Assignment 1 – Smooth permutations

Before you begin

We are using GitHub Classrooms for this assignment. Before you begin with the assignment, please link your GitHub account to the AFP classroom and create a repo for this assignment. You can do so by following this link

The deadline for the assignment is 2023-02-15 @ 23:59.

After the assignment deadline you will be assigned two peers to review. The deadline for the peer reviews is 2023-02-22 @ 23:59. More information about how to conduct the peer review can be found here.


In this assignment we want to build a library to generate smooth permutations. Given a list of integers xs and an integer d, a smooth permutation of xs with maximum distance d is a permutation in which the difference of any two consecutive elements is less than d.

A naïve implementation just generates all the permutations of a list,

split []     = []
split (x:xs) = (x, xs) : [(y, x:ys) | (y, ys) <- split xs]

perms []     = [[]]
perms xs     = [(v:p) | (v, vs) <- split xs, p <- perms vs]

and then filters out those which are smooth,

smooth n (x:y:ys) = abs (y - x) < n && smooth n (y:ys)
smooth _ _        = True

smoothPerms :: Int -> [Int] -> [[Int]]
smoothPerms n xs = filter (smooth n) (perms xs)

Exercise 1 – Packaging and documentation (1 pt)

  1. Create a library smoothies which exports perms and smoothPerms from a module SmoothPermsSlow. You should be able to install the package by just running cabal install in it.
  2. Document the exported functions using Haddock.

Exercise 2 – Testsuite (1 pt)

  1. Write a SmoothPermsTest module with a comprehensive set of properties to check that smoothPerms works correctly.
  2. Integrate your testsuite with Cabal using tasty (here is how you do so).

Exercise 3 – Implementation with trees (3 pt)

The initial implementation of smoothPerms is very expensive. A better approach is to build a tree, for which it holds that each path from the root to a leaf corresponds to one of the possible permutations, next prune this tree such that only smooth paths are represented, and finally use this tree to generate all the smooth permutations from. Expose this new implementation in a new SmoothPermsTree module.

  1. Define a data type PermTree to represented a permutation tree.
  2. Define a function listToPermTree which maps a list onto this tree.
  3. Define a function permTreeToPerms which generates all permutations represented by a tree.

    At this point the perms functions given above should be the composition of listToPermTree and permTreeToPerms.

  4. Define a function pruneSmooth, which leaves only smooth permutations in the tree.
  5. Redefine the function smoothPerms.

Integrate this module in the testsuite you developed in the previous exercise.

Exercise 4 – Unfolds (3 pts)

Recall the definition of unfoldr for lists,

unfoldr :: (s -> Maybe (a, s)) -> s -> [a]
unfoldr next x = case next x of
                   Nothing     -> []
                   Just (y, r) -> y : unfoldr next r

We can define an unfold function for binary trees as well:

data Tree a = Leaf a | Node (Tree a) (Tree a)
            deriving Show

unfoldTree :: (s -> Either a (s, s)) -> s -> Tree a
unfoldTree next x = case next x of
                      Left  y      -> Leaf y
                      Right (l, r) -> Node (unfoldTree next l) (unfoldTree next r)

Define the following functions in a new module UnfoldUtils, which should not be exposed by your package. Define the functions using unfoldr or unfoldTree, as required.

  1. iterate :: (a -> a) -> a -> [a]. The call iterate f x generates the infinite list [x, f x, f (f x), ...].
  2. map :: (a -> b) -> [a] -> [b].
  3. balanced :: Int -> Tree (), which generates a balanced binary tree of the given height.
  4. sized :: Int -> Tree Int, which generates any tree with the given number of nodes. Each leaf in the returned tree should have a unique label.

Define a new module SmoothPermsUnfold with an unfoldPermTree function which generates a PermTree as defined in the previous exercise. Then use that unfoldPermTree to implement a new version of listToPermTree and smoothPerms.

Recap of modules

By the end of exercise 4, you should have a package with the following modules:

Exercise 5 – Proofs (2 pts)

Write the following proofs as comments in the UnfoldUtils module.

  1. Prove using induction and equational reasoning that the version of map you defined using unfoldr coincides with the definition of map by recursion.
  2. We define the size of a binary tree as the number of internal nodes.

     size (Leaf _)   = 0
     size (Node l r) = 1 + size l + size r

    What is the size of a balanced tree as generated by balanced? Prove your result using induction and equational reasoning.