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Comment by cup-of-tea | Hacker News Reader

As someone who has learnt and loves Lisp it's really confusing to me that REPL has come to mean simply command interface.

This is the REPL:

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

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The key for me is that a REPL reads, then evaluates (generally involving printing), then starts over (looping). Bash for example fits this criteria. I'm as much of a fan of lisp as anyone, but it doesn't need to be literally printing the result of `eval` to be a REPL in my eyes.

bash doesn't print the result of an evaluation, the commands print if the want

it's more a PEL: Parse Execute Loop

In that case, most lisp 'repls' don't satisfy your definition either.

Most Lisp REPLS print a result somewhere.

The P stands for print. A REPL prints by definition.

Then what is this?

    (defun add (a b)
           (+ a b))

This is Lisp code defining a function ADD.

If you enter it at a Lisp REPL, the REPL will read the expression, evaluate it and store it as a function. The result value is the name of the defined function. The REPL will then print the result, here the name of the function: ADD.

  CL-USER 13 > (defun add (a b)
                 (+ a b))
  ADD

As you can see, the function does not print the Lisp symbol 'ADD', it's printed by the REPL. That's the P-component of R E P L, REPL. P stands for PRINT. This means that the REPL will always print a result - unless a Lisp function does not have a result - which is rare.

If we execute the function, the REPL prints the result of the function:

  CL-USER 14 > (add 17 25)
  42

Here the result is the number 42.

That's some Lisp code; if you enter it at a REPL, it prints the result of evaluating it:

    CL-USER> (defun add (a b) (+ a b))
    ADD

That is an expression which evaluates to the symbol add. It also has an important side-effect of defining a function.

Or for those who read left to right:

Read Eval Print Loop

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