Process the -parameter flag

[lukemaurer]

The flags for parameterized modules are already being parsed, but -parameter is similar to the load path in that it needs to be set up before typechecking begins. Without this feature, editing any module in a parameterized library gives an error for every reference to a parameter or to another parameterized module.

This feature sets up the parameters at the same time the load path is initialized, which is also when the compiler does it. The parameters are kept in Env.persistent_env, which is already handled by the Local_store mechanism, so they should stay in sync with all the other bits of global state lying around.

[liam923]

I think this needs to get rebased for the tests to pass on CI

[lukemaurer]

I think this needs to get rebased for the tests to pass on CI

Yeah, I was expecting the tests to merge before being run? Anyway, they should pass now

[liam923]

Thanks for the easy-to-read tests.

I'd like to see a couple tests that demonstrate that Merlin does the right thing when files/configuations change. Maybe:

• The parameter changes (ie, p.mli is edited and then recompiled)
• The set of parameters change (ie, start passing -parameter Q to a file that had been using -parameter P)

I'd also like to see a test where the -parameter flag is passed via a .merlin file. .merlin files are how we pass configuration to Merlin in our build system. To do this, you'll want to put a file named .merlin in the same directory as the .ml file. The contents should probably be:

S .
B .
FLG -parameter P

And then you won't need to pass -parameter on the command line. Note that you can create the .merlin file in the test (and then delete it) so that it isn't sitting around for all the tests that explicitly pass the flag.