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4

I'm learning Kotlin and I have some trouble with functions.
I'm trying to create something like a functional interface with a generic parameter.
In Java I would create something like this:

@FunctionalInterface
public interface Foo<T extends Bar> 
 String something(T arg);

Then I can use this somewhere else like this (given that Person extends Bar:

Foo<Person> f = p -> p.toString();

How do you write this with Kotlin?

The first thing I tried was to use type-aliases like this:

typealias Foo<T> = (T) -> String

However, it stopped working when I added the bound to the type parameter:

typealias Foo<T: Bar> = (T) -> String // Error: Bounds are not allowed on type alias parameters

The second approach was to write an interface that extends the function type:

interface Foo<T: Bar> : (T) -> String

However, now I don't know how to instantiate a lambda function from with this. It works when I create class from it like this:

class Something: Foo<Person> 
 override fun invoke(p: Person): String 
 return p.toString()
 


val f = Something()

But this is a big overhead and I'm sure there has to be a better solution.

So how can I define a function signature that can be reused by many functions that supports generic parameters with bounds in kotlin?

kotlin

share|improve this question

asked Mar 25 at 19:10

Manuel MaukyManuel Mauky

94511 gold badge99 silver badges1515 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  This question and answer has closely related info: stackoverflow.com/questions/48284994/…
  
  – Mike Hill
  Mar 25 at 20:08
  
  

  
  
  
  

add a comment | 

4

I'm learning Kotlin and I have some trouble with functions.
I'm trying to create something like a functional interface with a generic parameter.
In Java I would create something like this:

@FunctionalInterface
public interface Foo<T extends Bar> 
 String something(T arg);

Then I can use this somewhere else like this (given that Person extends Bar:

Foo<Person> f = p -> p.toString();

How do you write this with Kotlin?

The first thing I tried was to use type-aliases like this:

typealias Foo<T> = (T) -> String

However, it stopped working when I added the bound to the type parameter:

typealias Foo<T: Bar> = (T) -> String // Error: Bounds are not allowed on type alias parameters

The second approach was to write an interface that extends the function type:

interface Foo<T: Bar> : (T) -> String

However, now I don't know how to instantiate a lambda function from with this. It works when I create class from it like this:

class Something: Foo<Person> 
 override fun invoke(p: Person): String 
 return p.toString()
 


val f = Something()

But this is a big overhead and I'm sure there has to be a better solution.

So how can I define a function signature that can be reused by many functions that supports generic parameters with bounds in kotlin?

kotlin

share|improve this question

asked Mar 25 at 19:10

Manuel MaukyManuel Mauky

94511 gold badge99 silver badges1515 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  This question and answer has closely related info: stackoverflow.com/questions/48284994/…
  
  – Mike Hill
  Mar 25 at 20:08
  
  

  
  
  
  

add a comment | 

I'm learning Kotlin and I have some trouble with functions.
I'm trying to create something like a functional interface with a generic parameter.
In Java I would create something like this:

@FunctionalInterface
public interface Foo<T extends Bar> 
 String something(T arg);

Then I can use this somewhere else like this (given that Person extends Bar:

Foo<Person> f = p -> p.toString();

How do you write this with Kotlin?

The first thing I tried was to use type-aliases like this:

typealias Foo<T> = (T) -> String

However, it stopped working when I added the bound to the type parameter:

typealias Foo<T: Bar> = (T) -> String // Error: Bounds are not allowed on type alias parameters

The second approach was to write an interface that extends the function type:

interface Foo<T: Bar> : (T) -> String

However, now I don't know how to instantiate a lambda function from with this. It works when I create class from it like this:

class Something: Foo<Person> 
 override fun invoke(p: Person): String 
 return p.toString()
 


val f = Something()

But this is a big overhead and I'm sure there has to be a better solution.

So how can I define a function signature that can be reused by many functions that supports generic parameters with bounds in kotlin?

kotlin

share|improve this question

asked Mar 25 at 19:10

Manuel MaukyManuel Mauky

94511 gold badge99 silver badges1515 bronze badges

@FunctionalInterface
public interface Foo<T extends Bar> 
 String something(T arg);

Foo<Person> f = p -> p.toString();

typealias Foo<T> = (T) -> String

typealias Foo<T: Bar> = (T) -> String // Error: Bounds are not allowed on type alias parameters

interface Foo<T: Bar> : (T) -> String

class Something: Foo<Person> 
 override fun invoke(p: Person): String 
 return p.toString()
 


val f = Something()

share|improve this question

asked Mar 25 at 19:10

Manuel MaukyManuel Mauky

94511 gold badge99 silver badges1515 bronze badges

share|improve this question

asked Mar 25 at 19:10

Manuel MaukyManuel Mauky

94511 gold badge99 silver badges1515 bronze badges

asked Mar 25 at 19:10

Manuel MaukyManuel Mauky

94511 gold badge99 silver badges1515 bronze badges

asked Mar 25 at 19:10

Manuel MaukyManuel Mauky

94511 gold badge99 silver badges1515 bronze badges

1 Answer
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4

Most of the time (always?) it is sufficient to define the type of the lambda in the parameter of the function that receives it.

For example:

open class Bar
class Person: Bar()

var f = p: Person -> p.toString() 

fun <T : Bar> withFoo(block: (T) -> String) 
fun <T : Bar> otherFoo(block: (T) -> String) 

fun main() 
 withFoo(f)
 otherFoo(f)

The same way the Kotlin documentation states:
"since Kotlin has proper function types, automatic conversion of functions into implementations of Kotlin interfaces is unnecessary and therefore unsupported."

See https://kotlinlang.org/docs/reference/java-interop.html#sam-conversions

share|improve this answer

edited Mar 27 at 16:53

answered Mar 25 at 19:44

Alexander EggerAlexander Egger

3,53911 gold badge1818 silver badges3131 bronze badges

add a comment | 

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4

Most of the time (always?) it is sufficient to define the type of the lambda in the parameter of the function that receives it.

For example:

open class Bar
class Person: Bar()

var f = p: Person -> p.toString() 

fun <T : Bar> withFoo(block: (T) -> String) 
fun <T : Bar> otherFoo(block: (T) -> String) 

fun main() 
 withFoo(f)
 otherFoo(f)

The same way the Kotlin documentation states:
"since Kotlin has proper function types, automatic conversion of functions into implementations of Kotlin interfaces is unnecessary and therefore unsupported."

See https://kotlinlang.org/docs/reference/java-interop.html#sam-conversions

share|improve this answer

edited Mar 27 at 16:53

answered Mar 25 at 19:44

Alexander EggerAlexander Egger

3,53911 gold badge1818 silver badges3131 bronze badges

add a comment | 

4

Most of the time (always?) it is sufficient to define the type of the lambda in the parameter of the function that receives it.

For example:

open class Bar
class Person: Bar()

var f = p: Person -> p.toString() 

fun <T : Bar> withFoo(block: (T) -> String) 
fun <T : Bar> otherFoo(block: (T) -> String) 

fun main() 
 withFoo(f)
 otherFoo(f)

The same way the Kotlin documentation states:
"since Kotlin has proper function types, automatic conversion of functions into implementations of Kotlin interfaces is unnecessary and therefore unsupported."

See https://kotlinlang.org/docs/reference/java-interop.html#sam-conversions

share|improve this answer

edited Mar 27 at 16:53

answered Mar 25 at 19:44

Alexander EggerAlexander Egger

3,53911 gold badge1818 silver badges3131 bronze badges

add a comment | 

Most of the time (always?) it is sufficient to define the type of the lambda in the parameter of the function that receives it.

For example:

open class Bar
class Person: Bar()

var f = p: Person -> p.toString() 

fun <T : Bar> withFoo(block: (T) -> String) 
fun <T : Bar> otherFoo(block: (T) -> String) 

fun main() 
 withFoo(f)
 otherFoo(f)

The same way the Kotlin documentation states:
"since Kotlin has proper function types, automatic conversion of functions into implementations of Kotlin interfaces is unnecessary and therefore unsupported."

See https://kotlinlang.org/docs/reference/java-interop.html#sam-conversions

share|improve this answer

edited Mar 27 at 16:53

answered Mar 25 at 19:44

Alexander EggerAlexander Egger

3,53911 gold badge1818 silver badges3131 bronze badges

open class Bar
class Person: Bar()

var f = p: Person -> p.toString() 

fun <T : Bar> withFoo(block: (T) -> String) 
fun <T : Bar> otherFoo(block: (T) -> String) 

fun main() 
 withFoo(f)
 otherFoo(f)

share|improve this answer

edited Mar 27 at 16:53

answered Mar 25 at 19:44

Alexander EggerAlexander Egger

3,53911 gold badge1818 silver badges3131 bronze badges

answered Mar 25 at 19:44

Alexander EggerAlexander Egger

3,53911 gold badge1818 silver badges3131 bronze badges

answered Mar 25 at 19:44

Alexander EggerAlexander Egger

3,53911 gold badge1818 silver badges3131 bronze badges