My project (which contains a variety of TH usages) builds correctly with a "Cabal install", but editing a number of modules will result in "build wrapper: phase 'C pre-processor' failed (exit code = 1)" errors.
These errors are flagged at the top of the editor (line 1) when the module source is scanned (e.g. when you type and pause or save a changed editor). The presence of the error effectively prevents all the normal syntax-driven editor functionality and all other warnings and errors. So, this is quite debilitating.
I can only reproduce when the project has not been built. I think this error comes if the files under build/autogen are not present. Is your workspace set to autobuild? If not, can you build the project in Eclipse and reopen the files in the editor to see if it's better? Here as soon as I build the project the error goes away.
This behaviour seems also to have changed since I updated last.
Now, if I create a new project and copy in a module that uses TH features, I get the error on line 1, but indeed once the project is rebuilt, then this problem goes away. This is much better. I can definitely live with this if the issue only occurs before the first build.
I'll be doing some more work in the project that previously manifested the continual reoccurrence of this issue when you made changers in editors for modules that imported the TH feature module. I'll report on the state of things over the next few days. It'll be easy to provide a .zip with a minimal project for analysis if this recurs.
Yes, there's not much we can do before the first build, since the TH compilation needs the header files generated by the build. I suppose in auto build mode you can't see the problem.
Argh. It's still happening and driving me batty. So far I'm drawing a blank on what the condition might be that seems to set it off. I'll have to spend more time with a isolated case to see if I can figure it out some more.