first run
[1 of 3] Compiling A                ( A.hs, A.o )
[2 of 3] Compiling B                ( B.hs, B.o )
[3 of 3] Compiling C                ( C.hs, C.o )
second run
[2 of 3] Compiling B                ( B.hs, B.o )
[3 of 3] Compiling C                ( C.hs, C.o ) [B changed]