# Type inference when map with generic function

**URL:** <https://discuss.kotlinlang.org/t/type-inference-when-map-with-generic-function/2038>\
**Category:** Language Design\
**Created:** [October 14, 2016, 8:43pm UTC](https://discuss.kotlinlang.org/t/type-inference-when-map-with-generic-function/2038 "2016-10-14T20:43:42Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![vjache](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/vjache/32/2170_2.png) [@vjache](https://discuss.kotlinlang.org/u/vjache)\
**Post date:** [October 14, 2016, 8:43pm UTC](https://discuss.kotlinlang.org/t/type-inference-when-map-with-generic-function/2038/1 "2016-10-14T20:43:42Z")

</div>

Hi !

here is my situation:

```
open class A {
    fun <T:A> subst(data:Map<String, String>) : T {
        return this.clone().doSubst(data) as T //I know this looks strange
    }
}

class B : A() {
     fun doSomethingElse() {}
}

fun main(args: Array<String>) {

    val coll:List<B> = listOf(B(),B(),B())
    val data = mapOf("x" to "y")
    val coll1:List<B> = coll.map { it.subst(data) } // type inference failed! (even with explicit :List<B>)
}

```

but if I define:

```
fun <T:A> T.subst2(data:Map<String, String>) : T {
    return this
}

```

then this works fine:

```
fun main(args: Array<String>) {

    val coll:List<B> = listOf(B(),B(),B())
    val data = mapOf("x" to "y")
    val coll1 = coll.map { it.subst2(data) } // type inference success! even without (:List<B>).
}

```

So, the extension function ‘subst2’ have an advantage having a chance to capture the type of receiver. It would be great to have some way for member function ‘subst’ to specify to capture (when it possible) the type of a subclass e.g.:

```
open class A {
        fun This.subst(data:Map<String, String>) : This {
            // do something with this
            return this
        }
}

```

I know this is a syntax of extension member function but I meant to capture the type of an original receiver (probably another syntax could be elaborated).

The example of potential usage is chaning:

```
val b = B()

b.subst(data).subst(data1).doSomethingElse()

```

P.S. Sorry if this thing is already discussed somewhere or is a completely crap.

---

<div class="post-metadata">

**Author:** ![ilya.gorbunov](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/ilya.gorbunov/32/1645_2.png) [@ilya.gorbunov](https://discuss.kotlinlang.org/u/ilya.gorbunov)\
**Post date:** [October 15, 2016, 11:23am UTC](https://discuss.kotlinlang.org/t/type-inference-when-map-with-generic-function/2038/2 "2016-10-15T11:23:51Z")

</div>

`This` type for members is unlikely to be supported, currently you have to resort to extension functions to gain similar functionality.  
See also [this thread](https://discuss.kotlinlang.org/t/this-type/1421).

---

<div class="post-metadata">

**Author:** ![vjache](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/vjache/32/2170_2.png) [@vjache](https://discuss.kotlinlang.org/u/vjache)\
**Post date:** [October 15, 2016, 2:21pm UTC](https://discuss.kotlinlang.org/t/type-inference-when-map-with-generic-function/2038/3 "2016-10-15T14:21:35Z")

</div>

Well, I see.