You don't need an interpreter for a REPL. Swift's REPL performs line-at-a-time compilation, running the program to that line and suspending it until you write and compile another line.
>You don't need an interpreter for a REPL. Swift's REPL performs line-at-a-time compilation
Isn't that another way to say "interpreter"?
For all practical purposes, yes. In the strictest technical sense it's a form of JIT compilation, but that can be considered an optimization or implementation technique. If the user doesn't perceive a separate compilation step, it's effectively an interpreter and only a dedicated pedant would "well actually" that it's compilation.
Not really. An interpreter doesn't convert the language syntax tree into machine code, instead it just performs the operations. It sounds like Swift's REPL actually converts the lines into opcodes and runs those.