Library Dependencies¶

Library dependencies are specified using (libraries ...) fields in library and executables stanzas.

For libraries defined in the current scope, you can either use the real name or the public name. For libraries that are part of the installed world, or for libraries that are part of the current workspace but in another scope, you need to use the public name. For instance: (libraries base re).

When resolving libraries, ones that are part of the workspace are always preferred to ones that are part of the installed world.

Alternative Dependencies¶

Sometimes, one doesn’t want to depend on a specific library but rather on whatever is already installed, e.g., to use a different backend, depending on the target.

Dune allows this by using a (select ... from ...) form inside the list of library dependencies.

Select forms are specified as follows:

(select <target-filename> from
 (<literals> -> <filename>)
 (<literals> -> <filename>)
 ...)

<literals> are lists of literals, where each literal is one of:

<library-name>, which will evaluate to true if <library-name> is available, either in the workspace or in the installed world
!<library-name>, which will evaluate to true if <library-name> is not available in the workspace or in the installed world

When evaluating a select form, Dune will create <target-filename> by copying the file given by the first (<literals> -> <filename>) case where all the literals evaluate to true. It is an error if none of the clauses are selectable. You can add a fallback by adding a clause of the form (-> <file>) at the end of the list.

Re-Exported Dependencies¶

A dependency foo may be marked as always re-exported using the following syntax:

(re_export foo)

For instance:

(library
 (name bar)
 (libraries (re_export foo)))

This states that this library explicitly re-exports the interface of foo. Concretely, when something depends on bar, it will also be able to see foo independently of whether implicit transitive dependencies are allowed or not. When they are allowed, which is the default, all transitive dependencies are visible, whether they are marked as re-exported or not.