Toplevel-light

Toplevel-light is a very lightweight OCaml toplevel program tailored for communication between OCaml REPL process and external editor. Before OCaml REPL reads from or writes to the terminal, it prints a single-line message that is wrapped with $ stating what kind of interaction is being made.

bash $ ./toplevel_light
$STDIN$

$STDIN$ states that the OCaml toplevel is now waiting for an input.

Let's assume that a user wrote let _ = print_endline "hello, world!" in 10;;. toplevel_light will print:

hello, world!

$SUCCESS$
- : int = 10
$STDIN$

$SUCCESS$ states that the statement is successfully evaluated by OCaml. The lines before $SUCCESS$ is the printed output of the statement, and the line after $SUCCESS$ is its value.

If the input was syntactically incorrect, $ERROR$ will appear.

$STDIN$
1 + "a";;
$ERROR$
Line 1, characters 4-7:
1 | 1 + "a";;
        ^^^
Error: This expression has type string but an expression was expected of type
         int
$STDIN$

How to build

Using OCaml 4.14 is recommended.

make

Message Kinds

• $STDIN$: The toplevel is waiting for input. This will appear in every input line.
• $SUCCESS$: The toplevel successfully evaluated the statement
• $EXCEPTION$: The statement raised an exception.
• $ERROR$: The toplevel could not parse the statement.

Message sequence:

If there is no #use or Topdir.dir_use:

$STDIN$ (can appear multiple times until one toplevel phrase is parsed) -> one of $SUCCESS$,$EXCEPTION$ and $ERROR$

If there is #use or Topdir.dir_use: $SUCCESS$,$EXCEPTION$ and $ERROR$ can appear multiple times.

Caveats

• The user should not print a line that is syntactically equal to one of the messages.
• The outputs printed to stderr cannot be delineated by the $..$ messages.