This chapter describes the toplevel system for OCaml, that permits
interactive use of the OCaml system
through a read-eval-print loop (REPL). In this mode, the system repeatedly
reads OCaml phrases from the input, then typechecks, compile and
evaluate them, then prints the inferred type and result value, if
any. The system prints a # (sharp) prompt before reading each
phrase.
Input to the toplevel can span several lines. It is terminated by ;; (a
double-semicolon). The toplevel input consists in one or several
toplevel phrases, with the following syntax:
A phrase can consist of a definition, like those found in
implementations of compilation units or in struct … end
module expressions. The definition can bind value names, type names,
an exception, a module name, or a module type name. The toplevel
system performs the bindings, then prints the types and values (if
any) for the names thus defined.
A phrase may also consist in a value expression
(section 11.7). It is simply evaluated
without performing any bindings, and its value is
printed.
Finally, a phrase can also consist in a toplevel directive,
starting with # (the sharp sign). These directives control the
behavior of the toplevel; they are listed below in
section 14.2.
Unix:
The toplevel system is started by the command ocaml, as follows:
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 14.2).
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 OCaml phrases
and executed as per the #use directive
described in section 14.2.
The evaluation outcode for each phrase are not displayed.
If the current directory does not contain an .ocamlinit file,
the file XDG_CONFIG_HOME/ocaml/init.ml is looked up according
to the XDG base directory specification and used instead (on Windows
this is skipped). If that file doesn’t exist then an [.ocamlinit] file
in the users’ home directory (determined via environment variable HOME) is
used if existing.
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 rlwrap. An improved toplevel, utop, is also available.
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 INTR 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 OCaml phrases and executed, as per the #use
directive (section 14.2). 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 OCaml 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 should be 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 OCaml, and Unix kernels usually do not handle nested #!
scripts. A better solution is to put the following as the first line
of the script:
#!/usr/local/bin/ocamlrun /usr/local/bin/ocaml
1 Options
The following command-line options are recognized by the ocaml command.
-absname
Force error messages to show absolute paths for file names.
-argsfilename
Read additional newline-terminated command line arguments from filename.
It is not possible to pass a scriptfile via file to the toplevel.
-args0filename
Read additional null character terminated command line arguments from
filename.
It is not possible to pass a scriptfile via file to the toplevel.
-Idirectory
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. See also option -nostdlib.
If the given directory starts with +, it is taken relative to the
standard library directory. For instance, -I +unix adds the
subdirectory unix of the standard library to the search path.
Directories can also be added to the list once
the toplevel is running with the #directory directive
(section 14.2).
-initfile
Load the given file instead of the default initialization file.
The default file is .ocamlinit in the current directory if it
exists, otherwise XDG_CONFIG_HOME/ocaml/init.ml or
.ocamlinit in the user’s home directory.
-labels
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.
-no-app-funct
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.
-noassert
Do not compile assertion checks. Note that the special form
assert false is always compiled because it is typed specially.
-nolabels
Ignore non-optional labels in types. Labels cannot be used in
applications, and parameter order becomes strict.
-noprompt
Do not display any prompt when waiting for input.
-nopromptcont
Do not display the secondary prompt when waiting for continuation
lines in multi-line inputs. This should be used e.g. when running
ocaml in an emacs window.
-nostdlib
Do not include the standard library directory in the list of
directories searched for source and compiled files.
-ppxcommand
After parsing, pipe the abstract syntax tree through the preprocessor
command. The module Ast_mapper, described in
chapter 29:
Ast_mapper
,
implements the external interface of a preprocessor.
-principal
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.
-rectypes
Allow arbitrary recursive types during type-checking. By default,
only recursive types where the recursion goes through an object type
are supported.
-safe-string
Enforce the separation between types string and bytes,
thereby making strings read-only. This is the default, and enforced
since OCaml 5.0.
-short-paths
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. Identifier names starting with an underscore _ or
containing double underscores __ incur a penalty of +10 when computing
their length.
-stdin
Read the standard input as a script file rather than starting an
interactive session.
-strict-sequence
Force the left-hand part of each sequence to have type unit.
-strict-formats
Reject invalid formats that were accepted in legacy format
implementations. You should use this flag to detect and fix such
invalid formats, as they will be rejected by future OCaml versions.
-unsafe
Turn bound checking off for array and string accesses (the v.(i) and
s.[i] constructs). Programs compiled with -unsafe are therefore
faster, but unsafe: anything can happen if the program
accesses an array or string outside of its bounds.
-unsafe-string
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. This option raises an error unconditionally
since OCaml 5.0.
-v
Print the version number of the compiler and the location of the
standard library directory, then exit.
-verbose
Print all external commands before they are executed,
Useful to debug C library problems.
-version
Print version string and exit.
-vnum
Print short version number and exit.
-no-version
Do not print the version banner at startup.
-wwarning-list
Enable, disable, or mark as fatal the warnings specified by the argument
warning-list.
Each warning can be enabled or disabled, and each warning
can be fatal or non-fatal.
If a warning is disabled, it isn’t displayed and doesn’t affect
compilation in any way (even if it is fatal). If a warning is
enabled, it is displayed normally by the compiler whenever the source
code triggers it. If it is enabled and fatal, the compiler will also
stop with an error after displaying it.
The warning-list argument is a sequence of warning specifiers,
with no separators between them. A warning specifier is one of the
following:
+num
Enable warning number num.
-num
Disable warning number num.
@num
Enable and mark as fatal warning number num.
+num1..num2
Enable warnings in the given range.
-num1..num2
Disable warnings in the given range.
@num1..num2
Enable and mark as fatal warnings in
the given range.
+letter
Enable the set of warnings corresponding to
letter. The letter may be uppercase or lowercase.
-letter
Disable the set of warnings corresponding to
letter. The letter may be uppercase or lowercase.
@letter
Enable and mark as fatal the set of warnings
corresponding to letter. The letter may be uppercase or
lowercase.
uppercase-letter
Enable the set of warnings corresponding
to uppercase-letter.
lowercase-letter
Disable the set of warnings corresponding
to lowercase-letter.
Alternatively, warning-list can specify a single warning using its
mnemonic name (see below), as follows:
+name
Enable warning name.
-name
Disable warning name.
@name
Enable and mark as fatal warning name.
Warning numbers, letters and names which are not currently defined are
ignored. The warnings are as follows (the name following each number specifies
the mnemonic for that warning).
1 comment-start
Suspicious-looking start-of-comment mark.
2 comment-not-end
Suspicious-looking end-of-comment mark.
3
Deprecated synonym for the ’deprecated’ alert.
4 fragile-match
Fragile pattern matching: matching that will remain complete even
if additional constructors are added to one of the variant types
matched.
5 ignored-partial-application
Partially applied function: expression whose result has function
type and is ignored.
6 labels-omitted
Label omitted in function application.
7 method-override
Method overridden.
8 partial-match
Partial match: missing cases in pattern-matching.
9 missing-record-field-pattern
Missing fields in a record pattern.
10 non-unit-statement
Expression on the left-hand side of a sequence that doesn’t have type
unit (and that is not a function, see warning number 5).
11 redundant-case
Redundant case in a pattern matching (unused match case).
12 redundant-subpat
Redundant sub-pattern in a pattern-matching.
13 instance-variable-override
Instance variable overridden.
14 illegal-backslash
Illegal backslash escape in a string constant.
15 implicit-public-methods
Private method made public implicitly.
16 unerasable-optional-argument
Unerasable optional argument.
17 undeclared-virtual-method
Undeclared virtual method.
18 not-principal
Non-principal type.
19 non-principal-labels
Type without principality.
20 ignored-extra-argument
Unused function argument.
21 nonreturning-statement
Non-returning statement.
22 preprocessor
Preprocessor warning.
23 useless-record-with
Useless record with clause.
24 bad-module-name
Bad module name: the source file name is not a valid OCaml module name.
25
Ignored: now part of warning 8.
26 unused-var
Suspicious unused variable: unused variable that is bound
with let or as, and doesn’t start with an underscore (_)
character.
27 unused-var-strict
Innocuous unused variable: unused variable that is not bound with
let nor as, and doesn’t start with an underscore (_)
character.
28 wildcard-arg-to-constant-constr
Wildcard pattern given as argument to a constant constructor.
29 eol-in-string
Unescaped end-of-line in a string constant (non-portable code).
30 duplicate-definitions
Two labels or constructors of the same name are defined in two
mutually recursive types.
31 module-linked-twice
A module is linked twice in the same executable. (since 4.00)
32 unused-value-declaration
Unused value declaration. (since 4.00)
33 unused-open
Unused open statement. (since 4.00)
34 unused-type-declaration
Unused type declaration. (since 4.00)
35 unused-for-index
Unused for-loop index. (since 4.00)
36 unused-ancestor
Unused ancestor variable. (since 4.00)
37 unused-constructor
Unused constructor. (since 4.00)
38 unused-extension
Unused extension constructor. (since 4.00)
39 unused-rec-flag
Unused rec flag. (since 4.00)
40 name-out-of-scope
Constructor or label name used out of scope. (since 4.01)
41 ambiguous-name
Ambiguous constructor or label name. (since 4.01)
42 disambiguated-name
Disambiguated constructor or label name (compatibility warning). (since 4.01)
43 nonoptional-label
Nonoptional label applied as optional. (since 4.01)
44 open-shadow-identifier
Open statement shadows an already defined identifier. (since 4.01)
45 open-shadow-label-constructor
Open statement shadows an already defined label or constructor. (since 4.01)
46 bad-env-variable
Error in environment variable. (since 4.01)
47 attribute-payload
Illegal attribute payload. (since 4.02)
48 eliminated-optional-arguments
Implicit elimination of optional arguments. (since 4.02)
49 no-cmi-file
Absent cmi file when looking up module alias. (since 4.02)
50 unexpected-docstring
Unexpected documentation comment. (since 4.03)
51 wrong-tailcall-expectation
Function call annotated with an incorrect @tailcall attribute. (since 4.03)
The default setting is -w +a-4-6-7-9-27-29-32..42-44-45-48-50-60.
It is displayed by -help.
Note that warnings 5 and 10 are not always triggered, depending on
the internals of the type checker.
-warn-errorwarning-list
Mark as fatal the warnings specified in the argument warning-list.
The compiler will stop with an error when one of these warnings is
emitted. The warning-list has the same meaning as for
the -w option: a + sign (or an uppercase letter) marks the
corresponding warnings as fatal, a -
sign (or a lowercase letter) turns them back into non-fatal warnings,
and a @ sign both enables and marks as fatal the corresponding
warnings.
Note: it is not recommended to use warning sets (i.e. letters) as
arguments to -warn-error
in production code, because this can break your build when future versions
of OCaml add some new warnings.
The default setting is -warn-error -a+31 (only warning 31 is fatal).
-warn-help
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.
Unix:
The following environment variables are also consulted:
OCAMLTOP_INCLUDE_PATH
Additional directories to search for compiled
object code files (.cmi, .cmo and .cma). The specified directories are
considered from left to right, after the include directories specified on the
command line via -I have been searched. Available since OCaml 4.08.
OCAMLTOP_UTF_8
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.
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.
The following directives control the toplevel behavior, load files in
memory, and trace program execution.
Note: all directives start with a # (sharp) symbol. This #
must be typed before the directive, and must not be confused with the
# prompt displayed by the interactive loop. For instance,
typing #quit;; will exit the toplevel loop, but typing quit;;
will result in an “unbound value quit” error.
General
#help;;
Prints a list of all available directives, with corresponding argument type
if appropriate.
#quit;;
Exit the toplevel loop and terminate the ocaml command.
Loading codes
#cd "dir-name";;
Change the current working directory.
#directory "dir-name";;
Add the given directory to the list of directories searched for
source and compiled files.
#remove_directory "dir-name";;
Remove the given directory from the list of directories searched for
source and compiled files. Do nothing if the list does not contain
the given directory.
#load "file-name";;
Load in memory a bytecode object file (.cmo file) or library file
(.cma file) produced by the batch compiler ocamlc.
#load_rec "file-name";;
Load in memory a bytecode object file (.cmo file) or library file
(.cma file) produced by the batch compiler ocamlc.
When loading an object file that depends on other modules
which have not been loaded yet, the .cmo files for these modules
are searched and loaded as well, recursively. The loading order
is not specified.
#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.
#use_output "command";;
Execute a command and evaluate its output as if it had been captured
to a file and passed to #use.
#mod_use "file-name";;
Similar to #use but also wrap the code into a top-level module of the
same name as capitalized file name without extensions, following
semantics of the compiler.
For directives that take file names as arguments, if the given file
name specifies no directory, the file is searched in the following
directories:
In script mode, the directory containing the script currently
executing; in interactive mode, the current working directory.
Directories added with the #directory directive.
Directories given on the command line with -I options.
The standard library directory.
Environment queries
#show_class class-path;;
#show_class_type class-path;;
#show_exception ident;;
#show_module module-path;;
#show_module_type modtype-path;;
#show_type typeconstr;;
#show_val value-path;;
Print the signature of the corresponding component.
#show ident;;
Print the signatures of components with name ident in all the
above categories.
Pretty-printing
#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.
#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).
#remove_printer printer-name;;
Remove the named function from the table of toplevel printers.
Tracing
#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.
Compiler options
#labels bool;;
Ignore labels in function types if argument is false, or switch back
to default behaviour (commuting style) if argument is true.
#ppx "file-name";;
After parsing, pipe the abstract syntax tree through the preprocessor
command.
#principal bool;;
If the argument is true, check information paths during
type-checking, to make sure that all types are derived in a principal
way. If the argument is false, do not check information paths.
#rectypes;;
Allow arbitrary recursive types during type-checking. Note: once
enabled, this option cannot be disabled because that would lead to
unsoundness of the type system.
#warn_error "warning-list";;
Treat as errors the warnings enabled by the argument and as normal
warnings the warnings disabled by the argument.
#warnings "warning-list";;
Enable or disable warnings according to the argument.
Toplevel phrases can refer to identifiers defined in compilation units
with the same mechanisms as for separately compiled units: either by
using qualified names (Modulename.localname), or by using
the open construct and unqualified names (see section 11.3).
However, before referencing another compilation unit, an
implementation of that unit must be present in memory.
At start-up, the toplevel system contains implementations for all the
modules in the the standard library. Implementations for user modules
can be entered with the #load directive described above. Referencing
a unit for which no implementation has been provided
results in the error Reference to undefined global `...'.
Note that entering open Mod merely accesses the compiled
interface (.cmi file) for Mod, but does not load the
implementation of Mod, and does not cause any error if no
implementation of Mod has been loaded. The error
“reference to undefined global Mod” will occur only when
executing a value or module definition that refers to Mod.
This section describes and explains the most frequently encountered
error messages.
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
The ocamlmktop command builds OCaml toplevels that
contain user code preloaded at start-up.
The ocamlmktop command takes as argument a set of .cmo and .cma
files, and links them with the object files that implement the OCaml toplevel.
The typical use is:
ocamlmktop -o mytoplevel foo.cmo bar.cmo gee.cmo
This creates the bytecode file mytoplevel, containing the OCaml toplevel
system, plus the code from the three .cmo
files. This toplevel is directly executable and is started by:
./mytoplevel
This enters a regular toplevel loop, except that the code from
foo.cmo, bar.cmo and gee.cmo is already loaded in memory, just as
if you had typed:
This section describes a tool that is not yet officially supported but may be found useful.
OCaml code executing in the traditional toplevel system uses the bytecode
interpreter. When increased performance is required, or for testing
programs that will only execute correctly when compiled to native code,
the native toplevel may be used instead.
For the majority of installations the native toplevel will not have been
installed along with the rest of the OCaml toolchain. In such circumstances
it will be necessary to build the OCaml distribution from source.
From the built source tree of the distribution you may use
make natruntop to build and execute a native toplevel. (Alternatively
make ocamlnat can be used, which just performs the build step.)
If the make install command is run after having built the native
toplevel then the ocamlnat program (either from the source or the
installation directory) may be invoked directly rather than using
make natruntop.