ocaml - Man Page

The OCaml interactive toplevel

Examples (TL;DR)


ocaml [ options ] [ object-files ] [ script-file ]


The ocaml(1) command is the toplevel system for OCaml, that permits interactive use of the OCaml system through a read-eval-print loop. In this mode, the system repeatedly reads OCaml phrases from the input, then typechecks, compiles and evaluates them, then prints the inferred type and result value, if any. The system prints a # (hash) prompt before reading each phrase.

A toplevel phrase can span several lines. It is terminated by ;; (a double-semicolon). The syntax of toplevel phrases is as follows.

The toplevel system is started by the command ocaml(1). Phrases are read on standard input, results are printed on standard output, errors on standard error. End-of-file on standard input terminates ocaml(1).

If one or more object-files (ending in .cmo or .cma) are given, they are loaded silently before starting the toplevel.

If a script-file is given, phrases are read silently from the file, errors printed on standard error. ocaml(1) exits after the execution of the last phrase.


The following command-line options are recognized by ocaml(1).


Show absolute filenames in error messages.


Do not try to show absolute filenames in error messages.

-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 +compiler-libs adds the subdirectory compiler-libs 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.

-init file

Load the given file instead of the default initialization file. See the "Initialization file" section below.


Labels are not ignored in types, labels may be used in applications, and labelled parameters can be given in any order.  This is the default.


Deactivates the applicative behaviour of functors. With this option, each functor application generates new types in its result and applying the same functor twice to the same argument yields two incompatible structures.


Do not compile assertion checks.  Note that the special form assert false is always compiled because it is typed specially.


Do not load any initialization file. See the "Initialization file" section below.


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


Do not display any prompt when waiting for input.


Do not display the secondary prompt when waiting for continuation lines in multi-line inputs.  This should be used e.g. when running ocaml(1) in an emacs(1) window.


Do not include the standard library directory in the list of directories searched for source and compiled files.

-open module

Opens the given module before starting the toplevel. If several -open options are given, they are processed in order, just as if the statements open! module1;; ... open! moduleN;; were input.

-ppx command

After parsing, pipe the abstract syntax tree through the preprocessor command. The module Ast_mapper(3) implements the external interface of a preprocessor.


Check information path during type-checking, to make sure that all types are derived in a principal way.  When using labelled arguments and/or polymorphic methods, this flag is required to ensure future versions of the compiler will be able to infer types correctly, even if internal algorithms change. All programs accepted in -principal mode are also accepted in the default mode with equivalent types, but different binary signatures, and this may slow down type checking; yet it is a good idea to use it once before publishing source code.


Do not check principality of type inference. This is the default.


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


Do no allow arbitrary recursive types during type-checking. This is the default.


Enforce the separation between types string and bytes, thereby making strings read-only. This is the default.


Do not use type information to optimize pattern-matching. This allows to detect match failures even if a pattern-matching was wrongly assumed to be exhaustive. This only impacts GADT and polymorphic variant compilation.


When a type is visible under several module-paths, use the shortest one when printing the type's name in inferred interfaces and error and warning messages.


Read the standard input as a script file rather than starting an interactive session.


Force the left-hand part of each sequence to have type unit.


Left-hand part of a sequence need not have type unit. This is the default.


When a type is unboxable (i.e. a record with a single argument or a concrete datatype with a single constructor of one argument) it will be unboxed unless annotated with [@@ocaml.boxed].


When a type is unboxable  it will be boxed unless annotated with [@@ocaml.unboxed]. This is the default.


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.


Identify the types string and bytes, thereby making strings writable. This is intended for compatibility with old source code and should not be used with new software.


Print version string and exit.


Print short version number and exit.


Do not print the version banner at startup.

-w warning-list

Enable or disable warnings according to the argument warning-list. See ocamlc(1) for the syntax of the warning-list argument.

-warn-error warning-list

Mark as fatal the warnings described by the argument warning-list. Note that a warning is not triggered (and does not trigger an error) if it is disabled by the -w option.  See ocamlc(1) for the syntax of the warning-list argument.

-color mode

Enable or disable colors in compiler messages (especially warnings and errors). The following modes are supported:

auto use heuristics to enable colors only if the output supports them (an ANSI-compatible tty terminal);

always enable colors unconditionally;

never disable color output.

The environment variable "OCAML_COLOR" is considered if -color is not provided. Its values are auto/always/never as above.

If -color is not provided, "OCAML_COLOR" is not set and the environment variable "NO_COLOR" is set, then color output is disabled. Otherwise, the default setting is auto, and the current heuristic checks that the "TERM" environment variable exists and is not empty or "dumb", and that isatty(stderr) holds.

-error-style mode

Control the way error messages and warnings are printed. The following modes are supported:

short only print the error and its location;

contextual like "short", but also display the source code snippet corresponding to the location of the error.

The default setting is contextual.

The environment variable "OCAML_ERROR_STYLE" is considered if -error-style is not provided. Its values are short/contextual as above.


Show the description of all available warning numbers.

- file

Use file as a script file name, even when it starts with a hyphen (-).

-help or --help

Display a short usage summary and exit.

Initialization File

When ocaml(1) is invoked, it will read phrases from an initialization file before giving control to the user. The default file is .ocamlinit in the current directory if it exists, otherwise XDG_CONFIG_HOME/ocaml/init.ml according to the XDG base directory specification lookup if it exists (on Windows this is skipped), otherwise .ocamlinit in the user's home directory (HOME variable). You can specify a different initialization file by using the -init file option, and disable initialization files by using the -noinit option.

Note that you can also use the #use directive to read phrases from a file.

Environment Variables


When printing string values, non-ascii bytes (>0x7E) are printed as decimal escape sequence if OCAMLTOP_UTF_8 is set to false. Otherwise they are printed unescaped.


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.


.ocamlinit lookup procedure (see above).

See Also

ocamlc(1), ocamlopt(1), ocamlrun(1).
The OCaml user's manual, chapter "The toplevel system".

Referenced By

camlp5(1), libnbd-release-notes-1.4(1), menhir(1), ocamlbuild(1), ocamlc(1), ocamldoc(1), ocp-indent(1), utop(1), utop-full(1).