Prerequisites in FLOLAC 2018

prerequisites.hs

-- Answers for Prerequisites in FLOLAC 2018

-- 1) myFst

myFst :: (a, b) -> a
myFst (x, _) = x

-- 2) myOdd

myOdd :: Int -> Bool
myOdd n = (mod n 2) /= 0

-- 3) Answers:
{-
  (a) Ord is a class of type where any two value in the type can be compared and get result
      as one of {LT, EQ, GT}, i.e. the type is totally ordered.
      
      qs :: Ord a => [a] -> [a]      
      Mean for all type a which is of class Ord, i.e. equipped with a totally ordered relation 
      for all of its elements. Function qs can accept a list of type a and produce another list
      of type a.
      
  (b) The type of (++) is 
      (++) :: [a] -> [a] -> [a] 
      -- i.e. take two lists of same type a, produce a combined list of same type a.
      But due to type declaration of qs and type inference by Haskell, the type become
      (++) :: Ord a => [a] -> [a] -> [a]
      
      The functionality of (++) is to append two list and maintain orders of all elements respectively.
      
  (c) Elements of ys are those elements in xs which are less or equal (<=) to x.
      Elements of zs are those elements in xs which are greater than x.  
      
  (d) The function qs is an implemetation of quick sort. 
      Where qs pick one element (the head of the list) then split the tail into two lists where one contains
      less-or-equal elements and the other contains strictly greater ones, the sort two listd recursively.

  (e) Please see following code
-}

qs :: Ord a => [a] -> [a] 
qs list = case list of 
            []  ->  []
            (x:xs) -> let 
                        ys = [ y | y <- xs, y <= x]
                        zs = [ z | z <- xs, x < z]
                      in
                        qs ys ++ [x] ++ qs zs

Well done.