Currently, hsc2hs (as shipped with GHC) cannot be used with just hsc2hs Foo.hsc as it cannot find HsFFI.h
The hsc2hs repo includes a shell script (yes, I know, no good on Windows) called hsc2hs.wrapper that already adds some default arguments. (nhc98 has a modified version of the script, adding a -I$(includedir))
Another option would be for the user to tell hsc2hs which compiler they're using, e.g. hsc2hs --compiler=/usr/bin/ghc Foo.hsc
On my system, hsc2hs is already installed as hsc2hs-ghc, in addition to the plain hsc2hs, so if I want the compiler-specific tool, I can use it directly. (nhc98 should do the same thing, i.e. install hsc2hs-nhc98, but I believe for historical reasons, it simply avoids installing hsc2hs at all, to avoid conflicts with the ghc one.) Regards, Malcolm