We have not reached consensus on this issue yet. Based on informal chats with interested people, here is another concrete proposal: Instead of adding isWindows, isBeOS, isNixOS etc we have a simple enumeration of the major OS flavour: data OSFlavour = Linux | Windows | MacOS | BSD | Solaris | Other String osFlavour :: OSFlavour So no "subsystem" as fields of the OS constructors. This keeps it simple and covers many use cases. Adding more specific feature tests would probably cover most of the other use cases where we need more information than we can divine from the OS kind. So if people want isWindows or isUnixish predicates they can implement them easily in terms of this enumeration. This avoids us having to centrally agree an interpretation of what isUnixish might mean (so no arguments about whether ghc built to target cygwin is sufficiently unixish). If people feel it is really necessary, a separate build platform string could be added. This would be the system/platform/toolchain that the compiler targets: compilerBuild :: String eg for GHC on windows this would be "mingw32", "cygwin" or whatever. But I wonder if that's really necessary and if whatever people are testing on the basis of the build flavour couldn't be decided better with more specific feature tests. So I'd advocate a simple coarse OS classification and additional specific feature tests. Duncan