Alex and new Bool primops

[Jan Stolarek]

I am refactoring new Bool primops (#6135). There was a discussion on core-libraries-commitee list and the decision was made that we should keep names of primops we used so far and change their type signature (right now HEAD has different names for new primops and reuses old names for compatibility wrappers). I've changed names of the primops and removed the wrappers but I can't bootstrap GHC because old primops are hardwired into Alex. I get this build error when attempting to build stage 2 compiler:

compiler/stage2/build/Lexer.hs:2348:34:
    Couldn't match expected type ‛Bool’ with actual type ‛Int#’
    In the first argument of ‛(&&)’, namely ‛(offset >=# 0#)’
    In the expression: (offset >=# 0#) && (check ==# ord_c)
    In the expression:
      if (offset >=# 0#) && (check ==# ord_c) then
          alexIndexInt16OffAddr alex_table offset
      else
          alexIndexInt16OffAddr alex_deflt s

compiler/stage2/build/Lexer.hs:2348:53:
    Couldn't match expected type ‛Bool’ with actual type ‛Int#’
    In the second argument of ‛(&&)’, namely ‛(check ==# ord_c)’
    In the expression: (offset >=# 0#) && (check ==# ord_c)
    In the expression:
      if (offset >=# 0#) && (check ==# ord_c) then
          alexIndexInt16OffAddr alex_table offset
      else
          alexIndexInt16OffAddr alex_deflt s

And indeed, looking at Lexer.hs I see:

alex_scan_tkn user orig_input len input s last_acc =
 (...) -- some irrelevant code here
                (!(new_s)) = if (offset >=# 0#) && (check ==# ord_c)
                          then alexIndexInt16OffAddr alex_table offset
                          else alexIndexInt16OffAddr alex_deflt s

So to compile GHC, Alex would need to generate different code for GHC 7.6.3 and below, and different for GHC 7.7 and above (or alternatively it could generate #if pragmas), but this means we'd need to update Alex. I created compatibility module within GHC called ExtsCompat46 and tried adding it to list of imported modules, but in generated Lexer.hs Alex adds a bunch of other modules among which is GHC.Exts, which conflicts with import of ExtsCompat46:

compiler/stage1/build/Lexer.hs:2349:41:
    Ambiguous occurrence `>=#'
    It could refer to either `ExtsCompat46.>=#',
                             imported from `ExtsCompat46' at compiler/parser/Lexer.x:79:1-19
                          or `GHC.Exts.>=#',
                             imported from `GHC.Exts' at compiler/stage1/build/Lexer.hs:75:1-15
                             (and originally defined in `ghc-prim:GHC.Prim')

Again, we would need updated version of Alex to generate correct code (also, this would break in the future after removing that compatibility module). I don't see a way out of this. Help?

Janek