Packing and Functors
We have recently worked on modifying the OCaml system to be able to pack a set of modules within a functor, parameterized on some signatures. This page presents this work, funded by Jane Street.
All the patches on this page are provided for OCaml version 3.12.1.
Installation of the modified OCaml system
The patch for OCaml 3.12.1 is available here:
ocaml+libfunctor-3.12.1.patch.gz (26 kB)
To use it, you can use the following recipe, that will compile and
install the patched version in
~% wget http://caml.inria.fr/pub/distrib/ocaml-3.12/ocaml-3.12.1.tar.gz ~% tar zxf ~/ocaml-3.12.1.tar.gz ~% cd ocaml-3.12.1 ~/ocaml-3.12.1% wget ocamlpro.com/code/ocaml+libfunctor-3.12.1.patch.gz ~/ocaml-3.12.1% gzip -d ocaml+libfunctor-3.12.1.patch.gz ~/ocaml-3.12.1% patch -p1 < ocaml+libfunctor-3.12.1.patch ~/ocaml-3.12.1% ./configure –prefix ~/ocaml+libfunctor-3.12.1 ~/ocaml-3.12.1% make coldstart ~/ocaml-3.12.1% make ocamlc ocamllex ocamltools ~/ocaml-3.12.1% make library-cross ~/ocaml-3.12.1% make bootstrap ~/ocaml-3.12.1% make all opt opt.opt ~/ocaml-3.12.1% make install ~/ocaml-3.12.1% cd ~ ~% export PATH=$HOME/ocaml+libfunctor-3.12.1/bin:$PATH
Note that it needs to bootstrap the compiler, as the format of object files is not compatible with the one of ocaml-3.12.1.
Usage of the lib-functor patch.
Now that you are equiped with the new system, you can start using it. The lib-functor patch adds two new options to the compilers ocamlc and ocamlopt:
-functor <interface_file>: this option is used to specify that the current module is compiled with the interface files specifying the argument of the functor. This option should be used together with -for-pack
, where is the name of the module in which the current module will be embedded.
-pack-functor <module>: this option is used to pack the modules. It should be used with the option -o <object_file> to specify in which module it should be embedded. The
specified with -pack-functor specifies the name of functor that will be created in the target object file.
If the interface x.mli contains :
type t val compare : t -> t -> int
and the files
xmap.ml contain respectively :
module T = Set.Make(X)
module T = Map.Make(X)
~/test% ocamlopt -c -for-pack Xx -functor x.cmi xset.ml ~/test% ocamlopt -c -for-pack Xx -functor x.cmi xmap.ml ~/test% ocamlopt -pack-functor MakeSetAndMap -o xx.cmx xset.cmx xmap.cmx
will construct a compiled unit whose signature is (that you can get
ocamlopt -i xx.cmi, see below) :
module MakeSetAndMap : functor (X : sig type t val compare : t -> t -> int end) -> sig module Xset : sig module T : sig type elt = X.t type t = Set.Make(X).t val empty : t val is_empty : t -> bool … end end module Xmap : sig module T : sig type key = X.t type ‘a t = ‘a Map.Make(X).t val empty : ‘a t val is_empty : ‘a t -> bool … end end end
Other extension: printing interfaces
OCaml only allows you to print the interface of a module or interface by compiling its source with the -i option. However, you don’t always have the source of an object interface (in particular, if it was generated by packing), and you might still want to do it.
In such a case, the lib-functor patch allows you to do that, by using the -i option on an interface object file:
~/test% cat > a.mli val x : int ~/test% ocamlc -c -i a.mli val x : int ~/test% ocamlc -c -i a.cmi val x : int
Other extension: packing interfaces
OCaml only allows you to pack object files inside another object file (.cmo or .cmx). When doing so, you can either provide an source interface (.mli) that you need to compile to provide the corresponding object interface (.cmi), or the object interface will be automatically generated by exporting all the sub-modules within the packed module.
However, sometimes, you would want to be able to specify the interfaces of each module separately, so that:
you can reuse most of the interfaces you already specified
you can use a different interface for a module, that the one used to compile the other modules. This happens when you want to export more values to the other internal sub-modules than you want to export to the user.
In such a case, the lib-functor patch allows you to do that, by using the -pack option on interface object files:
test% cat > a.mli val x : int test% cat > b.mli val y : string test% ocamlc -c a.mli b.mli test% ocamlc -pack -o c.cmi a.cmi b.cmi test% ocamlc -i c.cmi module A : sig val x : int end module B : sig val y : string end
ocp-pack to pack source files
Installation of ocp-pack
Download the source file from:
ocp-pack-1.0.1.tar.gz (20 kB, GPL Licence, Copyright OCamlPro SAS)
Then, you just need to compile it with:
~% tar zxf ocp-pack-1.0.1.tar.gz ~% cd ocp-pack-1.0.1 ~/ocp-pack-1.0.1% make ~/ocp-pack-1.0.1% make install
ocp-pack can be used to pack source files of modules within just one
source file. It allows you to avoid the use of the
-pack option, that
is not always supported by all ocaml tools (for example,
ocp-pack tries to provide the correct locations
to the compiler, so errors are not reported within the generated
source file, but within the original source files.
It supports the following options:
% ocp-pack -help Usage: ocp-pack -o target.ml [options] files.ml* Options: -o <filename.ml> generate filename filename.ml -rec use recursive modules all .ml files must have a corresponding .mli file -pack-functor <modname> create functor with name <modname> -functor <filename.mli> use filename as an argument for functor -mli output the .mli file too .ml files without .mli file will not export any value -no-ml do not output the .ml file -with-ns use directory structure to create a hierarchy of modules -v increment verbosity –version display version information
ocp-pack automatically detects interface sources and implementation
sources. When only the interface source is available, it is assumed
that it is a type-only module, i.e. no val items are present inside.
Here is an example of using
ocp-pack to build the ocamlgraph package:
test% ocp-pack -o graph.ml lib/bitv.ml lib/heap.ml lib/unionfind.ml src/sig.mli src/dot_ast.mli src/sig_pack.mli src/version.ml src/util.ml src/blocks.ml src/persistent.ml src/imperative.ml src/delaunay.ml src/builder.ml src/classic.ml src/rand.ml src/oper.ml src/path.ml src/traverse.ml src/coloring.ml src/topological.ml src/components.ml src/kruskal.ml src/flow.ml src/graphviz.ml src/gml.ml src/dot_parser.ml src/dot_lexer.ml src/dot.ml src/pack.ml src/gmap.ml src/minsep.ml src/cliquetree.ml src/mcs_m.ml src/md.ml src/strat.ml test% ocamlc -c graph.ml test% ocamlopt -c graph.ml
The -with-ns option can be used to automatically build a hierarchy of modules. With that option, sub-directories are seen as sub-modules. For example, packing a/x.ml, a/y.ml and b/z.ml will give a result like:
[code language=”fsharp”] module A = struct module X = struct … end module Y = struct … end end module B = struct module Z = struct … end end [/code]
Packing modules as functors
-functor options provide the same behavior
as the same options with the lib-functor patch. The only difference is
-functor takes the interface source as argument, not the
Packing recursive modules
When trying to pack modules with
ocp-pack, you might discover that
your toplevel modules have recursive dependencies. This is usually
achieved by types declared abstract in the interfaces, but depending
on each other in the implementations. Such modules cannot simply
To handle them,
ocp-pack provides a
-rec option. With that option,
modules are put within a module rec construct, and are all required to
be accompagnied by an interface source file.
Moreover, in many cases, OCaml is not able to compile such recursive modules:
For typing reasons: recursive modules are typed in an environment containing only an approximation of other recursive modules signatures
For code generation reasons: recursive modules can be reordered depending on their shape, and this reordering can generate an order that is actually not safe, leading to an exception at runtime
To solve these two issues in most cases, you can use the following patch (you can apply it using the same recipe as for lib-functor, and even apply both patches on the same sources):
With this patch, recursive modules are typed in an environment that is enriched progressively with the final types of the modules as soon as they become available. Also, during code generation, a topological order is computed on the recursive modules, and the subset of modules that can be initialized using in that topological order are immediatly generated, leaving only the other modules to be reordered.
OCamlPro is a R&D lab founded in 2011, with the mission to help industrial users benefit from state-of-the art programming languages like OCaml and Rust.
We design, create and implement custom ad-hoc software for our clients. We also have a long experience in developing and maintaining open-source tooling for OCaml, such as Opam, TryOCaml, ocp-indent, ocp-index and ocp-browser, and we contribute to the core-development of OCaml, notably with our work on the Flambda optimizer branch.
Another area of expertise is that of Formal Methods, with tools such as our SMT Solver Alt-Ergo (check our Alt-Ergo Users'). We also provide vocational trainings in OCaml and Rust, and we can build courses on formal methods on-demand. Please reach out, we'll be delighted to discuss your challenges: firstname.lastname@example.org or book a quick discussion.
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