High Quality Content by WIKIPEDIA articles! A read–eval–print loop (REPL) is a simple, interactive computer programming environment. The term is most usually used to refer to a Lisp interactive environment, but can be applied to command line shells and similar environments for programming languages such as APL, BASIC, F#, Haskell, J, Perl, PHP, Prolog, Python, R, Ruby, Scala, Smalltalk, Standard ML, Tcl, et al. Synonyms include interactive toplevel and language shell.