ReadEvalPrintLoop

ReadEvalPrintLoop (commonly abbreviated REPL) is an interactive programming environment that reads a line or block of user input, evaluates it, prints the result, and then repeats. It is a fundamental component of many interpreted languages and is often exposed as a command-line shell or an integrated development environment's console.

In a typical REPL session, the loop begins with reading input, parsing and evaluating expressions or statements, and printing the outcome. If the input produces side effects, those effects are executed. The loop continues until the user issues an exit command or terminates the process. Modern REPLs may support features such as multi-line input, syntax highlighting, command history, tab completion, and exception reporting that helps debugging.

Origin and usage: The concept emerged from the Lisp family of languages in the 1960s and 1970s, where interactive computation and incremental development were central. The term REPL is commonly associated with Lisp systems, but the model has since spread to many languages and tools as a general interactive programming interface.

Examples: Python, Ruby, JavaScript (Node.js), Haskell (ghci), R, Julia, and Lisp/Scheme implementations all provide REPLs. Some languages provide a separate standalone executable for the REPL, while others offer an integrated shell within an IDE or notebook environment.

Security and design considerations: A REPL runs arbitrary code and can reveal or modify the program state, so it is typically restricted to trusted environments. Performance overhead and the potential for incomplete or partial code evaluation can affect behavior compared to batch compilation or execution. See also: REPL, interactive programming, shell.