How to use the lambda in kotlin ->

Question

Have a question about -> in kotlin.

   fun test0 (a: String, combine: (b: String) -> String ): String {
        return combine(a)
   }

There is a example function above, how to use this function? I dont know how to pass the parm combine.

Have tried follow:

    private fun getFold(a: String): String {
        return a
    }

    fun test() {
        var a: String = "a"
        val test1 = test0 (a,  combine = getFold(a))
    }

Sync error in combine = getFold(a)) ,say:

Type mismatch. Required:(String) → String Found: String

So how to pass the parm combine?

gidds · Accepted Answer · 2021-12-30 11:14:04Z

5

In your example, combine is a parameter of type (b: String) -> String, which is a function taking a single String and returning a String.

(In fact, you don't need to name the parameter b; it could simply be written (String) -> String.)

There are three main ways to pass a function as a parameter:

As a lambda. This is perhaps the most common: you provide the function body directly, in braces. For example:

val test1 = test0(a, { it })

Here the lambda is { it }. (it is a keyword you can use in lambdas which take a single parameter, and refers to that parameter. An equivalent might be { a -> a }.)

In fact, because the function parameter is the last parameter, Kotlin lets you move the lambda after the parens:

val test1 = test0(a){ it }

This means exactly the same, but in some cases it can read better (by making the function call look like language syntax; functions like map and filter are often called that way).

As an anonymous function. This is similar to a lambda, but the syntax is more like defining a normal function; it's more verbose, but gives you more control. In this case you could use a normal function body:

val test2 = test0(a, fun(b: String): String { return b })

Or an expression body:

val test2 = test0(a, fun(b: String) = b)

As a callable reference. This is for when you have an existing method or function that you want to use. (You might already have a suitable function in your project, or in a library; or the code might be too long to fit neatly into a lambda.)

It's done using the :: operator, e.g.:

val test3 = test0(a, ::getFold)

You can reference a method, top-level function, extension function, property, or constructor in this way. (If it's not a top-level function, you may need to tell the compiler which object/class it belongs to — just as you would when calling it directly — e.g. myInstance::methodName.)

It looks like this is what the code in the question is trying to do. However, the lambda syntax is usually simpler and clearer, so consider that if you don't need the existing function for any other reason.

(The error message in the question is because combine = getFold(a) will call getFold(), and try to assign its result to the combine parameter. That result is a String, not a function type, hence the type mismatch.)

edited Dec 30, 2021 at 11:14

answered Dec 30, 2021 at 10:44

gidds

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2 Comments

Endzeit Over a year ago

Very clean answer. Maybe you can extend the lambda answer with an example for the training lambda syntax, e.g. test0(a) { it } instead of test0(a, { it }), which seems to be common practice.

gidds Over a year ago

@Endzeit Thanks for the idea! (I hope it doesn't make the answer too complex.)

Nikolai Shevchenko · Accepted Answer · 2021-12-30 09:28:46Z

2

test0(a, combine = ::getFold)

or

test0(a, combine = this::getFold)

answered Dec 30, 2021 at 9:28

Nikolai Shevchenko

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1 Comment

Mark Rotteveel Over a year ago

Please don't post only code as answer, but also provide an explanation what your code does and how it solves the problem of the question. Answers with an explanation are usually more helpful and of better quality, and are more likely to attract upvotes.

bylazy · Accepted Answer · 2021-12-30 12:27:52Z

1

Or like this:

combine = {getFold(it)}

edited Dec 30, 2021 at 12:27

answered Dec 30, 2021 at 9:33

bylazy

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2 Comments

Tenfour04 Over a year ago

That should be it instead of a.

Mark Rotteveel Over a year ago

Please don't post only code as answer, but also provide an explanation what your code does and how it solves the problem of the question. Answers with an explanation are usually more helpful and of better quality, and are more likely to attract upvotes.

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