Haskell – Using ghci to find type

ghcihaskell

When I do something simple in ghci, like the following:

let x = 7 + 2

I expect ghci to give a response of the type that x holds, like:

x :: Integer

When I run ghci, I do not get that the above line. How do I get that response?

To show types automatically use :set +t:

μ> :set +t
μ> let x = 7 + 2
x :: Integer
μ>

GHCi now has a multiline-input mode, enabled with :set +m. For example,

Prelude> :set +m
Prelude> let fac 0 = 1
Prelude|     fac n = n * fac (n-1)
Prelude|
Prelude> fac 10
3628800

Is there a way provide type declarations in GHCi?

let numUniques' :: (Eq a) => [a] -> Int; numUniques' = length . nub

Or is there another way to define functions like these which doesn't require type declarations?

If you turn off the monomorphism restriction with -XNoMonomorphismRestriction, it will infer the right type.