Gabriel Scherer announced an experiment to host OCaml compiler pull requests on GitHub for six months. There is a general feeling that GitHub would be a more modern hosting platform than the venerable but reliable Mantis setup that has in place for over a decade, but the only way to find out for sure is by trying it out for a while.
One of the great benefits of using GitHub is their excellent API to easily automate workflows around issues and pull requests. After a suggestion from Jeremy Yallop and David Sheets over lunch, I decided to use this to make it easier to locally apply compiler patches. OPAM has a great compiler switch feature that lets you run simultaneous OCaml installations and swap between them easily. For instance, the default setting gives you access to:
$ opam switch
system C system System compiler (4.01.0)
-- -- 3.11.2 Official 3.11.2 release
-- -- 3.12.1 Official 3.12.1 release
-- -- 4.00.0 Official 4.00.0 release
-- -- 4.00.1 Official 4.00.1 release
-- -- 4.01.0 Official 4.01.0 release
-- -- 4.01.0beta1 Beta1 release of 4.01.0
I used my GitHub API bindings to
knock up a script that converts every GitHub pull request into a custom
compiler switch. You can see these by passing the --all option to
opam switch, as follows:
$ opam switch --all
-- -- 4.02.0dev+pr10 Add String.{split,rsplit}
-- -- 4.02.0dev+pr13 Add String.{cut,rcut}.
-- -- 4.02.0dev+pr14 Add absolute directory names to bytecode format for ocamldebug to use
-- -- 4.02.0dev+pr15 replace String.blit by String.unsafe_blit
-- -- 4.02.0dev+pr17 Cmm arithmetic optimisations
-- -- 4.02.0dev+pr18 Patch for issue 5584
-- -- 4.02.0dev+pr2 Parse -.x**2. (unary -.) as -.(x**2.). Fix PR#3414
-- -- 4.02.0dev+pr20 OCamlbuild: Fix the check of ocamlfind
-- -- 4.02.0dev+pr3 Extend record punning to allow destructuring.
-- -- 4.02.0dev+pr4 Fix for PR#4832 (Filling bigarrays may block out runtime)
-- -- 4.02.0dev+pr6 Warn user when a type variable in a type constraint has been instantiated.
-- -- 4.02.0dev+pr7 Extend ocamllex with actions before refilling
-- -- 4.02.0dev+pr8 Adds a .gitignore to ignore all generated files during `make world.opt'
-- -- 4.02.0dev+pr9 FreeBSD 10 uses clang by default, with gcc not available by default
-- -- 4.02.0dev+trunk latest trunk snapshot
Testing the impact of a particular compiler switch is now pretty straightforward. If you want to play with Stephen Dolan’s optimized arithmetic operations, for instance, you just need to do:
$ opam switch 4.02.0dev+pr17
$ eval `opam config env`
And your local environment now points to the patched OCaml compiler. For
the curious, the scripts to generate the OPAM pull requests are in my
avsm/opam-sync-github-prs
repository. It contains an example of how to query active pull requests,
and also to create a new cross-repository pull request (using the git
jar binary from my GitHub
bindings). The scripts run daily for now, and delete switches once the
corresponding pull request is closed. Just run opam update to retrieve
the latest switch set from the upstream OPAM package
repository.