bash-2.03$ cat old infix 5 \\ bash-2.03$ cat -vet old infix 5 \\ $ $ bash-2.03$ cat new infix 5 \\ -- bash-2.03$ cpp-2.95 old # 1 "old" infix 5 \\ bash-2.03$ cpp-2.95 new # 1 "new" infix 5 \\ -- bash-2.03$ cpp-3.4.1 old # 1 "old" # 1 "<built-in>" # 1 "<command line>" # 1 "old" old:1:10: warning: backslash and newline separated by space infix 5 \ bash-2.03$ cpp-3.4.1 new # 1 "new" # 1 "<built-in>" # 1 "<command line>" # 1 "new" infix 5 \\ -- bash-2.03$ The new behavior is documented in cpp.info of the gcc-3.4.1 distribution. Line 212 of hugs98-Nov2003/fptools/libraries/base/Data/List.hs expects the old behavior.