ia:
I'm looking for a way of specifying language extensions in a way which will work in all versions of GHC from 6.4 onwards.
GHC 6.4 does not support the LANGUAGE pragma. Specifying language options in the OPTIONS_GHC pragma starts to produce deprecation warnings in 6.10, and will presumably eventually fail altogether.
Any sort of preprocessing at the beginning of a file appears to insert enough noise to inhibit recognition of file-header pragmas. (Cpphs doesn't appear to help here.) Attempting preprocessing inside a leading OPTIONS_GHC pragma fails because it apparently attempts to interpret the pragma *before* invoking the preprocessor.
The only remaining option I can see is to have a completely separate version of the file for GHC 6.4. Have I missed anything?
Could you talk a bit about why you need to support 6.4 onwards -- that seems like quite a burden. Is the effort worth it?