Apparently in going to Mingw 64 the packages "system-fileio" and "unix-compat" no longer compiled because some Windows routines were not available in their expected "names". I reported the issues and John Milikin quickly patched "system-fileio" (thanks John, if you're listening). The unix-compat issue is still outstanding (not sure if it's still being maintained), but I was able to patch it by adding underscores to elements like "snprintf". ( _snprintf works, _stat instead of stat, etc.) These were the same types of changes that worked for system-fileio. Note that this approach may break Win32. The next issue is when linking with yaml (ghc.exe: panic!). This was reported as a GHC bug 2 weeks ago (#9080) by maun, who was trying to build GHCJS. I get the exact same error in building Yesod. Because of Yesod, I think #9080 should be promoted to high priority, and it would be great if someone could bring unix-compat up to date. Thanks Stuart