How I can return a function within itself it s like def f x

Question

How I can return a function within itself it s like def f x print x return f then I can do f 1 2 3

jw · Answer

`return f`

Ifvwm · Answer

< jw> then what its type is when the function return itself it won t produce an infinite type

jw · Answer

don t know use an expression body and find out

jw · Answer

it gets rejected by the compiler

Ifvwm · Answer

it seems we can t do that in any static type check language

streetsofboston · Answer

Yup the return type of `f` would be infinitely recursive ` Int gt Int gt Int gt Int gt `

Ifvwm · Answer

< streetsofboston> but if we ve known the times what it would be called then we can get the result type manually fun f x Int Int gt Int gt Any then we can call f 3 5 7

jw · Answer

to be fair recursive types are allowed

streetsofboston · Answer

< Ifvwm> But then you cannot do `return f` unless `f` returns a plain `Any` itself

streetsofboston · Answer

< jw> How do you declare such a recursive type in Kotlin

jw · Answer

Ifvwm · Answer

< streetsofboston> ``` fun test x String String gt String gt Any println x return test test 1 2 3 ```

streetsofboston · Answer

But ` test` has the type ` String gt String gt String gt Any ` Can that be coerced cast to ` String gt String gt Any` If so then ` String gt String gt String gt Any ` is a sub type of ` String gt String gt Any`

Ifvwm · Answer

actually I really don t know about that sub type stuff I m from Haskell there s no such thing like type inheritance

Ifvwm · Answer

a gt b gt a is a gt b gt a in type level

streetsofboston · Answer

grinning In other words If so is a ` String gt String gt String gt Any ` safely assignable to a ` String gt String gt Any`

streetsofboston · Answer

But in this case there s one extra String Is this true as well `a gt a gt a gt b` is `a gt a gt b`

Ifvwm · Answer

< streetsofboston> yes but when it s concreted then no

streetsofboston · Answer

Then how can test be returned as a return value to itself in Kotlin

Ifvwm · Answer

a is a type variable genetic in java

Ifvwm · Answer

a can be a gt a even a gt a gt a but Int gt Int and Int gt Int gt Int are definitely not the same

Ifvwm · Answer

< streetsofboston> actually I don t know why it can

streetsofboston · Answer

You can construct the type paramater `A` with a concrete type `Int` If `A gt A` is `A` shouldn t `Int gt Int` be `Int` after constructing it with an `Int`

Ifvwm · Answer

< streetsofboston> that s polymorphic

Ifvwm · Answer

Int gt Int and Int gt Int gt Int gt Int both are a gt a I think

streetsofboston · Answer

Or is this possible because of type erasure in JVM In JVM ` A gt A` is ambiguous with any other lambda that looks like ` A gt `

streetsofboston · Answer

Now I m very curious grinning

Ifvwm · Answer

I don t know type erasure in java well but that Haskell guys tell me Java s generics is very different with Haskell s type variables also some people call Java s generics is a false polymorphic

streetsofboston · Answer

It s almost midnight here I m going to ` Z gt Z gt Z gt Z gt Z ` zzz

Ifvwm · Answer

< streetsofboston> good night

Big Chungus · Answer

``` lateinit var f String gt String gt Any f = print it f f 1 2 3 ```

molikuner · Answer

Not quite a function but works like it <https pl kotl in tFUO00goe> ``` object test operator fun invoke i Int = also println i fun main test 0 1 2 3 4 ```