Homework 5

Download the skeleton files Deque.elm, LazyListPlus.elm, and EnumerateRB.elm, and use them as a starting point for the following problems. Look for all occurrences of TODO in comments, which point out where you should implement your solutions. Once you are done, follow the submission instructions below.

Problem 1: Deques

6.1.1 --- Okasaki, Exercise 5.1a (50 points)

A double-ended queue, or deque (pronounced "deck"), allows adding and removing elements from both ends of the data structure. In this problem, you will adapt the FastQueue strategy from Chapter 5 to implement the Deque abstraction. In particular, use the representation

type Deque a = D { front : List a, back : List a }

and maintain the invariant that front and back are both non-empty whenever there are at least two elements in the Deque.

Implement the following three operations (which we referred to in the Queue context as enqueue, dequeue, and peek, respectively):

addBack     : a -> Deque a -> Deque a
removeFront : Deque a -> Maybe (Deque a)
peekFront   : Deque a -> Maybe a

In addition, implement the following three analogous operators:

addFront    : a -> Deque a -> Deque a
removeBack  : Deque a -> Maybe (Deque a)
peekBack    : Deque a -> Maybe a

Implement and use a helper function

check : List a -> List a -> Deque a

that enforces the invariant by checking whether either list is empty and, if so, splitting the other in half and reversing one of the halves.

All operations should run in O(1) amortized time assuming, as in Chapter 5, that values are not used persistently (but you are not asked to prove it).

Hint: The frontmost and backmost element of a Deque may be the same.

Problem 2: Lazy Lists

In this problem, you will implement LazyList analogues of a couple more standard List functions. You will need to download LazyList.elm and elm-package.json and place them in the directory where you are working.

6.2.1 (50 points)

First, implement the following functions.

map    : (a -> b) -> LazyList a -> LazyList b
concat : LazyList (LazyList a) -> LazyList a

Your implementations should be as lazy as possible. In particular, for any f and xs, the expression map f xs should evaluate very quickly even if, for example, head (map f xs) does not.

Next, use map and concat to define the following.

concatMap : (a -> LazyList b) -> LazyList a -> LazyList b

Finally, implement the following function that computes the Cartesian product of two LazyLists and transforms each its pairs using the input function.

cartProdWith : (a -> b -> c) -> LazyList a -> LazyList b -> LazyList c

Problem 3 (OPTIONAL): Enumerating Red-Black Trees

In Homework 2, you defined functions that compute all trees of a certain height. In this problem, you will compute all valid red-black trees of a given black-height.

We will use the same representation for RedBlackTrees that store Ints from class:

type Color = R | B
type Tree  = E | T Color Tree Int Tree

6.3.1 (0 points)

Write a function

rbTrees : Int -> List Tree

such that rbTrees bh returns all of the Trees t (with the dummy data value 0 stored in each node) such that check bh == True, where check is defined as follows.

check bh t =
  color t == B && noRedRed t && blackHeight t == Just bh

Notice that compared to the rb predicate from class, here we drop the bso requirement because we are only concerned with generating trees that satisfy the color and black-height invariants.

You may want to define the following helper function.

cartProdWith : (a -> b -> c) -> List a -> List b -> List c

As a sanity check:

> import EnumerateRB exposing (..)

> rbTrees 0
[E] : List EnumerateRB.Tree

> rbTrees 1
[T B E 0 E] : List EnumerateRB.Tree

> List.map (List.length << rbTrees) [0..3]
[1,1,4,400] : List Int

> List.map (\i -> List.all (check i) (rbTrees i)) [0..3]
[True,True,True,True] : List Bool


> rbTrees 4
FATAL ERROR: JS Allocation failed - process out of memory

6.3.2 (0 points)

Since these Lists get very large very quickly, define the following version that generates valid red-black trees lazily; you will want to use your LazyListPlus functions from the previous problem.

rbTrees' : Int -> LazyList Tree

Things should be a bit better when you're done:

> import LazyList exposing (..)

> rbTrees' 4
Lazy <function> : Lazy.Lazy (LazyList.LazyListCell EnumerateRB.Tree)

> rbTrees' 4 |> take 10 |> toList |> List.all (check 4)
True : Bool

> rbTrees' 4 |> take 1000 |> toList |> List.all (check 4)
True : Bool

> rbTrees' 5 |> take 1000 |> toList |> List.all (check 5)
True : Bool

Grading and Submission Instructions

Start by navigating to the folder where you checked out your repo. Next, create a subfolder for this assignment and populate it with the skeleton code:

% svn mkdir hw5
% cd hw5
% wget http://www.classes.cs.uchicago.edu/current/22300-1/assignments/hw5/Deque.elm
% wget http://www.classes.cs.uchicago.edu/current/22300-1/assignments/hw5/LazyListPlus.elm
% wget http://www.classes.cs.uchicago.edu/current/22300-1/assignments/hw5/EnumerateRB.elm
% wget http://www.classes.cs.uchicago.edu/current/22300-1/assignments/hw5/elm-package.json
% wget http://www.classes.cs.uchicago.edu/current/22300-1/lectures/Laziness/LazyList.elm

If wget or a similar tool (such as curl) is not available on your machine, download and save the skeleton files provided above in some other way. Then add only these files to your repo:

% svn add Deque.elm
% svn add LazyListPlus.elm

Make sure you choose the same exact names for directories and files that you create. Once you are ready to submit:

% svn commit -m "hw5 submission"

You can resubmit as many times as you wish, and we will grade the most recent versions submitted. Late days, if any, will be computed based on your submission times.

As a sanity check, you can visit the Web-based frontend for your repository to make sure that you have submitted your files as intended: