Friends I still cannot build GHC on Windows. On Ben's advice I dumped my current msys setup and re-installed msys64 as described here https://ghc.haskell.org/trac/ghc/wiki/Building/Preparation/Windows I cloned an entirely new GHC repository. Then 'sh validate'. Same result as before: any attempt to run GHCi fails with an unresolved symbol. bash$ c:/code/HEAD-1/inplace/bin/ghc-stage2 --interactive GHCi, version 7.11.20151026: http://www.haskell.org/ghc/ :? for help ghc-stage2.exe: unable to load package `ghc-prim-0.4.0.0' ghc-stage2.exe: C:\code\HEAD-1\libraries\ghc-prim\dist-install\build\HSghc-prim-0.4.0.0.o: unknown symbol `_stg_upd_frame_info' How could I actually find what the problem is? Trying random things and hoping the problem goes away clearly is not working. Very stuck and deeply frustrated Thanks Simon