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