a lambda for each element

val squares = List(size = 6) { it * it } 
// same as:
val squares = listOf(0, 1, 4, 9, 16, 25)

No ternary oeprator: I would add that the fact there is no ternary operator is just the logical conclusion of everything being an expression. Once you get that, using if/else or when just makes sense.

https://github.com/Zhuinden/guide-to-kotlin/wiki/1.)-Major-Syntax-Differences#there-is-no-condition--true--false-ternary-operator

Do you think it'd be a good idea to have a section on generics in Kotlin?

• we must specify argument list with the name of the argument first, **semi-**colon, then the type

should be

• we must specify argument list with the name of the argument first, colon, then the type

where T: Blah, T: Meh