Haskell Program to subtract two numbers

Program

main = do
 let a = 20
 let b = 10
 putStrLn "Difference of two numbers is:"
 print (a - b)

Output

$ ghc subtract_two_numbers.hs 
[1 of 1] Compiling Main             ( subtract_two_numbers.hs, subtract_two_numbers.o )
Linking subtract_two_numbers ...
$ ./subtract_two_numbers 
Difference of two numbers is:
10