Add information on how to use the REPL

Author: pnathan

env~repl.md

@@ -11,5 +11,54 @@ In this article we'll assume that you've configured your Lisp
 environment correctly and now are trying to write Lisp code
 (Congratulations)!
 
-Let's say that you have your `emacs` and `SLIME` set up. Then, let's
-load our SLIME. `M-x slime`.  This will create a REPL buffer for you.
+Let's say that you have your `emacs` and `SLIME` set up. Then, open a
+file and load our SLIME - run `M-x slime` when the file is opened.
+This will create a REPL buffer for you associated to the file.
+
+Here's a function to put in the file (or write your own):
+
+```Commonlisp
+(defun heads (&rest lists)
+  "Gets the first elements of the lists"
+  (mapcar #'car lists))
+```
+
+Then place the cursor in the function and keyboard in the combination
+`C-cC-c`.
+
+In my REPL window the annotation `; compiling (DEFUN HEADS ...)`
+appears. We then can run `heads` and see its output, just as if we had
+written heads on the prompt.
+
+Not only that, the SLIME REPL offers lookup of symbol completion based
+upon what exists in the Lisp image: typing `hea` and then `TAB` in the
+REPL will attempt to complete heads (or present you with a list of
+possible completions).
+
+I frequently use `M-x slime-eval-buffer` to reload everything in my
+file (just checking to ensure that it works correctly).
+
+Let's invert this and use our actual file as a sort of REPL. Put an
+invocation of `heads` in your source file and run `C-xC-e` - this
+actually evaluates the form and outputs the result to the
+minibuffer. If you instead run `C-cC-j`, SLIME copies the form into
+the REPL and executes it. And, if you run `M-x
+slime-eval-print-last-expression`, then the expression will be
+evaluated and printed out into your executing buffer. This is very
+handy for scratch calculations and experimentation[1].
+
+If you're working with large amounts of data, then the dynamic
+variables at global scope `*print-length*` and `*print-depth*` are
+very handy. When set to `nil`, then the length and recursive depth of
+data objects are printed to infinite length and depth; when given an
+integer, the Common Lisp printer obeys that integer as a limit on the
+length and depth of the printing.
+
+
+[1] P.s., an elisp snippet to make that command easier is:
+
+```elisp
+(define-key slime-mode-map
+    "\C-x\C-j"
+    'slime-eval-print-last-expression)
+```