Haskell assignment type

Question

From the haskell.org website: every expression has a type. So what is the type of

main = putStrLn "Hello, World!"

I would like to know not what the type of main is, but what the type of the whole line is. Also, is it possible to get this type somehow in ghci? I tried

:t (main = putStrLn "Hello, World!")

but this does not work.

Answer 1

main = putStrLn "Hello, World!" is not an expression, and thus does not have a type. It's a definition, which assigns a value of type IO () (resulting from the evaluation of putStrLn :: String -> IO ()) to the name main.

Following the definition, you can find the type of main:

> :t main
main :: IO ()