(Sent to glasgow-haskell-users rather than haskell, as it is GHC-specific.) If we were writing a C library rather than a Haskell library, we could make "head" a macro which included an appropriate message referring to __FILE__ and __LINE__. The equivalent in Glasgow Haskell would be to make head inline, and include a primHaskellFile (and if possible, though I doubt it, primHaskellLine) which got replaced at a late stage by the actual name of the current module. This would be useful for locating other error messages as well. For example I have a debug action which (if debugging is turned on) prints out messages to a file; if we had a primHaskellFile I could make this debug message automatically refer to the calling module, which would be nice.
