Just as an addendum, this just happened again, but I got new errors this time. Make gave the usual: Linking build/test/RunTests ... Undefined symbols: "_UiziTrackC_d28LF", referenced from: _s28UI_info in TrackC.o "_UiziRulerC_dk8C", referenced from: _skev_info in RulerC.o ld: symbol(s) not found collect2: ld returned 1 exit status But then reloading that same module in ghci gave a new one:
:r [ 23 of 124] Compiling Ui.UiMsg ( Ui/UiMsg.hs, interpreted )
Ui/UiMsg.hs:51:31: Can't find interface-file declaration for type constructor or class Key.Modifier Probable cause: bug in .hi-boot file, or inconsistent .hi file Use -ddump-if-trace to get an idea of which file caused the error In the type `[Key.Modifier]' In the definition of data constructor `Mouse' In the data type declaration for `Data' The odd thing is that after restarting ghci and loading there was no problem, so I didn't get a chance to try its -ddump-if-trace suggestion. In this case before the reload I had added an instance declaration to a type in the Key module, though oddly it wasn't the referenced Modifier type. Removing the referenced .o files and recompiling was enough to fix the link error.