thp-web/md/learn/error-handling/null.md

Nullable types

All datatypes in THP disallow the usage of null by default. To represent null we must use nullable types, represented by the question mark ? character.

For instance, a POST request may have a username parameter, or it may not. This can be represented with an String?.

String? new_username = POST::get("username")

When we have a Type? we cannot use it directly. We must first check if the value is null, and then use it.

if new_username != null {
    // Here `new_username` is automatically casted to String
}

We must check explicitly that the value is not null. Doing if new_username {} alone is not allowed.

Usage

To create a nullable type we must explicitly annotate the type.

val favorite_color = null      // Error, we must define the type

String? favorite_color = null  // Ok

Other examples:

fun get_first(Array[String?] values) -> String? {}

val result = get_first([])

Optional chaining

If you have a Type? and you wish to access a field of Type if it exists, you can use the optional chaining operator.

Person? person = ...

val name = person?.name

• If person is null, person?.name will return null
• If person is not null, person?.name will return name

Elvis operator

The Elvis operator ?? is used to give a default value in case a null is found.

// This is a function that may return a Int
fun get_score() -> Int? {...}

val test_score = get_score() ?? 0

For the above code:

• If get_score() is not null, ?? will return get_score()
• If get_score() is null, ?? will return 0

You can use the Elvis operator to return early

val username = get_username() ?? return