Haskell CS 421 LogoCS 421 — Programming Languages

MP 1 — Basic Haskell


We will be using Haskell throughout the course to implement other programming languages. That being said, the focus of this course is not on Haskell, but rather on studying programming languages in general. You need to understand basic Haskell before we can proceed with the rest of the course though; this MP will test that understanding.


  • Write recursive functions and definitions.
  • Implement some set-theoretic functionality using Haskell lists.
  • Use higher-order functions to compactly represent common code patterns.
  • Use and write Algebraic Data Types (ADTs) with some associated operators.

Useful Reading

If you are stuck on some problems, perhaps it’s time to read some of Learn You a Haskell for Great Good. I would recommend reading the whole book eventually, but if you’re crunched for time Chapter 3 on Types and Typeclasses, Chapter 4 on Syntax in Functions, and Chapter 8 on Making Your Own Types and Typeclases seem the most relevant. If you’re still stuck on recursion problems, check out Chapter 5 on Recursion.

Getting Started

Relevant Files

In the directory app you’ll find Main.hs with all the relevant code. The first line module Main where says that what follows (the rest of the file in this case) belongs to the Main module. Haskell has a module system which allows for extensive code re-use. Saying something like import Data.List imports the List module, for example.

module Main where

main :: IO ()
main = return ()

Running Code

First, you have to stack init (you only need to do this once):

$ stack init

To run your code, start GHCi with stack ghci:

$ stack ghci
 - more output -
Ok modules loaded: Main.

Testing Your Code

You can run the test-suite provided you have supplied type declarations for each function and have provided the Algebraic Data Types at the end of the problem-set correctly. To do so, just run stack test

$ stack test

It will tell you which test-suites you pass, fail, and have exceptions on. To see an individual test-suite (so you can run the tests yourself by hand to see where the failure happens), look in the file test/Tests.hs.

I Can’t Do Problem X

You can ask for help on Piazza or in office hours. If you’re really stuck and don’t have time to fix it, you must still put in the type declaration and define it as undefined so that it still compiles with our autograder. Not doing so will result in loss of extra points from your total score. Remember that course policy is that code which doesn’t compile receives a zero.

For example, if you cannot complete problem app :: [a] -> [a] -> [a], make sure you put the following code into your submission:

app :: [a] -> [a] -> [a]
app = undefined


Builtins: In general you cannot use the Haskell built-in functions. Especially if we say that a function “should behave exactly like the Haskell built-in”, you cannot use that Haskell built-in.

Pattern-matching: You should try to use pattern-matching whenever possible (it’s more efficient and easier to read). If you are using the functions fst :: (a,b) -> a or snd :: (a,b) -> b (for tuples) head :: [a] -> a or tail :: [a] -> [a] (for lists) chances are you can do it with pattern matching. We will take off points if you use built-ins when it’s possible to pattern-match.

Helpers: : You may write your own helper functions (inside or outside a where clause). All the restrictions about allowable code (no built-ins/using pattern matching) still apply to your helper functions.


For these problems, you may not use higher-order functions. Instead, you should use recursion to implement the stated functionality. If you do use higher-order functions (or do not use recursion), you will receive no points.


Write a function mytake :: Int -> [a] -> [a] which takes the first n elements of a list, or the whole list if there are not n elements. It should behave exactly like the Haskell built-in take :: Int -> [a] -> [a].

*Main> mytake 4 [2,4,56]
*Main> mytake 3 []
*Main> mytake 1 ["hello", "world"]
*Main> mytake (-3) [1,2,3]


Write a function mydrop :: Int -> [a] -> [a] which drops the first n elements of a list, or the whole list if there are not n elements. It should behave exactly like the Haskell built-in drop :: Int -> [a] -> [a].

*Main> mydrop 3 [2,4,56,7]
*Main> mydrop 3 []
*Main> mydrop 1 ["hello", "world"]
*Main> mydrop (-3) [1,2,3]


Write a function rev :: [a] -> [a] which reverses the input list. To get credit, your solution must run in linear time. If you use the (++) list append operator, chances are your solution is running in quadratic time. This function should behave exactly like the Haskell built-in reverse :: [a] -> [a].

*Main> rev [1,2,3]
*Main> rev []
*Main> rev ["hello", "world"]


Write a function app :: [a] -> [a] -> [a] which appends two lists. This function should behave like the Haskell built-in (++) :: [a] -> [a] -> [a], and should run in linear time (in the size of the first list).

*Main> app [] [1,2,3]
*Main> app [1,2,3] []
*Main> app [4,5] [1,2,3]
*Main> app ["hello", "world"] ["and", "goodbye"]
*Main> app "hello" "world"


Write a function inclist :: Num a => [a] -> [a] which adds 1 to each element of the input list.

*Main> inclist [1,2,3,4]
*Main> inclist [-2,4,5,1]
*Main> inclist []
*Main> inclist [2.3, 4.5, 7.6]


Write a function sumlist :: Num a => [a] -> a which adds all the elements of the input list.

*Main> sumlist []
*Main> sumlist [1,2,3]
*Main> sumlist [-3,2,5]
*Main> sumlist [3.3,2.8,-1.2]


Write a function myzip :: [a] -> [b] -> [(a,b)] which zips up the elements of two lists. The resulting list should be the same length as the shorter of the two input lists.

*Main> myzip [1,2,3] []
*Main> myzip [] [1,2,3]
*Main> myzip [1,2,3] ["hello", "world"]
[(1,"hello"), (2,"world")]


Write a function addpairs :: (Num a) => [a] -> [a] -> [a] which zips up two lists using the addition operator (+). You must use your function myzip.

*Main> addpairs [1,2,3] []
*Main> addpairs [1,2,3] [4,5,6]
*Main> addpairs [1.2,3.4] [-1.2,8.9,7.6]


Write a (constant) function ones :: [Integer] which produces an infinite list of the integer 1.

*Main> take 15 ones
*Main> take 0 ones


Write a (constant) function nats :: [Integer] which produces an infinite list of all the natural numbers starting at 0. It is OK for this function if you do not use recursion.

*Main> take 15 nats
*Main> take 0 nats


Write a (constant) function fib :: [Integer] which produces an infinite list of the Fibonacci series starting with numbers 0 and 1. You can (and should) use your addpairs function here. This is the one place in the assignment where it really makes sense to use tail :: [a] -> [a].

*Main> take 10 fib
*Main> take 0 fib

Set Theory

We will be using Haskell lists to represent abstract mathematical sets. Recall that in a set there are no duplicates, which means that our lists should have that property as well. To make this simpler, we’ll be storing sorted lists, which means we can only create sets of things that are orderable (notice the Ord a type constraint in the type declarations).

As long as your set-theoretic interface functions do not create duplicates and always return sorted lists, then all of our nice set-theoretic properties will hold. You may assume that the input sets (Haskell lists) to these functions will also be sorted and will not contain duplicates.

You may use higher-order functions or recursion as you see fit in this section.


Write a function add :: Ord a => a -> [a] -> [a] which will add an element to the set. Remember that it must add it in the correct place to ensure that the list remains sorted. It should run in linear time (in the size of the list).

*Main> add 3 []
*Main> add 3 [1,2]
*Main> add 3 [1,3,5]
*Main> add 3 [1,5,8,9]
*Main> add "hello" ["goodbye", "world"]
["goodbye", "hello", "world"]


Write a function union :: Ord a => [a] -> [a] -> [a] which unions two input sets (Haskell lists). This should look similar to the “merge” step of merge-sort, and should run in linear time (in the added sizes of the input sets). You may use the add function defined above, but if you do it probably will not run in linear time, so it’s probably better not to.

*Main> union [] []
*Main> union [1,2,3] []
*Main> union [] [1,2,3]
*Main> union [1,2,3] [1,2,3]
*Main> union ["goodbye", "world"] ["humans", "smell"]
["goodbye", "humans", "smell", "world"]


Write a function intersect :: Ord a => [a] -> [a] -> [a] which intersects two input sets. This should run in linear time (in the size of the input sets).

*Main> intersect [] [1,2,3]
*Main> intersect [1,2,3] []
*Main> intersect [1,2,3] [3,4,45,89]
*Main> intersect ["cruel", "hello", "world"] ["good", "hello", "world"]
["hello", "world"]


Write a function powerset :: Ord a => [a] -> [[a]] which calculates the powerset of the input set. Because the output is also a set, it must preserve our set properties, including that there are no duplicate elements and the elements are lexicographically sorted. Using the functions union and add that you’ve already defined is useful here.

*Main> powerset []
*Main> powerset [1,2]
*Main> powerset ["goodbye", "hello", "world"]
[ [], ["goodbye"], ["goodbye", "hello"], ["goodbye", "hello", "world"]
, ["goodbye", "world"], ["hello"], ["hello", "world"], ["world"]

Higher Order Functions

For these problems, you must use higher-order functions. No recursion allowed!


Write a function inclist' :: Num a => [a] -> [a] which increments each element of an input list.

*Main> inclist' [1,2,3,4]
*Main> inclist' [-2,4,5,1]
*Main> inclist' []
*Main> inclist' [2.3, 4.5, 7.6]


Write a function sumlist' :: (Num a) => [a] -> a which adds all the elements of the input list.

*Main> sumlist' []
*Main> sumlist' [1,2,3]
*Main> sumlist' [-3,2,5]
*Main> sumlist' [3.3,2.8,-1.2]

Algebraic Data Types

If you haven’t already you may want to read Chapter 8 of Learn you a Haskell, Making Our Own Types and Typeclasses. If Chapter 8 is a little bit over your head, try out Chapter 3 Types and Typeclasses first.

We won’t be making any Typeclasses in this MP, but we will be using and making Types. Specifically, we’ll be making Algebraic Data Types, because that’s what Haskell supports. Below are two Algebraic Data Types we supply for you to use in this assignment.

data List a = Cons a (List a)
            | Nil
  deriving (Show, Eq)

data Exp = IntExp Integer
         | PlusExp [Exp]
         | MultExp [Exp]
  deriving (Show, Eq)

The above code declares two new type constructors, List and Exp. List is a type constructor that takes one type as an argument (for example, if you said List Int that would be a different type than List String or List Double). Exp takes no type arguments.

It also declares several data constructors, two for List a and three for Exp. Their types are given below:

-- List data constructors
Cons :: a -> List a -> List a
Nil  :: List a

-- Exp data constructors
IntExp  :: Integer -> Exp
PlusExp :: [Exp] -> Exp
MultExp :: [Exp] -> Exp

Notice how the above type declarations are written as if the data constructors are functions. In fact, you can think of them as functions! Cons is a function that takes two arguments, (an a and a List a) and constructs a List a. If that doesn’t make sense to you, perhaps you need to read Chapters 3 and 8 of Learn You a Haskell as mentioned above.

The nice thing about algebraic data-constructors is that we can pattern match on them (see Chapter 4 of Learn You a Haskell for pattern matching). Suppose we wanted to make a function double :: List Int -> List Int which doubles each element of the input list. We can just ask ourselves “what should we do for each of the ways that a List Int can be constructed?”

double :: List Int -> List Int
double Nil        = Nil
double (Cons i l) = Cons (2*i) (double l)

Notice here that I’ve told Haskell “if the list is constructed as a Nil, then just produce Nil again” (this is the base-case). I’ve also told Haskell, “if the list is constructed as a Cons i l (where i :: Int, l :: List Int), then multiply the i by 2 and double the rest of the list” (the recursive case). Because I’ve exhausted all of the data-constructors for List Int, I know that Haskell will be able to apply double to any List Int. If you get a “non-exhaustive patterns” error, it means you haven’t told Haskell how to handle all of the ways something of your input type can be constructed.

You’ll be writing a few functions which manipulate the ADTs given above.


Write a function list2cons :: [a] -> List a which converts a Haskell list into our List type. Do this recursively (not using higher-order functions).

*Main> list2cons []
*Main> list2cons [3,2,5]
Cons 3 (Cons 2 (Cons 5 Nil))
*Main> list2cons ["hello", "world"]
Cons "hello" (Cons "world" Nil)
*Main> list2cons "hello"
Cons 'h' (Cons 'e' (Cons 'l' (Cons 'l' (Cons 'o' Nil))))


Write a function cons2list :: List a -> [a] which converts our List type into the Haskell list type. Do this recursively.

*Main> cons2list Nil
*Main> cons2list (Cons 3 (Cons 4 Nil))
*Main> cons2list (Cons "goodbye" (Cons "world" Nil))
["goodbye", "world"]


Write a function eval :: Exp -> Integer which evaluates the integer expression represented by its input. You may use recursion and higher-order functions.

*Main> eval (IntExp 3)
*Main> eval (PlusExp [])
*Main> eval (MultExp [])
*Main> eval (PlusExp [MultExp [IntExp 3, IntExp 5], PlusExp [IntExp 3], IntExp 5])
*Main> eval (MultExp [IntExp 3, IntExp 45, IntExp (-2), PlusExp [IntExp 2, IntExp 5]])


Write a function list2cons' :: [a] -> List a which converts a Haskell list into our List type. You are required to use higher-order functions for this, no recursion.

*Main> list2cons' []
*Main> list2cons' [3,2,5]
Cons 3 (Cons 2 (Cons 5 Nil))
*Main> list2cons' ["hello", "world"]
Cons "hello" (Cons "world" Nil)
*Main> list2cons' "hello"
Cons 'h' (Cons 'e' (Cons 'l' (Cons 'l' (Cons 'o' Nil))))


Write an ADT BinTree a which represents a binary tree that stores things of type a at internal nodes, and stores nothing at the leaves.

You must add deriving (Show) to the data declaration so that GHCi can print your datatype (as we’ve done above for List a and Exp). Not doing so will result in a loss of points.

The data-constructors must have the following types (and names):

Node :: a -> BinTree a -> BinTree a -> BinTree a
Leaf :: BinTree a


Write a function sumTree :: Num a => BinTree a -> a which takes a BinTree a (where a is a Num) from the previous problem and sums all the elements of its nodes.

*Main> sumTree Leaf
*Main> sumTree (Node 3 Leaf (Node 5 (Node 8 Leaf Leaf) Leaf))
*Main> sumTree (Node (-4) Leaf Leaf)


Write an ADT SimpVal which represents the values that a simple programming language can have. We’ll have IntVal for integers, BoolVal for booleans, StrVal for strings, and ExnVal for exceptions.

You must add deriving (Show) to the data declaration so that GHCi can print your datatype (as we’ve done above for List a and Exp). Not doing so will result in a loss of points.

The data-constructors must have the following types (and names):

IntVal  :: Integer -> SimpVal
BoolVal :: Bool    -> SimpVal
StrVal  :: String  -> SimpVal
ExnVal  :: String  -> SimpVal


Write a function liftIntOp :: (Integer -> Integer -> Integer) -> SimpVal -> SimpVal -> SimpVal which will take an operator over integers (like (+) :: Integer -> Integer -> Integer) and turn it into an operator over SimpVals. If the inputs are not IntVal, raise an exception by returning ExnVal "not an IntVal!".

*Main> liftIntOp (+) (IntVal 3) (IntVal 4)
IntVal 7
*Main> liftIntOp (*) (IntVal 2) (IntVal (-5))
IntVal (-10)
*Main> liftIntOp (+) (BoolVal True) (IntVal 3)
ExnVal "not an IntVal!"
*Main> liftIntOp (+) (IntVal 5) (StrVal "hello")
ExnVal "not an IntVal!"
*Main> liftIntOp (+) (StrVal "hello") (ExnVal "not an IntVal!")
ExnVal "not an IntVal!"