Virtual libraries & variants

Virtual libraries correspond to dune’s ability to compile parameterized libraries and delay the selection of concrete implementations until linking an executable.

The feature introduces two kinds of libraries: virtual and implementations. A virtual library corresponds to an interface (although it may contain partial implementation). An implementation of a virtual library fills in all unimplemented modules in the virtual library.

The benefit of this partition is that other libraries may depend and compile against the virtual library and only select concrete implementations for these virtual libraries when linking executables. An example where this might be useful would be a virtual, cross platform, clock library. This library would have clock.unix and implementations. Executable using clock or libraries that use clock would conditionally select one of the implementations, depending on the target platform.

Virtual library

To define a virtual library, a virtual_modules field must be added to an ordinary library stanza and the version of the dune language must be at least 1.5. This field defines modules for which only an interface would be present (mli only):

 (name clock)
 ;; clock.mli must be present, but must not be
 (virtual_modules clock))

Apart from this field, the virtual library is defined just like a normal library and may use all the other fields. A virtual library may include other modules (with or without implementations), which is why it’s not a pure “interface” library.

Note: the virtual_modules field is not merged in modules, which represents the total set of modules in a library. If a directory has more than one stanza and thus a modules field must be specified, virtual modules still need to be added in modules.


An implementation for a library is defined as:

 (name clock_unix)
 ;; must be present, but clock.mli must not be
 (implements clock))

The name field is slightly different for an implementation than it is for a normal library. The name is just an internal name to refer to the implementation, it does not correspond to any particular module like it does in the virtual library.

Other libraries may then depend on the virtual library as if it was a regular library:

 (name calendar)
 (libraries clock))

But when it comes to creating an executable, we must now select a valid implementation for every virtual library that we’ve used:

 (name birthday-reminder)
  clock_unix ;; leaving this dependency will make dune loudly complain


This feature is still under development and may change with new dune releases. You need to write (using library_variants 0.2) in your dune-project file to unlock it.

When building a binary, implementations can be selected using a set of variants rather than individually specifying implementations.

An example where this is useful is providing JavaScript implementation. It would be tedious to select the JS implementation for every single virtual library. Instead, such implementations could select a js variant. Here’s the syntax:

 (name foo)
 (libraries time filesystem)
 (variants js))

An implementation can specify which variant it corresponds to using the variant option. Say for example that time is a virtual library. Its JS implementation would have the following configuration:

 (name time-js)
 (implements time)
 (variant js))

The list of available variants is computed while building the virtual library. This means only variant implementations that are part of the same project are implicitly taken into account. It’s possible to declare an external implementation by using the external_variant stanza in the virtual library scope.

 (variant foo)
 (implementation lib-foo)
 (virtual_library vlib))

This will add lib-foo to the list of known implementations of vlib.

Default implementation

This feature is also guarded by (using library_variants ...).

A virtual library may select a default implementation, which is enabled after variant resolution, if no suitable implementation has been found.

 (name time)
 (virtual_modules time)
 (default_implementation time-js))


The current implementation of virtual libraries suffers from a few limitations. Some of these are temporary.

  • It is not possible to link more than one implementation for the same virtual library in one executable.
  • It is not possible for implementations to introduce new public modules. That is, modules that aren’t a part of the virtual library’s cmi. Consequently, a module in an implementation either implements a virtual module or is private.
  • It’s not possible to load virtual virtual libraries into utop. As a result, any directory that contains a virtual library will not work with $ dune utop. This is an essential limitation, but it would be best to somehow skip these libraries or provide an implementation for them when loading a toplevel.
  • Virtual libraries must be defined using dune. It’s not possible for dune to implement virtual libraries created outside of dune. On the other hand, virtual libraries and implementations defined using dune should be usable with findlib based build systems.
  • It is not possible for a library to be both virtual and implement another library. This isn’t very useful, but technically, it could be used to create partial implementations. It is possible to lift this restriction if there’s enough demand for this.