83 lines
1.7 KiB
Markdown
83 lines
1.7 KiB
Markdown
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# Nullable types
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All datatypes in THP disallow the usage of `null` by default.
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To represent `null` we must use nullable types, represented
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by the question mark `?` character.
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For instance, a POST request may have a `username` parameter,
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or it may not. This can be represented with an `String?`.
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```thp
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String? new_username = POST::get("username")
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```
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When we have a `Type?` we cannot use it directly. We must first
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check if the value is null, and then use it.
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```thp
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if new_username != null {
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// Here `new_username` is automatically casted to String
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}
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```
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We must check explicitly that the value is not null. Doing
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`if new_username {}` alone is not allowed.
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## Usage
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To create a nullable type we must explicitly annotate the type.
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```thp
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val favorite_color = null // Error, we must define the type
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String? favorite_color = null // Ok
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```
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Other examples:
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```thp
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fun get_first(Array[String?] values) -> String? {}
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val result = get_first([])
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```
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## Optional chaining
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If you have a `Type?` and you wish to access a field of `Type` if it exists,
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you can use the optional chaining operator.
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```thp
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Person? person = ...
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val name = person?.name
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```
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- If `person` is null, `person?.name` will return `null`
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- If `person` is not null, `person?.name` will return `name`
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## Elvis operator
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The Elvis operator `??` is used to give a default value in case a `null` is found.
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```thp
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// This is a function that may return a Int
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fun get_score() -> Int? {...}
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val test_score = get_score() ?? 0
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```
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For the above code:
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- If `get_score()` is not null, `??` will return `get_score()`
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- If `get_score()` *is* null, `??` will return `0`
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You can use the Elvis operator to return early
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```thp
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val username = get_username() ?? return
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```
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