Is Lisp the only language with REPL?

Question

There are languages other than Lisp (ruby, scala) that say they use REPL (Read, Eval, Print, Loop), but it is unclear whether what is meant by REPL is the same as in Lisp. H

Answer 1

I think it is interesting to compare two approaches. A bare bones REPL loop in a Lisp system would look like this:

(loop (print (eval (read))))

Here are two actual Forth implementations of a REPL loop. I'm leaving nothing out here -- this is the full code to these loops.

: DO-QUIT   ( -- )  ( R:  i*x -- )
    EMPTYR
    0 >IN CELL+ !   \ set SOURCE-ID to 0
    POSTPONE [
    BEGIN           \ The loop starts here
        REFILL      \ READ from standard input
    WHILE
        INTERPRET   \ EVALUATE  what was read
        STATE @ 0= IF ."  OK" THEN  \ PRINT
        CR
    REPEAT
;

: quit
  sp0 @ 'tib !
  blk off
  [compile] [
  begin
    rp0 @ rp!
    status
    query           \ READ
    run             \ EVALUATE
    state @ not
    if ." ok" then  \ PRINT
  again             \ LOOP
;

Lisp and Forth do completely different things, particularly in the EVAL part, but also in the PRINT part. Yet, they share the fact that a program in both languages is run by feeding its source code to their respective loops, and in both cases code is just data (though in Forth case it is more like data is also code).

I suspect what anyone saying only LISP has a REPL is that the READ loop reads DATA, which is parsed by EVAL, and a program is created because CODE is also DATA. This distinction is interesting in many respects about the difference between Lisp and other languages, but as far as REPL goes, it doesn't matter at all.

Let's consider this from the outside:

1. READ -- returns input from stdin
2. EVAL -- process said input as an expression in the language
3. PRINT -- print EVAL's result
4. LOOP -- go back to READ

Without going into implementation details, one can't distinguish a Lisp REPL from, for example, a Ruby REPL. As functions, they are the same.