Spring 2023 Haskell Exam

Problem 1

Write a main function that accepts a single command-line argument representing the current color of a stoplight, which is one of "green", "yellow", or "red". It prints the color that the light will switch to next. For example:

> runhaskell main.hs green
yellow
> runhaskell main.hs red
green
> runhaskell main.hs Red
?
> runhaskell main.hs flung
?

> runhaskell main.hs green
yellow
> runhaskell main.hs red
green
> runhaskell main.hs Red
?
> runhaskell main.hs flung
?

If the argument is any other string or uses a different case, print "?".

Problem 2

Write function mapFromTo that accepts a list of any type and a transformation function. It returns a list of pairs. The first element of each pair is the corresponding element from the original list. The second element is the value to which it transforms. For example:

> mapFromTo length ["a", "an", "the"]
[("a", 1), ("an", 2), ("the", 3)]

> mapFromTo length ["a", "an", "the"]
[("a", 1), ("an", 2), ("the", 3)]

Problem 3

Write function days that accepts a year, a month, and a list of dates. Each date is a 3-tuple of a year, month, and day of the month. The function returns the days of the month for those dates that fall in the given year and month. For example:

> days 2023 3 [(2023, 3, 27), (2023, 4, 1), (2023, 3, 15)]
[27, 15]
> days 2023 4 [(2023, 3, 27), (2023, 4, 1), (2023, 3, 15)]
[1]
> days 2023 5 [(2023, 3, 27), (2023, 4, 1), (2023, 3, 15)]
[]

> days 2023 3 [(2023, 3, 27), (2023, 4, 1), (2023, 3, 15)]
[27, 15]
> days 2023 4 [(2023, 3, 27), (2023, 4, 1), (2023, 3, 15)]
[1]
> days 2023 5 [(2023, 3, 27), (2023, 4, 1), (2023, 3, 15)]
[]

Problem 4

Write a main function that reads in a file whose path is passed as a command-line argument. It prints the number of lines in the file. For example:

> cat vowels.txt
A
E
I
O
U
> runhaskell main.hs vowels.txt
5

> cat vowels.txt
A
E
I
O
U
> runhaskell main.hs vowels.txt
5

Problem 5

Define a Distance data type with two variants. Meters holds a distance in meters. Feet holds a distance in feet. All fields are of type Double.

Define also a Metric typeclass with a function toMetric that turns a value of an implementing type into its metric variant.

Make Distance implement the Metric typeclass. Consider 1 foot to be equivalent to 0.3048 meters. If the value is already metric, it is returned unchanged.

Problem 6

Write function braid that accepts two lists of the same type. It returns a pair of lists that are like the parameters, but every other element has been swapped between the two lists. For example:

> braid "abcd" "1234"
("a2c4", "1b3d")
> braid "dime" "pulpit"
("dump", "pile")

> braid "abcd" "1234"
("a2c4", "1b3d")
> braid "dime" "pulpit"
("dump", "pile")

Only elements that have a partner in the other list are included in the braid, as you can see in the second example.

Recursion is helpful here. How can you call your recursive braid function to achieve alternation?