[Caml-list] [ANN] Merlin 3.4.0 : introducing external configuration readers

["Ulysse Gérard" <ulysse@u31.fr>]

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I am glad to announce, on behalf of the Merlin team, the release of Merlin 3.4.0
which brings some major changes in the way configuration is handled.

As you might know, Merlin reads its configuration from the closest .merlin file
to the source file being edited. These files tell merlin where to find other
source files and build artifacts, but also which flags should be passed to the
compiler, which syntax extensions are enabled and which packages are used by the
project.

In this setting the configuration is the same for all the source files of a
folder, regardless of their specificities. In other words, the configuration
loaded for a single source file contains the union of the dependencies of this
file and of all its siblings which is not an optimal behavior.

Starting with version 3.4.0 merlin will ship with two packages: merlin and
dot-merlin-reader which, as the name suggests, reads configuration from .merlin
files. Both are necessary for proper function.

When a .merlin file is present in the source folder the Merlin server will start
a dot-merlin-reader process and communicate with it via standard input and
output following a simple protocol. These processes are halted with the server.

This change should not have any visible impact on users’ workflows as long as
the dot-merlin-reader binary is correctly installed and in the path. (which
should be the case in opam-based setups)

This change in itself will not solve the granularity problem mentioned earlier,
but it paves the way for such improvements: in a near-future Dune will stop
generating .merlin files and Merlin will obtain file-based configuration
directly from the build system using the same protocol as the one used by
dot-merlin-reader. 

Changelog
   + merlin binary
       - fix completion of pattern matchings with exception patterns (#1169)
       - delegate configuration reading to external programs via a simple protocol
        and create a new package dot-merlin-reader with a binary that reads
        .merlin files. (#1123, #1152)

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