Ruby 3.0 should be enabled, see this to learn how to do it

Please organize the structure of test fixture as following (although it has been mentioned in the attached link, I'm repeated here again)

describe "<This message should describe the categories of test groups inside this block>" do
  it "<A short message describing this test group>" do
    expect(...).to eq(...) #Assertions
    #Or Test.assert_equals(user_response, reference_response)
  end
end

Julia translation

If there isn't one already, why not add a test case to see whether or not a solution correctly returns a new array, per the instructions? A few of the Java solutions I saw were destructive.

haskell still lacking its random tests

Add random tests:

import Test.QuickCheck
import Test.Hspec

main :: IO ()
main = hspec $ do
  describe "merge" $
    it "should merge basic examples" $ do
      merge [2,2,0,0] `shouldBe` [4,0,0,0]
      
    it "should also shift" $ do
      -- ...  
    
    it "should only merge once per cell" $ do
      -- ...
    
    it "should work for other list sizes" $ do
      -- ...
    
    it "should work for random lists" $ property $ \xs ->
      merge xs `shouldBe` solution xs
      
    it "should work for random (mergeable) lists" $ property $ 
      forAll (listOf1 $ elements [1,2,4,8]) \xs ->
        merge xs `shouldBe` solution xs
  
  where 
    solution = -- your solution here.