Multi-line commands in GHCi

Most of the time, you can rely on type inference to work out a signature for you. In your example, the following is sufficient:

Prelude> let addTwo x y = x + y

If you really want a definition with a type signature, or your definition spans over multiple lines, you can do this in ghci:

Prelude> :{
Prelude| let addTwo :: Int -> Int -> Int
Prelude|     addTwo x y = x + y 
Prelude| :}
Prelude> addTwo 4 7
11

Note that you can also squeeze this onto one line:

Prelude> let addTwo :: Int -> Int -> Int ; addTwo x y = x + y

You can find out more about interacting with ghci on the Interactive evaluation at the prompt section of the documentation.