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Kotlin
fold and scan
`fold` accumulates from a seed across all items. `runningFold` / `scan` yields every intermediate accumulator value — running totals, cumulative metrics.
@admin
Kotlin
groupBy and partition
`groupBy` returns a `Map<Key, List<T>>` keyed by what the lambda returns. `partition` is a special-case `groupBy` for booleans — returns `(matching, nonMatching)` Pair.
Kotlin
Sequences — Lazy Collections
A `List` materializes every intermediate `map`/`filter` result. A `Sequence` processes ONE element through the whole pipeline at a time — better for big inputs or short-circuiting (`first`, `take`).
Kotlin
StateFlow — Observable State
`StateFlow<T>` is a hot Flow holding a single current value. Perfect for UI state in Compose/Android — always has a value, conflates intermediate values, multi-subscriber.
Kotlin
Enums with Properties and Methods
Kotlin enums can carry constructor properties and define methods — including abstract methods overridden per constant. Way more expressive than Java enums.
Kotlin
Sealed Interfaces (Kotlin 1.5+)
Like sealed classes, but for interfaces — and a class can implement multiple sealed interfaces. Cleaner state modeling when you want "this is both a Loadable and a Cacheable".
Kotlin
ViewModel + StateFlow + Compose
Standard Android pattern: ViewModel holds state in a `StateFlow`, Compose collects it as state. Survives config changes; isolates business logic from UI.
Kotlin
kotlin.time — Duration and measureTime
`kotlin.time.Duration` is a typesafe time amount with friendly literal syntax (`5.seconds`, `2.minutes`). `measureTime { }` benchmarks a block.
Kotlin
map / filter / reduce
The three classic functional combinators. Each takes a lambda and returns a new collection (or a single value for reduce). Chain freely — intermediate allocations only matter at very large scale (use `Sequence` then).
Kotlin
Custom Exception + Sealed Result Pattern
For library / service code, define your own exception hierarchy. Pair with `sealed` + `when` for type-safe error handling at the call site.
Kotlin
Result Type — Safe Error-Returning APIs
`Result<T>` wraps "success or thrown exception" without forcing the caller into try/catch. `runCatching { ... }` is the standard producer.
Kotlin
java.time — When You Need JVM Interop
On the JVM, `java.time` is fine — and often necessary for interop with Java APIs. Kotlin's extension methods make it ergonomic enough that you might never reach for kotlinx-datetime.
Kotlin
Infix Functions
A single-arg method/extension can be called without dot or parens when marked `infix`. Lets you write DSL-style code like `5 shouldBe 5` or `key to value`.
Kotlin
Ktor Server — Middleware (Plugins)
Plugins wrap every request — logging, CORS, auth, rate limiting. Install with `install(Plugin) { config }` at the application level.
Kotlin
Exposed — Kotlin-First ORM
JetBrains' Exposed combines a SQL DSL with a lightweight ORM. Either style is fully type-safe; both compile to plain SQL.
Kotlin
Ktor Client — GET Request
Ktor is JetBrains' Kotlin-first HTTP client. Multiplatform (JVM, JS, Native). Coroutine-native — every call is a suspend function.
Kotlin
repeat, also lateinit and isInitialized
`repeat(n) { i -> ... }` for N-times loops where you don't care about index naming. `lateinit var` lets you declare a non-null var without initializing it — common for DI / framework injection.