Hi, I'm a little confused about lambda expression....

# getting-started

[xun su]

Hi, I'm a little confused about lambda expression. say we have a expression below

val return5 = {5}

I think it is equal to

fun return5(): Int {
  // note return here
  return 5
}

if so, please explain the code bellow:

fun test(fn: () -> Unit) {}

val foo =
    fun(name: String): String {
      return name
    }

test { foo } // why no error ?

I mean ,test accept a parameter , which is a function with the type ()->Unit , but I think test { foo } is equals to

test(
      fun() {
        return foo
      })

Apparently it mismatch the type of parameter fn of the function test , the case above is () -> String -> String , but why no error here ?
``` ```

[ephemient]

they are not completely equal

[ephemient]

as for your second question:

test(foo)

will error

[Vampire]

If the method you implement using the lambda requires a return value, the last expression in the lambda is used as the return value.

If the method does not have a return value, it just also works.

[ephemient]

when you write { foo }, it doesn't actually mean `fun() { return foo }`: because the return type of Unit is inferred from context, Kotlin ignores the type of the last value of the lambda

[ephemient]

Unit is special-cased

[ephemient]

otherwise writing functions like

list.forEach { item -> otherList.add(item) }

would be very annoying to write, because add returns a Boolean while forEach doesn't want any result from its lambda

[xun su]

"because the return type of Unit is inferred from context", in this case , "context" mean I've defined fn:()->Unit ?, @ephemient

[Vampire]

Yes

[ephemient]

https://kotlinlang.org/spec/statements.html#coercion-to-kotlin.unit

[xun su]

thank you all! really appreciate it!😀