Multiline commands in GHCi
Use let
:
Prelude> :{
Prelude let addTwo :: Int > Int > Int
Prelude addTwo x y = x + y
Prelude :}
Prelude> addTwo 2 3
5
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
.
As of GHCI version 8.0.1, let
is no longer required to define functions on the REPL.
So this should work fine for you:
λ: addTwo x y = x + y
λ: addTwo 1 2
3
λ: :t addTwo
addTwo :: Num a => a > a > a
Haskell's typeinference provides generalized typing that works for floats as well:
λ: addTwo 2.0 1.0
3.0
If you must provide your own typing, it seems you'll need to use let
combined with multiline input (use :set +m
to enable multiline input in GHCI):
λ: let addTwo :: Int > Int > Int
 addTwo x y = x + y

λ: addTwo 1 2
3
But you'll get errors if you try to pass anything but an Int
because of your nonpolymorphic typing:
λ: addTwo 2.0 1.0
<interactive>:34:8: error:
• No instance for (Fractional Int) arising from the literal ‘2.0’
• In the first argument of ‘addTwo’, namely ‘2.0’
In the expression: addTwo 2.0 1.0
In an equation for ‘it’: it = addTwo 2.0 1.0
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.