How to provide explicit type declarations for functions when using GHCi?

Note that you can also avoid the monomorphism restriction simply by adding "points" (i.e. explicit variables) back to your expression. So this also gives the correct type:

let numUniques x = length . nub $ x

The GHC User's Guide shows two additional ways to achieve this. This subsection introduces the :{ ... :} construct, which can be used as follows:

> :{
| numUniques :: (Eq a) => [a] -> Int
| numUniques = length . nub
| :}

Alternatively, you can enable multiline mode:

> :set +m
> let
| numUniques :: (Eq a) => [a] -> Int
| numUniques = length . nub
| 

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.