[Haskell] Strange "let"

Hi, I'm a Haskell newbie. I was trying several things with GHCi and found out that the expression "let a + b = 3" does not generate any errors. Prelude> let a + b = 3 Prelude> a <interactive>:1: Variable not in scope: `a' Prelude> b <interactive>:1: Variable not in scope: `b' Prelude> What does "let a + b = 3" mean in this case? I also tries with GHC compiler using the code below, but it generates no error. main :: IO () main = do let a + b = 3 putStrLn "Hello" Could somebody answer this? Jinwoo