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
implementations. Executable using
clock or libraries that use
would conditionally select one of the implementations, depending on the target
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
;; clock.mli must be present, but clock.ml must not be
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.
virtual_modules field is not merged in
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
An implementation for a library is defined as:
;; clock.ml must be present, but clock.mli must not be
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:
But when it comes to creating an executable, we must now select a valid implementation for every virtual library that we’ve used:
clock_unix ;; leaving this dependency will make dune loudly complain
Variants were an experimental feature that were removed in Dune 2.6.
A virtual library may select a default implementation, which is enabled after variant resolution if no suitable implementation has been found.
The default implementation must live in the same package as the virtual
library. In the example above, that would mean that the
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
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.