[Haskell] Strange "let"

[Jinwoo Lee]

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