Kotlin

Variance — in vs out

`out T` (covariant) → producer of T; can be assigned to a wider type. `in T` (contravariant) → consumer of T; can be assigned to a narrower type. Without modifiers, generics are invariant (strict match).

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Kotlin

JSON Annotations — Rename, Default, Optional

Per-field annotations let you map Kotlin camelCase ↔ JSON snake_case, set defaults for missing fields, and skip nulls.

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Kotlin

Type-Safe Builder DSL

`@DslMarker` + lambdas with receivers let you build statically-typed DSLs that look like declarative configuration. The HTML / Gradle Kotlin DSL style.

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Kotlin

tailrec — Tail-Call Optimization

Mark a recursive function `tailrec` and the compiler rewrites it as a loop — no stack frame per call, no StackOverflowError on deep recursion. Function must end with itself as the FINAL expression.

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PHP

Dump Variable to Browser (var_dump replacement)

A nicer var_dump-style dumper that wraps output in <pre> with monospaced font for browser inspection. Drop-in replacement for var_dump while debugging.

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TypeScript

Type-Safe Event Emitter

A 30-line type-safe mini EventEmitter. The event-name → payload map is enforced at compile time, so emit and on calls can't drift apart.

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Python

Walk Directory Tree (Filtered)

Recursively yield every file under a directory matching a predicate. Built on pathlib.rglob; the predicate gives you precise control without listing the whole tree up front.

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Bash

Clean Up Merged Branches

Delete every local branch that's been merged to main. Filters out main/master and the current branch as a safety net.

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TypeScript

Conditional Types Basics

`T extends U ? X : Y` lets types branch on shape. Combine with `infer` to extract pieces of complex types — the building block under `ReturnType`, `Parameters`, and most utility libraries.

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Bash

Sort an Array

Bash itself doesn't sort arrays — you pipe through `sort`. readarray captures the sorted output back into an array, preserving each element verbatim (including spaces).

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Bash

Check Command Exists Before Using It

`command -v` is the portable, fast way to check whether a binary is on $PATH. Avoid `which` (its behavior varies between systems).

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Bash

Disk Space Alert

Cron-friendly script that checks disk usage on each filesystem and emails (or webhooks) if any partition exceeds a threshold.

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Bash

Rotate Logs Manually

If a service doesn't hook into logrotate, you can still rotate a log file yourself in one safe pattern: rename current → .1, truncate the original, signal the process to re-open if needed.

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Bash

docker exec — Drop Into Running Container

`docker exec` is your debugger. Get a shell, inspect env, tail logs, run one-off commands inside a live container.

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Go

errors.Join — Aggregate Multiple Errors

Go 1.20+ ships `errors.Join`, which combines multiple errors into one. Stop accumulating errors in a slice and stringifying yourself — use the standard library.

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Go

sync/atomic — Lock-Free Counter

For simple counters and flags, atomic ops are much faster than `sync.Mutex`. Go 1.19+ added typed wrappers (`atomic.Int64`) — clearer than the raw functions.

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SQL

CREATE TABLE with Constraints

Declare data integrity rules right in the schema — primary keys, foreign keys, unique constraints, NOT NULL, CHECK constraints, defaults. The DB enforces them so application bugs can't corrupt your data.

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Kotlin

Path Operations with java.nio.file

Modern Kotlin code uses `java.nio.file.Path` over the legacy `File`. Operator overloads + Kotlin extensions make it ergonomic.

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Kotlin

Parameterized Tests with JUnit 5

JUnit 5 + Kotlin: `@ParameterizedTest` runs the same test body with multiple input rows. `@CsvSource` for inline data, `@MethodSource` for complex args.

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Kotlin

kotlinx-datetime — Multiplatform Dates

`kotlinx-datetime` is the multiplatform date/time library — works on JVM, JS, Native. `Instant` (UTC moment), `LocalDate`, `TimeZone` are the building blocks.

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PHP

Recursive Directory Walker

Yield every file under a directory using a generator and SPL's RecursiveIteratorIterator. Filter by extension or any other predicate without slurping all paths into memory first.

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Bash

Download with Progress Bar

Show a progress bar for long downloads. wget gives a great built-in bar; curl needs an explicit flag.

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Bash

Tar Archive with Timestamp

Pack up a directory with a date-stamped filename for easy backups. -z for gzip, -j for bzip2, -J for xz (smallest, slowest).

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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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Go

Signal Handling with signal.NotifyContext

Go 1.16+ `signal.NotifyContext` returns a context that's canceled on the listed signals. Cleaner than the legacy `signal.Notify(channel)` dance for graceful shutdown.

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PHP

Generate Identicon Avatar

Render a deterministic 5x5 symmetric "identicon" placeholder avatar from any seed (usually a user's email or username). No external service — pure GD.

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Bash

Sum a Column with awk

Awk's default field splitter is whitespace; pass a single character with -F. Perfect for summing the Nth column of a log file or CSV.

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Bash

Get Public IP Address

Several free services return your public IP as plain text. Useful when scripts running behind NAT need to know what address the outside world sees.

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Bash

Check Systemd Service Is Running

`systemctl is-active` is the right tool — exit code 0 if running, non-zero otherwise. Combine with restart-on-fail for poor-man's supervision.

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Bash

Create User With Sensible Defaults

Provision a new user account with a home directory, default shell, and sudo membership. Idempotent — re-running on an existing user is a no-op.

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