Main.hs is now:
main = putStrLn "Hello from A"
Compiling and running Main.hs:
[1 of 2] Compiling Main             ( Main.hs, Main.o )
[2 of 2] Linking Main
Hello from A
Main.hs is now:
main = putStrLn "Hello from B"
Compiling and running Main.hs:
[1 of 2] Compiling Main             ( Main.hs, Main.o ) [Source file changed]
[2 of 2] Linking Main [Objects changed]
Hello from B
Touching Main.hs
Compiling and running Main.hs:
Hello from B