i thnk than the secondtest is wrong, in the set of minor words say "The In" should will be"the in"!?, whatever Anyway, with all the cases that I tried it works but when I want to send it I get an error but I don't know which is the test that makes my code fail

Personally, I had the basic idea right away... stacking partially applied functions and using something like id as the initial value, but I couldn't get the form right and resorted to looking it up in the wiki. I guess I cheated, but I plan to come back to this one in the future and see if I can get it without peeking.

I didn't build the Haskell vesion of this kata. I will attempt to reach out to the creator since Haskell is very literally a different language to me.

In the Haskell version of the problem, you use both Int and Integer:

countBy :: Integer -> Int -> [Integer]

I'm not sure what "types" you are referring to. Can you be more specific?

Why mix Int and Integer in the types? I think they should both be the same.

Here's a test case that uses random integer lists. (I forget the syntax to make it work for any list of Eq.)

module RemoveDuplicates.Test where
import Data.List (nub)
import RemoveDuplicates (unique)
import Test.Hspec

main = hspec $ do
  describe "Duplicates removed from array" $ do
    it "should work for random arrays" $
      property $ \a -> unique a == (nub a :: [Int])

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Not new, it's in base

http://hackage.haskell.org/package/base-4.7.0.1/docs/Data-Monoid.html#v:-60--62-

Hayoo search:

http://hayoo.fh-wedel.de/?query=%28%3C%3E%29

I have since added a new Kata: Hughes' List. This uses a technique very similar to the one required here and may be a little bit easier to solve than this one.

It'd help a lot if you could explain that <> is a new synonym for mappend (I don't think it was originally the case, right?)

I searched for <> with Hoogle, and it couldn't find this synonym:

http://www.haskell.org/hoogle/?hoogle=%3C%3E

It could use a handful of tests.

It's an infix synonym: http://hackage.haskell.org/package/base-4.7.0.1/docs/Data-Monoid.html#v:-60--62-

You can search the haskell docs with Hayoo!
http://hayoo.fh-wedel.de/

Yes.