Advanced topics

This section describes some details of Jbuilder for advanced users.

META file generation

Jbuilder uses META files from the findlib library manager in order to interoperate with the rest of the world when installing libraries. It is able to generate them automatically. However, for the rare cases where you would need a specific META file, or to ease the transition of a project to Jbuilder, it is allowed to write/generate a specific one.

In order to do that, write or setup a rule to generate a META.<package>.template file in the same directory as the <package>.opam file. Jbuilder will generate a META.<package> file from the META.<package>.template file by replacing lines of the form # JBUILDER_GEN by the contents of the META it would normally generate.

For instance if you want to extend the META file generated by Jbuilder you can write the folliwing META.foo.template file:

# JBUILDER_GEN
blah = "..."

Using a custom ppx driver

You can use a custom ppx driver by putting it as the last library in (pps ...) forms. An example of alternative driver is ppx_driver. To use it instead of ocaml-migrate-parsetree.driver-main, simply write ppx_driver.runner as the last library:

(preprocess (pps (ppx_sexp_conv ppx_bin_prot ppx_driver.runner)))

Driver expectation

Jbuilder will invoke the executable resulting from linking the libraries given in the (pps ...) form as follows:

ppx.exe <flags-written-by-user> --dump-ast -o <output-file> \
  [--cookie library-name="<name>"] [--impl|--intf] <source-file>

Where <source-file> is either an implementation (.ml) or interface (.mli) OCaml source file. The command is expected to write a binary OCaml AST in <output-file>.

Additionally, it is expected that if the executable is invoked with --as-ppx as its first argument, then it will behave as a standard ppx rewriter as passed to -ppx option of OCaml. This is for two reasons:

  • to improve interoperability with build systems other than Jbuilder
  • so that it can be used with merlin

Findlib integration and limitations

Jbuilder uses META files to support external libraries. However, it doesn’t export the full power of findlib to the user, and especially it doesn’t let the user specify predicates.

The reason for this limitation is that so far they haven’t been needed, and adding full support for them would complicate things quite a lot. In particular, complex META files are often hand-written and the various features they offer are only available once the package is installed, which goes against the root ideas jbuilder is built on.

In practice, jbuilder interpret META files assuming the following set of predicates:

  • mt: what this means is that using a library that can be used with or without threads with jbuilder will force the threaded version
  • mt_posix: forces the use of posix threads rather than VM threads. VM threadws are deprecated and are likely to go away soon
  • ppx_driver: when a library acts differently depending on whether it is linked as part of a driver or meant to add a -ppx argument to the compiler, choose the former behavior

Cross Compilation

Jbuilder allows for cross compilation by defining build contexts with multiple targets. Targets are specified by adding a targets field to the definition of a build context.

targets takes a list of target name. It can be either:

  • native which means using the native tools that can build binaries that run on the machine doing the build
  • the name of an alternative toolchain

Note that at the moment, there is no official support for cross-compilation in OCaml. Jbuilder supports the two following opam-cross-x repositories:

To build Windows binaries using opam-cross-windows, write windows in the list of targets. To build Android binaries using opam-cross-android, write android in the list of targets.

For example, the following workspace file defines three different targets for the default build context:

(context (default (targets (native windows android))))

This configuration defines three build contexts:

  • default
  • default.windows
  • default.android

Note that the native target is always implicitly added when not present. However, when implicitly added jbuilder build @install will skip this context, i.e. default will only be used for building executables needed by the other contexts.

With such a setup, calling jbuilder build @install will build all the packages three times.

Note that instead of writing a jbuild-workspace file, you can also use the -x command line option. Passing -x foo to jbuilder without having a jbuild-workspace file is the same as writing the following jbuild-workspace file:

(context (default (targets (foo))))

If you have a jbuild-workspace and pass a -x foo option, foo will be added as target of all context stanzas.

How does it work?

In such a setup, binaries that need to be built and executed in the default.windows or default.android contexts as part of the build, will no longer be executed. Instead, all the binaries that will be executed will come from the default context. One consequence of this is that all preprocessing (ppx or otherwise) will be done using binaries built in the default context.

To clarify this with an example, let’s assume that you have the following src/jbuild file:

(executable ((name foo)))
(rule (with-stdout-to blah (run ./foo.exe)))

When building _build/default/src/blah, jbuilder will resolve ./foo.exe to _build/default/src/foo.exe as expected. However, for _build/default.windows/src/blah jbuilder will resolve ./foo.exe to _build/default/src/foo.exe

Assuming that the right packages are installed or that your workspace has no external dependencies, jbuilder will be able to cross-compile a given package without doing anything special.

Some packages might still have to be updated to support cross-compilation. For instance if the foo.exe program in the previous example was using Sys.os_type, it should instead take it as a command line argument:

(rule (with-stdout-to blah (run ./foo.exe -os-type ${os_type})))

Classical ppx

classical ppx refers to running ppx using the -ppx compiler option, which is composed using Findlib. Even though this is useful to run some (usually old) ppx’s which don’t support drivers, Jbuilder does not support preprocessing with ppx this way. but a workaround exists using the ppxfind tool.