Kotlin

Safe Call Chain + Elvis Default

`?.` is the safe-call operator — short-circuits to `null` at the first null in a chain. Pair with `?:` (Elvis) for "this or a default". Replaces nested if-not-null ladders.

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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`.

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Rust

Rc and Arc — Shared Ownership

`Rc<T>` lets multiple owners share read-only access in single-threaded code; `Arc<T>` is the thread-safe version. Pair with `RefCell` / `Mutex` when you need shared MUTABLE access.

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Kotlin

Default Arguments + Named Parameters

Default values eliminate most overload sets. Named args make call sites self-documenting and let you skip middle parameters without thinking about positions.

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Kotlin

Higher-Order Functions

Functions that take or return other functions. Foundation for streams, callbacks, DSLs — and an alternative to interfaces with a single method.

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Python

Deprecation Warning Decorator

Mark a function as deprecated so callers get a DeprecationWarning the first time it's called. Includes the replacement function name in the message so callers know what to switch to.

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Kotlin

Sealed Classes — Closed Type Hierarchies

`sealed` restricts subclassing to the same module. Combined with exhaustive `when`, the compiler enforces handling of every variant — refactor-friendly state machines.

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TypeScript

Exhaustive `assertNever`

Compile-time guarantee that every variant of a union is handled. When you add a new variant later, every switch missing that case becomes a type error. The runtime version is a safety net.

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Kotlin

buildString / buildList / buildMap

Stdlib builders that give you a mutable scope, then return an immutable result. Replaces `StringBuilder().apply { ... }.toString()` boilerplate; the result type is the read-only one.

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Kotlin

Inline Value Classes (kotlin 1.5+)

`@JvmInline value class` wraps a single value with a distinct type at compile time but no runtime overhead — at runtime it's just the underlying primitive/object. Replaces typedef tricks and prevents UserId vs PostId mix-ups.

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TypeScript

Discriminated Unions with Exhaustive Switch

Discriminated (tagged) unions give you compiler-checked state machines. Pair with `assertNever` to force every new variant to be handled at every switch site — refactors stop being scary.

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Java

AtomicInteger / LongAdder

Lock-free atomics for shared counters. `AtomicInteger` for low contention; `LongAdder` for high contention (shards internally — much faster under heavy parallel load).

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Bash

Heredoc with Variable Interpolation Control

Heredocs are the cleanest way to embed multi-line text. Unquote the delimiter to allow variable expansion; QUOTE it to keep the body literal (no $foo expansion).

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Kotlin

Coroutine Cancellation

Cancellation is cooperative — your coroutine must check via `ensureActive()`, `yield()`, or any other suspending call. CPU-busy loops without a suspend point are NOT cancellable.

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Kotlin

coroutineScope and Structured Concurrency

`coroutineScope { }` waits for ALL its children before returning. If any child throws, the others are cancelled. The cornerstone of structured concurrency — no leaked coroutines.

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Kotlin

Flow — Cold Async Streams

`Flow<T>` is a coroutine-based reactive stream — like Sequence but async. Cold (each collector restarts the producer) and respects cancellation. The backbone of modern Kotlin/Android reactive code.

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Kotlin

launch and Jobs

`launch` starts a coroutine and returns a `Job` you can join, cancel, or check status on. Fire-and-forget or join-when-ready style.

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Kotlin

Coroutines — suspend Function Basics

`suspend fun` can be paused and resumed without blocking a thread. Call only from another `suspend` function or a coroutine builder (`launch`, `runBlocking`, `async`).

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Kotlin

withContext — Switching Dispatchers

`withContext(dispatcher)` suspends the calling coroutine, runs the block on the given dispatcher, returns the result. Use `Dispatchers.IO` for blocking I/O, `Default` for CPU work, `Main` for UI updates.

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SQL

NULLS FIRST / NULLS LAST

Control where NULLs land in an `ORDER BY`. PostgreSQL/Oracle default NULLs to LAST in ASC, FIRST in DESC; MySQL/SQL Server flip it. Be explicit if it matters.

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Kotlin

val vs var — Prefer Immutability

`val` is read-only (can't reassign the reference); `var` is mutable. Default to `val` everywhere — Kotlin's style guide recommends it, and a Compiler warning fires if a `var` is never reassigned.

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Go

Variadic Functions

`...T` in the last parameter slot accepts zero or more T values. Pass a slice by suffixing with `...` to spread it. Used everywhere from `fmt.Println` to custom builders.

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Kotlin

Nullable Types — String? and the ? Operator

Kotlin distinguishes `String` (never null) from `String?` (may be null) in the type system. The compiler refuses to compile code that could deref a possibly-null value — the famous "no more NullPointerException" feature.

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Go

Pointers — When and Why

Use pointers when you want to mutate the callee's value, share a large struct without copying, or distinguish "no value" via nil. Go has no pointer arithmetic — much safer than C.

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Kotlin

Lambdas and Function Types

Kotlin functions are first-class values. A lambda is `{ args -> body }`; its type is `(InputTypes) -> ReturnType`. Single-argument lambdas can use the implicit `it` parameter.

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Kotlin

when Expressions — Powerful switch

Kotlin's `when` is an expression (returns a value), supports ranges, type checks, multiple values per branch, and arbitrary boolean conditions. Replaces nested if/else AND traditional switch.

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Go

defer / panic / recover

`defer` runs a statement when the enclosing function returns — LIFO order. `panic` aborts; `recover` (inside a deferred func) catches a panic and converts it back to a normal return. Use sparingly.

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Go

Methods and Receivers

Methods are functions with a receiver argument. Use pointer receivers when you mutate, when the struct is large, OR for consistency (mixing pointer + value receivers on the same type is a common bug source).

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Go

Multiple Return Values + Named Returns

Go functions can return multiple values — most idiomatically a `(result, error)` pair. Named returns let you document the meaning of each value AND enable naked returns in short functions.

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HTML

Picture Element — Art Direction

`<picture>` lets you serve DIFFERENT images at different breakpoints — not just different sizes of the same crop. Perfect for a wide desktop banner that becomes a square mobile poster.

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