The LFE REPL

Most operating systems have a command interpreter or shell -- Unix and Linux have many, while Windows has the Command Prompt. Likewise, Erlang has a shell where you can directly write bits of Erlang code and evaluate (run) them to see what happens. LFE has more than a shell: it's a full REPL (read-eval-print loop) like other Lisps, and it can do more than the Erlang shell can (including defining proper functions and Lisp macros).

Starting the LFE REPL

In your system terminal window where you changed directory to the clone of the LFE repository, you can start the LFE REPL by typing the following:

./bin/lfe

At which point you will see output something like this:

Erlang/OTP 26 [erts-14.0.2] [source] [64-bit] [smp:10:10] [ds:10:10:10] [async-threads:1] [jit] [dtrace]

   ..-~.~_~---..
  (      \\     )    |   A Lisp-2+ on the Erlang VM
  |`-.._/_\\_.-':    |   Type (help) for usage info.
  |         g |_ \   |
  |        n    | |  |   Docs: http://docs.lfe.io/
  |       a    / /   |   Source: http://github.com/lfe/lfe
   \     l    |_/    |
    \   r     /      |   LFE v2.1.2 (abort with ^G)
     `-E___.-'

lfe>

Interactive LFE Code

Now let's multiply two numbers in the REPL by typing (* 2 21) at the > prompt:

lfe> (* 2 21)

Lisp stands for "LISt Processor" because nearly everything in Lisp is really just a list of things -- including the code itself. The lists in Lisps are created with parentheses, just like the expression above. As you can see, the multiplication operator goes first, followed by its arguments -- this is called prefix notation, due to the operator coming first.

In order to tell the REPL that you want it to evaluate your LFE code, you need to his the <ENTER> or <RETURN> key.

42
lfe>

It has correctly given you the answer: 42.

Now let's try a more complex calculation:

lfe> (* 2 (+ 1 2 3 4 5 6))

This expression has one nested inside the other. The first one to be executed is the inner-most, in this case, the addition operation. Just like we saw before, the operator comes first (the "addition" operator, in this case) and then all of the numbers to be added.

Hit <ENTER> to get your answer:

42
lfe>

Defining Variables and Functions

The LFE REPL allows you do set variables and define functions. Let's define a variable called multiplier:

lfe> (set multiplier 2)
2
lfe>

When we set the value for that variable, the REPL provided feedback on the expression entered, showing us the value. Now we can use it just like the number for which it stands:

lfe> (* multiplier (+ 1 2 3 4 5 6))
42
lfe>

The set form lets you define a variable; the defun form lets you define a function. Enter this in the REPL:

lfe> (defun double (x)
    (* 2 x))

Now try it:

lfe> (double 21)
42
lfe>

As we can see, this function multiplies any given number by 2.

Leaving the REPL

To exit the REPL and shutdown the underlying Erlang system which started when you executed ./bin/lfe, simply exit:

lfe> (exit)
ok

At which point you will be presented with your regular system terminal prompt.

There are two other ways in which you may leave the REPL:

  • Hitting ^c twice in a row, or
  • Hitting ^g then q