Haskell – How to enable language extensions from within GHCi

ghcihaskelllanguage-extension

I'm trying to enable XRankNTypes in GHCi. How do I do this?

:set -XRankNTypes. The -X part is a compiler switch, by the way, extension is called just RankNTypes.

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.