Yeah, really great approach!

Nice and concise! You can even leave out the null check which is quite ugly by just stating "this?.second == x - 1". :-)

Nice kata but shouldn't test consider empty array case?
Or maybe add "non empty" to description?

Wow! Looks like autogenerated code. What a shame!

Haskell tests are wrong because they violates promise given in kata specification.
Spec: you will always receive a valid array containing a random assortment of direction letters ('n', 's', 'e', or 'w' only).

Test:
it "should reject infinite lists" $ do
property $ forAll infiniteList $ \walk ->
isValidWalk walk ??! "this infinite walk should have been rejected"

So test generates infinite lists of arbitrary Char instead of direction letters one.
Please fix either test or specification.