first run
[1 of 4] Compiling B                ( B.hs, B.o )
[2 of 4] Compiling C                ( C.hs, C.o )
[3 of 4] Compiling Main             ( Main.hs, Main.o )
[4 of 4] Linking Main
5
[1 of 1] Compiling B                ( B.hs, nothing ) [Source file changed]
second run
[1 of 4] Compiling B                ( B.hs, B.o ) [Missing object file]
[4 of 4] Linking Main [Objects changed]
15