Cppo: cpp for OCaml
Cppo is an equivalent of the C preprocessor for OCaml programs. It allows the definition of simple macros and file inclusion.
- more OCaml-friendly than cpp
- easy to learn without consulting a manual
- reasonably fast
- simple to install and to maintain
Cppo is a preprocessor for programming languages that follow lexical rules compatible with OCaml.
Cppo supports a number of directives. A directive is a
# sign placed
at the beginning of a line, possibly preceded by some whitespace, and followed
by a valid directive name or by a number:
BLANK* "#" BLANK* ("define"|"undef" |"if"|"ifdef"|"ifndef"|"else"|"elif"|"endif" |"include" |"warning"|"error" |"ext"|"endext") ...
Directives can be split into multiple lines by placing a backslash
the end of the line to be continued. In general, any special character
can used as a normal character by preceding it with backslash.
This is how a source file
hello.ml can be included.
Relative paths are searched first in the directory of the current file
and then in the search paths added on the command line using
-I, if any.
This is a simple macro that doesn't take an argument ("object-like macro" in the cpp jargon):
#define Ms Mississippi match state with Ms -> true | _ -> false
After preprocessing by cppo, the code above becomes:
match state with Mississippi -> true | _ -> false
If needed, defined macros can be undefined. This is required prior to redefining a macro:
An important distinction with cpp is that only previously-defined macros are accessible. Defining, undefining or redefining a macro has no effect on how previous macros will expand.
Macros can take arguments ("function-like macro" in the cpp
jargon). Both in the definition (
#define) and in macro application the
opening parenthesis must stick to the macro's identifier:
#define debug(args) if !debugging then Printf.eprintf args else () debug("Testing %i" (1 + 1))
is expanded into:
if !debugging then Printf.eprintf "Testing %i" (1 + 1) else ()
Here is a multiline macro definition. Newlines occurring between tokens must be protected by a backslash:
#define repeat_until(action,condition) \ action; \ while not (condition) do \ action \ done
All user-definable macros are constant. There are however two
predefined variable macros:
__LINE__ which take the value
of the position in the source file where the macro is being expanded.
#define loc (Printf.sprintf "File %S, line %i" __FILE__ __LINE__)
Macros can be defined on the command line as follows:
# preprocessing only cppo -D 'VERSION 1.0' example.ml # preprocessing and compiling ocamlopt -c -pp "cppo -D 'VERSION 1.0'" example.ml
Here is a quick reference on conditionals available in cppo. If you
are not familiar with
refer to the corresponding section in the cpp manual.
#ifndef VERSION #warning "VERSION is undefined" #define VERSION "n/a" #endif #ifndef VERSION #error "VERSION is undefined" #endif #if OCAML_MAJOR >= 3 && OCAML_MINOR >= 10 ... #endif #ifdef X ... #elif defined Y ... #else ... #endif
The boolean expressions following
#elif may perform arithmetic
operations and tests over 64-bit ints.
defined... followed by an identifier, returns true if such a macro exists
Arithmetic comparisons used in boolean expressions:
Arithmetic operators over signed 64-bit ints:
Macro identifiers can be used in place of ints as long as they expand to an int literal or a tuple of int literals, e.g.:
#define one 1 #if one + one <> 2 #error "Something's wrong." #endif #define VERSION (1, 0, 5) #if VERSION <= (1, 0, 2) #error "Version 1.0.2 or greater is required." #endif
Version strings (http://semver.org/) can also be passed to cppo on the
command line. This results in multiple variables being defined, all
sharing the same prefix. See the output of
cppo -help (copied at the
bottom of this page).
$ cppo -V OCAML:`ocamlc -version` #if OCAML_VERSION >= (4, 0, 0) (* All is well. *) #else #error "This version of OCaml is not supported." #endif
# 2 "<stdin>" (* All is well. *)
Source file location
Location directives are the same as in OCaml and are echoed in the output. They consist of a line number optionally followed by a file name:
# 123 # 456 "source"
Warnings and error messages can be produced by the preprocessor:
#ifndef X #warning "Assuming default value for X" #define X 1 #elif X = 0 #error "X may not be null" #endif
Calling an external processor
Cppo provides a mechanism for converting sections of a file using
and external program. Such a section must be placed between
$ cat foo ABC #ext lowercase DEF #endext GHI #ext lowercase KLM NOP #endext QRS $ cppo -x lowercase:'tr "[A-Z]" "[a-z]"' foo # 1 "foo" ABC def # 5 "foo" GHI klm nop # 10 "foo" QRS
In the example above,
lowercase is the name given on the
command-line to external command
'tr "[A-Z]" "[a-z]"' that reads
input from stdin and writes its output to stdout.
The following characters can be escaped by a backslash when needed:
( ) , #
# is used for method calls. It is usually not a problem
because in order to be interpreted as a preprocessor directive, it
must be the first non-blank character of a line and be a known
directive. If an object has a define method and you want
# to appear
first on a line, you would have to use
Line directives in the usual format supported by OCaml are correctly interpreted by cppo.
Comments and string literals constitute single tokens even when they span across multiple lines. Therefore newlines within string literals and comments should remain as-is (no preceding backslash) even in a macro body:
#define welcome \ "********** *Welcome!* ********** "
CONCAT() is a predefined macro that takes two arguments, removes any
whitespace between and around them and fuses them into a single identifier.
The result of the concatenation must be a valid identifier of the
form [A-Za-z_][A-Za-z0-9_]+ or [A-Za-z], or empty.
#define x 123 CONCAT(z, x)
However the following is illegal:
#define x 123 CONCAT(x, z)
because 123z does not form a valid identifier.
CONCAT(a,b) is roughly equivalent to
a##b in cpp syntax.
STRINGIFY() is a predefined macro that takes one argument,
removes any leading and trailing whitespace, reduces each internal
whitespace sequence to a single space character and produces
a valid OCaml string literal.
#define TRACE(f) Printf.printf ">>> %s\n" STRINGIFY(f); f TRACE(print_endline) "Hello"
is expanded into:
Printf.printf ">>> %s\n" "print_endline"; print_endline "Hello"
STRINGIFY(x) is the equivalent of
#x in cpp syntax.
An ocamlbuild plugin is available. To use it, you can call ocamlbuild
with the argument
-plugin-tag package(cppo_ocamlbuild) (only since
With Oasis :
OCamlVersion: >= 4.01 AlphaFeatures: ocamlbuild_more_args XOCamlbuildPluginTags: package(cppo_ocamlbuild)
After that, you need to add in your
let () = Ocamlbuild_plugin.dispatch (fun hook -> Ocamlbuild_cppo.dispatcher hook ; )
By default the plugin will apply cppo on all files ending in
cppo.mlpack, in order to produce
.mlpack files. The following tags are available:
- The tag
cppo_I(foo)can behave in two way:
foois a directory, it's equivalent to
foois a file, it adds
fooas a dependency and apply
-V OCAML:VERSION, where
VERSIONis the version of OCaml that ocamlbuild uses.
Detailed command-line usage and options
Usage: ./cppo [OPTIONS] [FILE1 [FILE2 ...]] Options: -D DEF Equivalent of interpreting '#define DEF' before processing the input -U IDENT Equivalent of interpreting '#undef IDENT' before processing the input -I DIR Add directory DIR to the search path for included files -V VAR:MAJOR.MINOR.PATCH-OPTPRERELEASE+OPTBUILD Define the following variables extracted from a version string (following the Semantic Versioning syntax http://semver.org/): VAR_MAJOR must be a non-negative int VAR_MINOR must be a non-negative int VAR_PATCH must be a non-negative int VAR_PRERELEASE if the OPTPRERELEASE part exists VAR_BUILD if the OPTBUILD part exists VAR_VERSION is the tuple (MAJOR, MINOR, PATCH) VAR_VERSION_STRING is the string MAJOR.MINOR.PATCH VAR_VERSION_FULL is the original string Example: cppo -V OCAML:4.02.1 -o FILE Output file -q Identify and preserve camlp4 quotations -s Output line directives pointing to the exact source location of each token, including those coming from the body of macro definitions. This behavior is off by default. -n Do not output any line directive other than those found in the input (overrides -s). -version Print the version of the program and exit. -x NAME:CMD_TEMPLATE Define a custom preprocessor target section starting with: #ext "NAME" and ending with: #endext NAME must be a lowercase identifier of the form [a-z][A-Za-z0-9_]* CMD_TEMPLATE is a command template supporting the following special sequences: %F file name (unescaped; beware of potential scripting attacks) %B number of the first line %E number of the last line %% a single percent sign Filename, first line number and last line number are also available from the following environment variables: CPPO_FILE, CPPO_FIRST_LINE, CPPO_LAST_LINE. The command produced is expected to read the data lines from stdin and to write its output to stdout. -help Display this list of options --help Display this list of options
See our contribution guidelines at https://github.com/mjambon/documents/blob/master/how-to-contribute.md