context¶

The (context ...) stanza declares a build context. The argument can be either default or (default) for the default build context, or it can be the description of an opam switch, as follows:

(context (opam (switch <opam-switch-name>)
               <optional-fields>))

<optional-fields> are:

(name <name>) is the subdirectory’s name for _build, where this build’s context artifacts will be stored.
(root <opam-root>) is the opam root. By default, it will take the opam root defined by the environment in which dune is run, which is usually ~/.opam.
(merlin) instructs Dune to use this build context for Merlin.
(generate_merlin_rules) instructs Dune to generate Merlin rules for this context, even if it is not the one selected via (merlin).
(profile <profile>) sets a different profile for a build context. This has precedence over the command-line option --profile.
(env <env>) sets the environment for a particular context. This is of higher precedence than the root env stanza in the workspace file. This field has the same options as the env stanza.
(toolchain <findlib_toolchain>) sets a findlib toolchain for the context.
(host <host_context>) chooses a different context to build binaries that are meant to be executed on the host machine, such as preprocessors.
(paths (<var1> <val1>) .. (<varN> <valN>)) allows you to set the value of any PATH-like variables in this context. If PATH itself is modified in this way, its value will be used to resolve workspace binaries, including finding the compiler and related tools. These variables will also be passed as part of the environment to any program launched by Dune. For each variable, the value is specified using the Ordered Set Language. Relative paths are interpreted with respect to the workspace root. See Finding the Root.
(fdo <target_exe>) builds this context with feedback-direct optimizations. It requires OCamlFDO. <target_exe> is a path-interpreted relative to the workspace root (see Finding the Root). <target_exe> specifies which executable to optimize. Users should define a different context for each target executable built with FDO. The context name is derived automatically from the default name and <target-exe>, unless explicitly specified using the (name ...) field. For example, if <target_exe> is src/foo.exe in a default context, then the name of the context is default-fdo-foo and the filename that contains execution counters is src/fdo.exe.fdo-profile. This feature is experimental and no backwards compatibility is implied.
By default, Dune builds and installs dynamically-linked foreign archives (usually named dll*.so). It’s possible to disable this by setting by including (disable_dynamically_linked_foreign_archives true) in the workspace file, so bytecode executables will be built with all foreign archives statically linked into the runtime system.

Both (default ...) and (opam ...) accept a targets field in order to setup cross compilation. See Cross-Compilation for more information.

Merlin reads compilation artifacts, and it can only read the compilation artifacts of a single context. Usually, you should use the artifacts from the default context, and if you have the (context default) stanza in your dune-workspace file, that is the one Dune will use.

For rare cases where this is not what you want, you can force Dune to use a different build contexts for Merlin by adding the field (merlin) to this context.