FLOLAC '18 preparation answer

prep.hs

{- 1 -}
myFst :: (a, b) -> a
myFst x = fst x

{- 2 -}
myOdd :: Int -> Bool
myOdd x = mod x 2 == 1

{- 3 -}
-- (a) Ord is for types that have an ordering.
--
-- (b) (++) :: [a] -> [a] -> [a]
--     ++ concatenates two list.
--
-- (c) ys is a set of all elements smaller than x,
--     while zs is a set of all elements larger than x.
--
-- (d) qs is quicksort with first element as pivot.
--
-- (e)
qs' :: (Ord a) => [a] -> [a]
qs' xs = 
    case xs of 
        [] -> []
        (x:xs) -> 
            let ys = [y | y <- xs, y <= x]
                zs = [y | y <- xs, y > x]
            in
                qs' ys ++ [x] ++ qs' zs

Well done.

(a) Ord is for types that have an ordering.

To be precise, every instance of Ord has a total order provided by compare:: a -> a -> Ordering.

... smaller than x, ...

should be "... smaller than or equal to x..."