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

Question

How to I define the equivalent of this function (taken from learnyouahaskell) inside GHCi?

import Data.List  

numUniques :: (Eq a) => [a] -> Int  
num         


        

Answer 1

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

Answer 2

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.

Answer 3

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
|