What’s coming in Dune 1.1.0?

Now that 1.0.0 has been released, the team took a break from dune affairs. However, now we’re back to work, and although we’re working at a more relaxed pace, we still have some interesting features in store. Most of these were already planned for quite a while, but simply had to be delayed to get 1.0.0 out of the door. Hopefully this post convinces the reader that it was worth the wait.

name(s) Fields are Now Optional

I’m sure many of you had to write stanzas of the form many times:

(executables
 (names foo bar)
 (public_names foo bar))

(library
 (name lib)
 (public_name lib))

In dune 1.1.0, we’ve decided to spare you from some of this boilerplate and allow the omission of the name(s) field when public_name(s) is present. The above can now be written as:

(executables (public_names foo bar))
(library (public_name lib))

Proper Multi Directory Sources for Libraries

Users have longed for the ability to define a library across many directories, and our standard response was to use #copy_files. While this isn’t so bad, we’ve introduced a more fluent approach. Simply include the following stanza your dune file:

(include_subdirs unqualified)

(library (public_name foo))

And now the library foo will be composed of the modules in all the sub directories where this stanza is defined. For now, only unqualified is supported, but we look forward to introduce qualified as well.

Per Context & Workspace Environments

The env stanza introduced in 1.0 was a great start in making project definitions more flexible. The ability to define various flags in a profile dependent way and to avoid repetition when many stanzas shared the same flags is quite useful. We’ve decided to extend this ability to allow setting the environment for particular contexts and entire workspaces.

For example, you might want to have a context to check if your code compiles with the -principal flag. This can be done with this workspace file:

(context
 (default
  (env
   (_ (flags (:standard -principal))))))

You can also set the environment for all contexts in an environment file with a toplevel environment stanza. Since such environment definitions can override each other, the priority is as follows:

  • The environment at the toplevel of the workspace has the lowest priority
  • Context specific environment overrides the workspace environment
  • Environments defined in dune flies override those defined in the workspace

Faster Compilation in Development Mode

I’m not sure how many users are familiar with the -opaque compiler option, so allow me to quote the manual:

-opaque

  Interface file compiled with this option are marked so that other
  compilation units depending on it will not rely on any implementation
  details of the compiled implementation. The native compiler will not access
  the .cmx file of this unit -- nor warn if it is absent. This can improve
  speed of compilation, for both initial and incremental builds, at the
  expense of performance of the generated code.

Dune 1.1.0 will include this flag to generate rules without extra .cmx dependencies. As the manual states, this should make compilation faster at the expense of cross module optimizations. Of course, this compilation mode will only be used when the profile is set to dev. In --profile release we will favor rules the old rules that optimize for runtime performance.

Support for ppx_import

To avoid preprocessing files more than once, dune restricted its ppx support to only those rewriters that are parsetree transformations without any other dependencies. While this covered a very wide set of plugins, it excluded some very useful ones, in particular: ppx_import. This is because ppx_import reads cmi files as part of its AST rewriting and must be ran in two stages (hence the performance penalty). To use such a ppx we’ve introduced the staged_pps constructor. Here’s an example:

(library
 (name foo)
 (preprocess (staged_pps ppx_import)))

Now this will not be usable until ppx_import supports the ppx driver interface. Hopefully this feature will create some incentive for the ppx_driver maintainers to do just that.

Finally, dune is still missing support to define such rewriters. This feature is marginally used so the dune team cannot prioritize it, but if anybody is interested in implementing this, don’t hesitate to contact us.

Reminder to update dune-project

To use some of the features above, you will need to bump your dune lang version. Simply update your dune-project file to read:

(lang dune 1.1)
...

Conclusion

I’ve omitted some minor features and bug fixes that are also included in 1.1.0. Please consult the changelog for a full list. As always, if anything isn’t clear regarding the features above, don’t hesitate to ask a question or to open a bug against the documentation.

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