The toplevel system is started by the command ocaml, as follows:

        ocaml options objects                # interactive mode
        ocaml options objects scriptfile        # script mode

options are described below. objects are filenames ending in .cmo or .cma; they are loaded into the interpreter immediately after options are set. scriptfile is any file name not ending in .cmo or .cma.

If no scriptfile is given on the command line, the toplevel system enters interactive mode: phrases are read on standard input, results are printed on standard output, errors on standard error. End-of-file on standard input terminates ocaml (see also the #quit directive in section 9.2).

On start-up (before the first phrase is read), if the file .ocamlinit exists in the current directory, its contents are read as a sequence of Objective Caml phrases and executed as per the #use directive described in section 9.2. The evaluation outcode for each phrase are not displayed. If the current directory does not contain an .ocamlinit file, but the user's home directory (environment variable HOME) does, the latter is read and executed as described below.

The toplevel system does not perform line editing, but it can easily be used in conjunction with an external line editor such as ledit or ocaml2 (see the Caml Hump). Another option is to use ocaml under Gnu Emacs, which gives the full editing power of Emacs (command run-caml from library inf-caml).

At any point, the parsing, compilation or evaluation of the current phrase can be interrupted by pressing ctrl-C (or, more precisely, by sending the sigintr signal to the ocaml process). The toplevel then immediately returns to the # prompt.

If scriptfile is given on the command-line to ocaml, the toplevel system enters script mode: the contents of the file are read as a sequence of Objective Caml phrases and executed, as per the #use directive (section 9.2). The outcome of the evaluation is not printed. On reaching the end of file, the ocaml command exits immediately. No commands are read from standard input. Sys.argv is transformed, ignoring all Objective Caml parameters, and starting with the script file name in Sys.argv.(0).

In script mode, the first line of the script is ignored if it starts with #!. Thus, it is theoretically possible to make the script itself executable and put as first line #!/usr/local/bin/ocaml, thus calling the toplevel system automatically when the script is run. However, ocaml itself is a #! script on most installations of Objective Caml, and Unix kernels usually do not handle nested #! scripts.

In addition to the text-only command ocaml.exe, which works exactly as under Unix (see above), a graphical user interface for the toplevel is available under the name ocamlwin.exe. It should be launched from the Windows file manager or program manager. This interface provides a text window in which commands can be entered and edited, and the toplevel responses are printed.

-I directory

Add the given directory to the list of directories searched for source and compiled files. By default, the current directory is searched first, then the standard library directory. Directories added with -I are searched after the current directory, in the order in which they were given on the command line, but before the standard library directory.

If the given directory starts with +, it is taken relative to the standard library directory. For instance, -I +labltk adds the subdirectory labltk of the standard library to the search path.

Directories can also be added to the search path once the toplevel is running with the #directory directive (section 9.2).

-nolabels

Ignore non-optional labels in types. Labels cannot be used in applications, and parameter order becomes strict.

-principal

Check information path during type-checking, to make sure that all types are derived in a principal way. All programs accepted in -principal mode are also accepted in default mode with equivalent types.

-rectypes

Allow arbitrary recursive types during type-checking. By default, only recursive types where the recursion goes through an object type are supported.

-unsafe

See the corresponding option for ocamlc, chapter 8. Turn bound checking off on array and string accesses (the v.(i) and s.[i] constructs). Programs compiled with -unsafe are therefore slightly faster, but unsafe: anything can happen if the program accesses an array or string outside of its bounds.

-w warning-list

Enable or disable warnings according to the argument warning-list.

The following environment variables are also consulted:

LC_CTYPE

If set to iso_8859_1, accented characters (from the ISO Latin-1 character set) in string and character literals are printed as is; otherwise, they are printed as decimal escape sequences (\ddd).

TERM

When printing error messages, the toplevel system attempts to underline visually the location of the error. It consults the TERM variable to determines the type of output terminal and look up its capabilities in the terminal database.

#quit;;

Exit the toplevel loop and terminate the ocaml command.

#labels bool;;

Ignore labels in function types if argument is false, or switch back to default behaviour (commuting style) if argument is true.

#warnings "warning-list";;

Enable or disable warnings according to the argument.

#directory "dir-name";;

Add the given directory to the list of directories searched for source and compiled files.

#cd "dir-name";;

Change the current working directory.

#load "file-name";;

Load in memory a bytecode object file (.cmo file) produced by the batch compiler ocamlc.

#use "file-name";;

Read, compile and execute source phrases from the given file. This is textual inclusion: phrases are processed just as if they were typed on standard input. The reading of the file stops at the first error encountered.

#install_printer printer-name;;

This directive registers the function named printer-name (a value path) as a printer for values whose types match the argument type of the function. That is, the toplevel loop will call printer-name when it has such a value to print.

The printing function printer-name should have type Format.formatter -> t -> unit, where t is the type for the values to be printed, and should output its textual representation for the value of type t on the given formatter, using the functions provided by the Format library. For backward compatibility, printer-name can also have type t -> unit and should then output on the standard formatter, but this usage is deprecated.

#remove_printer printer-name;;

Remove the named function from the table of toplevel printers.

#trace function-name;;

After executing this directive, all calls to the function named function-name will be ``traced''. That is, the argument and the result are displayed for each call, as well as the exceptions escaping out of the function, raised either by the function itself or by another function it calls. If the function is curried, each argument is printed as it is passed to the function.

#untrace function-name;;

Stop tracing the given function.

#untrace_all;;

Stop tracing all functions traced so far.

#print_depth n;;

Limit the printing of values to a maximal depth of n. The parts of values whose depth exceeds n are printed as ... (ellipsis).

#print_length n;;

Limit the number of value nodes printed to at most n. Remaining parts of values are printed as ... (ellipsis).

Cannot find file filename

The named file could not be found in the current directory, nor in the directories of the search path.

If filename has the format mod.cmi, this means you have referenced the compilation unit mod, but its compiled interface could not be found. Fix: compile mod.mli or mod.ml first, to create the compiled interface mod.cmi.

If filename has the format mod.cmo, this means you are trying to load with #load a bytecode object file that does not exist yet. Fix: compile mod.ml first.

If your program spans several directories, this error can also appear because you haven't specified the directories to look into. Fix: use the #directory directive to add the correct directories to the search path.

This expression has type t₁, but is used with type t₂

See section 8.4.

Reference to undefined global mod

You have neglected to load in memory an implementation for a module with #load. See section 9.3 above.

        ocamlmktop -o mytoplevel foo.cmo bar.cmo gee.cmo

        ./mytoplevel

        #load "foo.cmo";;
        #load "bar.cmo";;
        #load "gee.cmo";;

        open Foo;;

-cclib libname

Pass the -llibname option to the C linker when linking in ``custom runtime'' mode. See the corresponding option for ocamlc, in chapter 8.

-ccopt option

Pass the given option to the C compiler and linker, when linking in ``custom runtime'' mode. See the corresponding option for ocamlc, in chapter 8.

-custom

Link in ``custom runtime'' mode. See the corresponding option for ocamlc, in chapter 8.

-I directory

Add the given directory to the list of directories searched for compiled object code files (.cmo and .cma).

-o exec-file

Specify the name of the toplevel file produced by the linker. The default is a.out.