Haskell Practice Prompts

Question 1

Use div, /, and mod on two ints, inline

Question 2

Make a character list, return the first element, and the last element

Answer 2

use head and tail

Question 3

Using a character list, make two new lists. Use the first half of the list for one, and the second half of the list for the other

Answer 3

Use drop and take

Question 4

use a function to determine the largest value in a list.

Question 5

Multiply two lists together

Question 6

Make a list containing 100 a’s, then output the 99th element from that list

Answer 6

use repeat or replicate, then use !! 99

Question 7

Make two lists, and combine them into tuples of each index

Answer 7

use zip

Question 8

Question: what are the two ways of writing multiline commands in ghci?

Answer 8

:{
–your stuff here
:}
or
:set +m

Question 9

What is a type class? Create a function that take an arbitrary list type as input

Question 10

How do you find the type of a function? Find the parameterized types of “zipwith”

Answer 10

This can be achieved using :t function

Question 11

What is a class constraint? Name some common class constraints, then implement one in a function.

Answer 11

(const a) => [a] -> a
class constraints place constraints on what values can be inputted as arbitrary data types. ex. Eq, Ord, Show, Num, Read, Enum

Question 12

implement where in a function

Question 13

implement let in a function

Question 14

implement multiple variables in a function using let

Answer 14

let a = 100; b = 200 in

Question 15

implement case

Answer 15

head’ :: [a] ‐> a;
head’ xs = case xs of
[] ‐> error “No head for empty lists!”;
(x:_) -> x

Question 16

Implement case using where instead

Answer 16

describeList :: [a] ‐> String
describeList ls = “The list is “ ++ what ls
where
what [] = “empty.”
what [x] = “has one element.”
what xs = “has many elements.”

Question 17

Design a recursive function

Question 18

use fold and lambda to count the number of a’s in a [Char]

Question 19

create a curried function, then use it

Question 20

using curried functions, divide two integers

Question 21

using concatenation, turn an Int list into a [Char] string

Question 22

implement a zipWith’ function using recursion

Question 23

use the map function, to apply a function to every element in a list

Question 24

implement filter’ using recursion

Question 25

use filter to filter out all a’s from a string

Question 26

What is the collatz chain?

Answer 26

chain :: Integer ‐> [Integer]
chain 1 = [1]
chain n
|evenn =n:chain(ndiv2) |True =n:chain(3*n+1)

Question 27

use a lambda function with map on a list

Question 28

use a case inside of a lambda function

Answer 28

week 4 p 30

Question 29

use foldr and foldl on the same problem of dividing a list of integers

Question 30

implement elem’

Answer 30

week4 p 38

Question 31

What does the dot operator do?

Answer 31

𝑓·𝑔𝑥 = f (g(x))
just places it inside the parentheses

Question 32

Implement a module and import it into ghci

Answer 32

module Shapes
( Point(..)
, Shape(..)
, area
, nudge
, baseCircle
, baseRect
) where

Question 33

implement a module and import it into another module

Answer 33

module Shapes
( Point(..)
, Shape(..)
, area
, nudge
, baseCircle
, baseRect
) where

Question 34

implement any to see if there are any 5’s in a list

Answer 34

Week 4 p.73

Question 35

Create a map from a list, then make a query

Answer 35

week 4 ~ p48

Question 36

Create your own data type, then use it in a function

Question 37

Make a type (object), then call the constructor to make one

Answer 37

data Car = Car {
company :: String,
model :: String,
year :: Int
} deriving (Show)
Call the constructor:
Car {company = “Ford”, model = “Mustang”, year = 1967}

Question 38

Make a data type that handles (Maybe a) as an input

Answer 38

data Maybe a = Nothing | Just a

Question 39

Make a type that contains a generic variable, then construct one, then call the generic variable.

Answer 39

data Car a = Car {
company :: String,
model :: String,
year :: a
} deriving (Show)

Question 40

Make a type, then parameterize that type as input for a function

Answer 40

tellCar :: (Show a) => Car a ‐> String
tellCar (Car {company = c, model = m, year = y}) =
“This “ ++ c ++ “ “ ++ m ++ “ was made in “ ++ show y

Question 41

Make a vector type, and then use it as inputs for vector dot product calculation

Answer 41

data Vector a = Vector a a a deriving (Show)

dotProd :: (Num a) => Vector a ‐> Vector a ‐> a
(Vector i j k) dotProd (Vector p q r) = (ip) + (jq) + (k*r)

Question 42

use the cons operator

Answer 42

5 Cons Empty
4 Cons (5 Cons Empty)
3 Cons (4 Cons (5 Cons Empty))

Question 43

Write an infix function

Answer 43

infixr 5 :‐:
data List a = Empty | a :‐: (List a)
deriving (Show, Read, Eq, Ord)

slide 66 week 5

Question 44

Concatenate two strings

Question 45

Make a tree data structure and insert a value

Question 46

Make a type class

Answer 46

class YesNo a where
yesno :: a ‐> Bool

instance YesNo Int where
yesno 0 = False
yesno _ = True

Question 47

Make a lambda function call

Question 48

What is a first class function? Implement one

Answer 48

First class functions pass functions in as arguments

Question 49

Write functions named inc, double, and square that increment, double, and square an argument n, respectively.

Question 50

Write a function that takes a value n. If n is even, the function returns n - 2, and if the number is odd, the function returns 3 × n + 1. To check whether the number is even, you can use either Haskell’s even function or mod (Haskell’s modulo function).

Question 51

Haskell has a function called repeat that takes a value and repeats it infinitely. Using the functions you’ve learned so far, implement your own version of repeat.

Question 52

Write a function subseq that takes three arguments: a start position, an end posi- tion, and a list. The function should return the subsequence between the start and end. For example:

Question 53

Write a function inFirstHalf that returns True if an element is in the first half of a list, and otherwise returns False.

Question 54

The tail function in Haskell returns an error when called on an empty list. Mod-
ify myTail so that it does handle the case of an empty list by returning the empty list.

Question 55

Implement your own version of reverse, which reverses a list.

Question 56

Use filter and length to re-create the elem function.

Question 57

Your isPalindrome function from lesson 6 doesn’t handle sentences with spaces or
capitals. Use map and filter to make sure the phrase “A man a plan a canal Panama” is recognized as a palindrome.

Question 58

In mathematics, the harmonic series is the sum of 1/1 + 1/2 + 1/3 + 1/4 …. Write a function harmonic that takes an argument n and calculates the sum of the series to n. Make sure to use lazy evaluation.

Question 59

What is the type signature for filter? How is it different from map?

Question 60

In Haskell, both tail and head have an error when called on an empty list. You can
write a version of tail that won’t fail but instead return an empty list when called on an empty list. Can you write a version of head that returns an empty list when called on an empty list? To answer this, start by writing out the type signatures of both head and tail.

Question 61

What is a type? What is a type class? How do they differ?

Answer 61

A type is like an object. A type class is the constraint placed on it, like Enum, Ord, or Show

Question 61

What is a type? What is a type class? How do they differ?

Answer 61

A type is like an object. A type class is the constraint placed on it, like Enum, Ord, or Show

Question 62

Define a letter type, an interest rate type, and an isFun type, Char, Double, and Bool respectively

Answer 62

letter :: Char
letter = ‘a’
interestRate :: Double
interestRate = 0.375
isFun :: Bool
isFun = True

Question 63

Define a function type, then a function definition

Answer 63

double :: Int -> Int
double n = n*2

Question 64

What is a type synonym? Implement one

Answer 64

A type synonym is a “rebranded” type. Like if I say
type car :: String
then car = “mustang”
now I can parameterize the car type in functions
myCar :: car -> String
myCar = car

Question 65

How do you make a new type? Make one

Answer 65

data Sex = Male | Female
data Covid = Pos | Neg
– Here we see that sexAndStatus calls its constructor using both Sex and Covid
data sexAndStatus = sexAndStatus Sex Covid

Question 66

use record syntax to make a Patient object, with sex, age, and height, create a Patient object, then use these types in a function

Question 67

Implement a function that returns a maybe variable, then force it to return nothing

Question 68

Make a Haskell script, compile it from terminal, then run It in terminal

Question 69

Make a Haskell script, compile it from terminal, then pipe it to a text file

Question 70

What is nub?

Answer 70

Removes duplicate elements from a list

Question 71

what is fmap?

Answer 71

Applies a function to a functor

Question 72

What is a functor?

Answer 72

things that can be mapped over, like maybes, Lists, Maps, and trees. These are all functors. Functors use data types that are wrapped in a type class, like a Maybe or Either.

A functor has to implement a fmap function, which tells it how it applies functions… This is because you can’t apply functions to a Just variable… It needs to look inside of the Just.

Question 73

Write and use a Haskell script that takes command arguments

Question 74

use read but dictate the returned type

Answer 74

read “5” :: Int

Question 75

put a where clause after a guard. for instance, area of a rectangle

Answer 75

area < 100
where area = x * y
other area = y * x

Question 76

Implement let.

Answer 76

cylinder :: (RealFloat a) => a -> a -> a
cylinder r h =
let sideArea = 2 * pi * r * h
topArea = pi * r ^2
in sideArea + 2 * topArea

Question 77

What is <$> equivalent to?

Answer 77

fmap

Question 78

What is <*> called?

Answer 78

Applicative functor

Question 79

Give an example of a list comprehension

Answer 79

[x*2 | x <- [1..10]]