IMO exploration of complex semantics and/or libraries is quite often a bit futile exercise without appropriate boilerplate/scaffolding. Which means ultimately,

IMO exploration of complex semantics and/or libraries is quite often a bit futile exercise without appropriate boilerplate/scaffolding. Which means ultimately, a good REPL is an advanced debugger with code sideloading and reloading/monkeypatching. Running in isolation (in context of the REPL, not the attached-to program) would be awesome, but not trivial (if possible at all) in many cases. To illustrate, suppose you have some network service (an API in todays talk) and want to play with some form of middleware or new endpoints. You need functional boilerplate to encode, sign, send a request and receive, decode, parse a response. You want to load those parts of the program and play around.