The New (Experimental) Yesod Development Server - Feedback Requested!
November 24, 2016
I'm guessing almost every Yesod user has - at some point - used the
yesod devel command, which launches a server which
auto-recompiles your source code on any file changes. This has been a
core part of the Yesod ecosystem for many years. Unfortunately, it's
had to be far more complicated than I'd have liked:
- Since it predates
addDependentFile(good work Greg Weber on getting that in!), it has some pretty complex logic around guessing which external files (like Hamlet files) should force a recompile. (Adding support for
addDependentFileto the current
yesod develis possible, but it's a non-trivial undertaking.)
- In order to ensure a consistent set of dependencies, it does some
real fancy footwork around intercepting arguments passed to
ghcand linker executables.
- In order to parse various files, it links against the
ghclibrary, tying it to a specific compiler version. This makes things difficult for users (don't accidentally use
yesodfrom GHC 7.10.3 with GHC 8.0.1!), and sometimes really painful for maintainers.
For a few months now, I've been meaning to greatly simplify
devel, but the maintenance burden finally gave me the excuse I needed
to bite the bullet and do it. The result is a
dramatic simplification of the code base. First
I'd like to ask for user feedback, and then I'll discuss some of the
details of implementation.
Please try it out!
Since this is such a big change, I'd really appreciate if others could give this a shot before I release it. There are two ways you can do this:
git clone https://github.com/yesodweb/yesod --branch 1304-stack-based-devel yesod-new-devel
stack install yesod-bin
- From your project directory, run
yesod devel. NOTE: do not use
stack exec -- yesod devel, you want to use the newly globally installed executable, not the one from your snapshot!
Add the following to your
- location: git: https://github.com/yesodweb/yesod commit: f3fc735a25eb3d5c051c761b59070eb9a0e4e156 subdirs: - yesod-bin extra-dep: true
Likely: add the following to your
- say-0.1.0.0 - typed-process-0.1.0.0
stack build yesod-bin
stack exec -- yesod devel
Use whichever method you feel most comfortable with. Please let me
know both successes and failures, and then I'll try to get this rolled
out. Comments would be great
on the Github pull request.
So far, in my limited testing, I've found that the new
runs faster than the current one, but that could very much be
confirmation bias speaking.
Note: there are a few removed features in this update, please see the changelog.
How it works
The big change - as the branch name implies - was depending entirely
on Stack for all of the heavy lifting. Stack already provides a
--file-watch command to automatically recompile, and uses GHC's own
addDependentFile information to track external file
dependencies. This cuts out the vast majority of the
complexity. There's no longer any need to depend on the
Cabal library code involved (making cross-version
support much simpler), and almost everything is handled by shelling
out to external executables.
I also got to redo the concurrency aspects of this using my absolute favorite package in the world: async. The result is, in my opinion, very straightforward. I also leveraged some of the newer libraries I've worked on, like safe-exceptions, typed-process, and say.
The code is (finally) well commented, so you can
jump in and look yourself. I've
a decent README,
an example of using
yesod devel with a non-Yesod project.