I noticed that T1969 is failing again, and decided to do a little investigation. The maximum residency when compiling T1969 with HEAD compared with 7.6.3 looks like this: 7.6.3: 10,473,920 HEAD: 13,783,536 This is with +RTS -h -i0.01 to avoid sampling errors as much as possible. The figures are pretty accurate, and the heap profiles confirm it: we're using a lot more heap now. -ddump-if-trace shows that HEAD is reading more interface files: Renamer stats: 10 interfaces read 6 type/class/variable imported, out of 1361 read 0 instance decls imported, out of 105 read 0 rule decls imported, out of 53 read vs. with 7.6.3: Renamer stats: 8 interfaces read 2 type/class/variable imported, out of 828 read 0 instance decls imported, out of 40 read 0 rule decls imported, out of 45 read The extra interface files are Control.Applicative and Control.Monad. So the question is, why are we now reading these on *every* compilation? (T1969 imports only the Prelude). Presumably this is something to do with the AMP warnings, but can't we lazily load these interfaces when they're needed? Cheers, Simon