1) Show all the functions (when the number is low), but place platform specific functions under separate headers: "Windows", "Linux/BSD/POSIX", "OS X", etc.
If a function isn't available on all OS's then all Hoogle would be encouraging you to do is break compatibility and stop me from using your software.
How about showing all functions, but indicating available platforms? That way, we won't go looking for functions that work on "my platform", but for functions that work on the most platforms. And if we're looking at a function that isn't portable, we'll know immediately - unless I'm explicitly in one-shot/one-platform mode, portability (or lack of) is rather important information for me. It would be ok to filter that info in some way (character codes, or green symbol if it is available on major platforms, red otherwise, with more detailed info at package level). One of the most appealing things about Haskell, usually more common with scripting languages, is that it is possible to write portable code in it. So we don't have to make extra efforts to "code for Windows" or "code for Unix" - we just have to make the small effort to ensure that we are not coding for any particular platform. The less platform-specific code, the less work when a piece of software starts being successful. It would be very nice to have this info at the package/hackage level, too, including transitive dependency chasing. That way, package authors will not accidentally depend on packages that have non-portable dependencies, but have the chance to prefere portable alternatives whenever possible (unix vs unix-compat, etc). And package users will see where "cross-platform" was sacrificed. Claus