The REPL
Also: read-eval-print loop, interactive shell
Read–Eval–Print Loop: a live conversation with a running program, born with Lisp.
A REPL — Read, Eval, Print, Loop — reads an expression you type, evaluates it, prints the result, and waits for the next one. It turns programming from a batch process (write, compile, run, read the printout) into a conversation with a live system.
The REPL was born with Lisp in the early 1960s and is one of computing's most quietly influential ideas. It made programming exploratory: you can poke at a running program, ask it questions, grow it incrementally, and keep your mental model and the machine's state in sync.
The REPL is the immediate ancestor of Smalltalk's live environment, of interactive notebooks, and of the feedback-loop obsession behind live programming. Its core promise — reduce the distance between thinking a thought and seeing it run — runs straight through to the future of programming.