1.Purs

newtype AtLeastTwoAndUnique a = AtLeastTwoAndUnique (Array a)

toArray :: forall a. AtLeastTwoAndUnique a -> Array a
toArray (AtLeastTwoAndUnique xs) = xs

first :: forall a. AtLeastTwoAndUnique a -> a
first = Unsafe.head

instance arbAtLeastTwoAndUnique :: (Arbitrary a) => Arbitrary (AtLeastTwoAndUnique a) where
  arbitrary = AtLeastTwoAndUnique <$> arbitrary `suchThat` (\xs -> unique xs && atLeastTwo xs)
    where
    unique xs = nub xs == xs
    atLeastTwo = (>= 2) <<< length

2.purs

newtype AtLeastTwoAndUnique a = AtLeastTwoAndUnique (Tuple (Tuple a a) (Array a))

toArray :: forall a. AtLeastTwoAndUnique a -> Array a
toArray (AtLeastTwoAndUnique (Tuple (Tuple x1 x2) xs)) = x1 : x2 : xs

first :: forall a. AtLeastTwoAndUnique a -> a
first (AtLeastTwoAndUnique (Tuple tuple _)) = fst tuple

instance arbAtLeastTwoAndUnique :: (Arbitrary a, Ord a, Show a) => Arbitrary (AtLeastTwoAndUnique a) where
  arbitrary = AtLeastTwoAndUnique <$> arbitrary `suchThat` unique
    where
    unique (Tuple (Tuple x1 x2) xs) = nub (x1 : x2 :xs) == (x1 : x2 : xs)