Multi-line commands in GHCi

I am having problem in entering multi-line commands in ghci.

The following 2-line code works from a file:

addTwo :: Int -> Int -> Int
addTwo x y = x + y

But when I enter in ghci, I get an error:

<interactive>:1:1: error:
    Variable not in scope: addTwo :: Int -> Int -> Int

I also tried putting the code inside :{ ... :}, but they are also not working for this example, because this is just appending the lines into one line, which should not be the case.

I am using WinGHCi, version 2011.2.0.1

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.

Solve this problem by firing up GHCI and typing :set +m:

Prelude> :set +m
Prelude> let addTwo :: Int -> Int -> Int
Prelude|     addTwo x y = x + y
Prelude| 
Prelude> addTwo 1 3
4

Boom.

What's going on here (and I'm talking mostly to you, person googling for help while working your way through Learn You A Haskell) is that GHCI is an interactive environment where you're changing bindings of function names on the fly. You have to wrap your function definitions in a let block, so that Haskell knows that you're about to define something. The :set +m stuff is shorthand for the multiline :{ code :} construct.

Whitespace is also significant in blocks, so you have to indent your function definition after your type definition by four spaces to account for the four spaces in let.