Null Safety Patterns

Let Scope

Run a small block only when a nullable value is present.

let `let` can transform a present value while safe calls and Elvis provide the null path.

Let Scope

raw
LetScope.kt
Replay: real traced execution (multi-file project)
fun main() {
    val raw: String? = "  oak  "
    val cleaned = raw?.let { value ->
        value.trim().toUpperCase()
    } ?: "EMPTY"

    println("raw=${raw ?: "none"}")
    println("cleaned=$cleaned")
}

fun main() {
    val raw: String? = null
    val cleaned = raw?.let { value ->
        value.trim().toUpperCase()
    } ?: "EMPTY"

    println("raw=${raw ?: "none"}")
    println("cleaned=$cleaned")
}

fun main() {
    val raw: String? = "  pine  "
    val cleaned = raw?.let { value ->
        value.trim().toUpperCase()
    } ?: "EMPTY"

    println("raw=${raw ?: "none"}")
    println("cleaned=$cleaned")
}

  1. raw ← oak , cleaned ← OAK

    1fun main() {2    val raw→   oak  : String? = "  oak  " //@raw=null, "  pine  "3    val cleaned→ OAK = raw  oak  ?.let { value ->4        value.trim().toUpperCase()5    } ?: "EMPTY"67    println("raw=${raw  oak   ?: "none"}")8    println("cleaned=$cleanedOAK")9}
    outputraw=  oak
cleaned=OAK

  1. raw ← null, cleaned ← EMPTY

    1fun main() {2    val raw→ null: String? = null3    val cleaned→ EMPTY = rawnull?.let { value ->4        value.trim().toUpperCase()5    } ?: "EMPTY"67    println("raw=${rawnull ?: "none"}")8    println("cleaned=$cleanedEMPTY")9}
    outputraw=none
cleaned=EMPTY

  1. raw ← pine , cleaned ← PINE

    1fun main() {2    val raw→   pine  : String? = "  pine  "3    val cleaned→ PINE = raw  pine  ?.let { value ->4        value.trim().toUpperCase()5    } ?: "EMPTY"67    println("raw=${raw  pine   ?: "none"}")8    println("cleaned=$cleanedPINE")9}
    outputraw=  pine
cleaned=PINE

Let or Empty

  1. raw may hold padded text or null.
  2. raw?.let { ... } runs the block only for present text.
  3. Inside let, the text is trimmed and uppercased.
  4. If raw is null, Elvis returns "EMPTY". | raw value | Flow | cleaned | | --- | --- | --- | | " oak " | run let | OAK | | null | skip let | EMPTY |

Exercise: LetScope.kt

Use let to trim and uppercase nullable text, then use EMPTY when the text is null