Virtual Libraries & Variants

Virtual libraries correspond to Dune’s ability to compile parameterised 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 on and compile against the virtual library, and they might 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 doesn’t 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


Variants were an experimental feature that were removed in Dune 2.6.

Default Implementation

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 default implementation must live in the same package as the virtual library. In the example above, that would mean that the time-js and time libraries must be in the same package

Before version 2.6, this feature was experimental and guarded under the library_variants language. In 2.6, this feature was promoted to the stable Dune language, and all uses of (using library_variants) are forbidden since 2.6.


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

  • It’s impossible to link more than one implementation for the same virtual library in one executable.

  • It’s 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 isn’t possible to load 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’s impossible for a library to be both virtual and implement another library. This isn’t very useful, but it could technically be used to create partial implementations. It is possible to lift this restriction if there’s enough demand.