The following provides a string → string preprocessor for Fortran programs that deals with label-delimited do-continue blocks of FORTRAN 77 era. With a traditional LR(1) parser, these are not easily (or not at all) parsable. Consider the valid FORTRAN 77 code:

    do j = 1,5
      do 17 i=1,10
        print *,i
      17 continue
    end do

Here the 'continue' acts as an 'end do' (not as a usual 'continue' statement) because it is labelled with the same label '17' as the 'do' statement which starts the block. Parsing this requires arbitrary look-ahead (e.g., LR(infinity)) which is provided by the following parsec parser, but not by the 'happy' parser generator.

This pre processor is currently quite heavy handed. It replaces 'continue' in the above program with 'end do'. E.g., the above program is transformed to:

    do j = 1,5
      do 17 i=1,10
        print *,i
      17 end do
    end do