Migration — dune documentation

# Migration

Dune was initially called Jbuilder. Up to mid-2018, the package was still called jbuilder which only installed a jbuilder binary. This document explain how the migration to Dune will happen.

## Timeline

The general idea is that the migration is gradual and existing Jbuilder projects don’t need to be updated all at once. We encourage users to switch their development repositories and continue their usual release cycle. There is no need to re-release existing packages just to switch to Dune immediately.

The plan is as follows:

### July 2018: release of Dune 1.0.0

First release of the opam package dune. The jbuilder package becomes a transitional package that depends on dune.

The dune package installs two binaries: dune and jbuilder. These two binaries are exactly the same and they work on both Jbuilder and Dune projects. Additionally they recognize both Jbuilder and Dune configuration files. The new Dune configuration files are described later in this document.

### January 2019: deprecation of Jbuilder

At this point, the jbuilder binary emits a warning on every startup inviting users to switch to dune. When encountering jbuild or other Jbuilder configuration files, both binaries emit a warning. The rest is unchanged.

During this period, it makes sense for projects to do new releases just to switch to Dune if none of their existing releases is using Dune.

### July 2019: support for Jbuilder is dropped

jbuilder is now a dummy executable that always exit with an error message on startup. dune no longer reads jbuild or other Jbuidler configuration files but still prints a warning when encountering them.

At this point, a conflict with newer versions of dune will be added to all opam packages that rely on the jbuilder binary or Jbuilder configuration files.

### January 2020: the jbuilder binary goes away

The dune package no longer installs a jbuilder binary. The rest is unchanged.

### Distant future

Once we are sure there are no more jbuild files out there, Dune will completely ignore jbuild and other Jbuilder configuration files.

## Check list

This section is a concise list of migration tasks that will be required to transition from jbuilder to dune.

### New configuration files

Until July 2019, dune will still read jbuild and other Jbuilder configuration files. There is no change in these files.

However, based on the experience acquired since the first release of Jbuilder, we made a few changes in the configuration files read by Dune. The most notable ones are the following:

- jbuild files are renamed simply dune
- projects now have a dune-project file at their root
- jbuild-ignore files are replaced by ignored_subdirs stanzas in dune files
- jbuild-workspace are replaced by dune-workspace files
- jbuild-workspace<suffix> files no longer mean anything

Following are detailed explanation of the differences between the Jbuilder configuration files and the Dune ones.

### dune-project files

These are a new kind of file. With Jbuilder, projects used to be identified by the presence of at least one <package>.opam file in a directory. This will still be supported until July 2019, however as Jbuilder evolved it became clear that we needed project files, so Dune introduces dune-project files to mark the root of projects.

Eventually, we are hoping that Dune will generate opam files. So users will only have to write a dune-project file.

The purpose of this file is to:

- delimit projects in larger workspaces
- set a few project-wide parameters, such as the name, the version of the Dune language in use or specification of extra features (plugins) used in the project

Eventually, for users who wish to do so it should be possible to centralize all the configuration of a project in this file.

### dune files

These are the same as jbuild files.

### dune-workspace

These are the same as jbuild-workspace files.

When looking for the root of the workspace, Jbuilder also looks for files whose name start with jbuild-workspace, such as jbuild-workspace.in. This rule will be kept until July 2019, however it is not preserved for dune-workspace files. I.e. a dune-workspace.in file means nothing.

This rule was only useful when we didn’t have project files.

### Variable syntax

`${foo} and $(foo)` are no longer valid variable syntax in dune files. Variables are defined as `%{foo}`. This change is done to simplify interoperability with bash commands which also use the `${foo}` syntax.

### `(files_recursively_in ..)` is removed

The `files_recursively_in` dependency specification is invalid in dune files. A source_tree stanza has been introduced to reflect the actual function of this stanza.

### Escape sequences

Invalid escape sequences of the form `\x` where `x` is a character other than `[0-9]`, `x`, `n`, `r`, `t`, `b` are not allowed in dune files.

### Comments syntax

Block comments of the form `#| ... |#` and comments of the form `#;` are not supported in dune files.

### Renamed variables

All existing variables have been lowercased for consistency. Other variables have always been renamed. Refer to this table for details:

Jbuild | Dune
--- | ---
`${@}` | `%{targets}`
`${^}` | `%{deps}`
`${path:file}` | `%{dep:file}`
`${SCOPE_ROOT}` | `%{project_root}`
`${ROOT}` | `%{workspace_root}`
`${findlib:..}` | `%{lib:..}`
`${CPP}` | `%{cpp}`
`${CC}` | `%{cc}`
`${CXX}` | `%{cxx}`
`${OCAML}` | `%{ocaml}`
`${OCAMLC}` | `%{ocamlc}`
`${OCAMLOPT}` | `%{ocamlopt}`
`${ARCH_SIXTYFOUR}` | `%{arch_sixtyfour}`
`${MAKE}` | `%{make}`

### Removed variables

`${path-no-dep:file}` and `${<}` have been removed.

A named dependency should be used instead of `${<}`. For instance the following jbuild file:

    (alias
     ((name   runtest)
      (deps   (input))
      (action (run ./test.exe %{<}))))

should be rewritten to the following dune file:

    (rule
     (alias  runtest)
     (deps   (:x input))
     (action (run ./test.exe %{x})))

### `# JBUILDER_GEN` renamed

`# DUNE_GEN` should be used instead of `# JBUILDER_GEN` in META templates.

### jbuild-ignore (deprecated)

`jbuild-ignore` files are deprecated and replaced by dirs (since 1.6) stanzas in `dune` files.