Railway Pattern

Question 1

Create the following ADTs in Python:

• A singly linked list
• A Maybe-like type, with “constructors” Just and Nothing
• An Either-like type, with “constructors” Left and Right
• A Validation-like type, with “constructors” Success and Failure. Because Python does not have higher-kinds, you may assume that Failures always hold a list of strings.

Then define methods on all these types so that they are all functors, applicatives and monads (Validation does not need to be a monad). fmap can be called map, <*> can be called ap, return can just be pure, and >>= can be called flatMap.

Due to Python’s inexpressive type system, you are free to omit type annotations.

Try not to look at Haskell’s definitions when doing this exercise to truly understand how these data structures work!

Example runs for each data structure follow:

Lists

Python interactive session

# lists
>>> my_list = Node(1, Node(2, Empty()))

# map
>>> my_list.map(lambda x: x + 1)
Node(2, Node(3, Empty()))

# pure
>>> List.pure(1)
Node(1, Empty())

# ap
>>> Node(lambda x: x + 1, Empty()).ap(my_list)
Node(2, Node(3, Empty()))

# flatMap
>>> my_list.flatMap(lambda x: Node(x, Node(x + 1, Empty())))
Node(1, Node(2, Node(2, Node(3, Empty()))))

Maybe

Python interactive session

>>> my_just = Just(1)
>>> my_nothing = Nothing()

# map
>>> my_just.map(lambda x: x + 1)
Just(2)
>>> my_nothing.map(lambda x: x + 1)
Nothing()

# pure
>>> Maybe.pure(1)
Just(1)

# ap
>>> Just(lambda x: x + 1).ap(my_just)
Just(2)
>>> Just(lambda x: x + 1).ap(my_nothing)
Nothing()
>>> Nothing().ap(my_just)
Nothing()
>>> Nothing().ap(my_nothing)
Nothing()

# flatMap
>>> my_just.flatMap(lambda x: Just(x + 1))
Just(2)
>>> my_nothing.flatMap(lambda x: Just (x + 1))
Nothing()

Either

Python interactive session

>>> my_left = Left('boohoo')
>>> my_right = Right(1)

# map
>>> my_left.map(lambda x: x + 1)
Left('boohoo')
>>> my_right.map(lambda x: x + 1)
Right(2)

# pure
>>> Either.pure(1)
Right(1)

# ap
>>> Left('sad').ap(my_right)
Left('sad')
>>> Left('sad').ap(my_left)
Left('sad')
>>> Right(lambda x: x + 1).ap(my_right)
Right(2)
>>> Right(lambda x: x + 1).ap(my_left)
Left('boohoo')

# flatMap
>>> my_right.flatMap(lambda x: Right(x + 1))
Right(2)
>>> my_left.flatMap(lambda x: Right(x + 1))
Left('boohoo')

Validation

Python interactive session

>>> my_success = Success(1)
>>> my_failure = Failure(['boohoo'])

# map
>>> my_failure.map(lambda x: x + 1)
Failure(['boohoo'])
>>> my_success.map(lambda x: x + 1)
Right(2)

# pure
>>> Validation.pure(1)
Right(1)

# ap
>>> Failure(['sad']).ap(my_success)
Failure(['sad'])
>>> Failure(['sad']).ap(my_failure)
Failure(['sad', 'boohoo'])
>>> Success(lambda x: x + 1).ap(my_success)
Success(2)
>>> Success(lambda x: x + 1).ap(my_failure)
Failure(['boohoo'])

Question 2

Question 2.1: Unsafe Sum

Recall Question 6 in Chapter 1 (Exercises) where we defined a function sumDigits in Haskell. Now write a function sum_digits(n) that does the same, i.e. sums the digits of a nonnegative integer , in Python. Example runs follow:

Python interactive session

>>> sum_digits(1234)
10
>>> sum_digits(99999)
45

Your Haskell definition should also run similarly:

GHCi session

ghci> sumDigits 1234
10
ghci> sumDigits 99999
45

Question 2.2: Safe Sum

Try entering negative integers as arguments to your functions. My guess is that something bad happens.

Let us make sum_digits safe. Re-define sum_digits so that we can drop the assumption that is nonnegative (but will still be an integer), correspondingly using the Maybe context to keep our function pure. Use the Maybe data structure that you have defined from earlier for the Python version, and use Haskell’s built-in Maybe to do so. Example runs follow:

Python interactive session

>>> sum_digits(1234)
Just(10)
>>> sum_digits(99999)
Just(45)
>>> sum_digits(-1)
Nothing

GHCi session

ghci> sumDigits 1234
Just 10
ghci> sumDigits 99999
Just 45
ghci> sumDigits (-1)
Nothing

Question 2.3: Final Sum

Now define a function final_sum(n) that repeatedly calls sum_digit until a single-digit number arises. Just like your safe implementation of sum_digit, final_sum should also be safe. Example runs follow:

>>> final_sum(1234)
Just(1)
>>> final_sum(99999)
Just(9)
>>> final_sum(-1)
Nothing()

GHCi session

ghci> finalSum 1234
Just 1
ghci> finalSum 99999
Just 9
ghci> finalSum (-1)
Nothing

Tip

Use do-notation in your Haskell implementation!

Question 3

Question 3.1: Splitting Strings

Define a function split that splits a string delimited by a character. This is very similar to s.split(c) in Python. However, the returned result should be a singly-linked list—in Python, this would be the singly-linked-list implementation you defined in Question 1, and in Haskell, this would be just [String].

Example runs follow:

Python interactive session

>>> split('.', 'hello. world!. hah')
Node('hello', Node(' world!', Node(' hah', Empty())))
>>> split(' ', 'a   b')
Node('this', Node('', Node('', Node('is', Empty()))))

GHCi session

ghci> split '.' "hello. world!. hah"
["hello"," world!"," hah"]
ghci> split ' ' "a   b"
["a","","","b"]

Note

The split function in Haskell was defined in the hands-on section in Chapter 4.4 (Railway Pattern#Validation).

Question 3.2: CSV Parsing

The Python csv library allows us to read CSV files to give us a list of rows, each row being a list of cells, and each cell is a string. Our goal is to do something similar using the list data structure.

A CSV-string is a string where each row is separated by \n, and in each row, each cell is separated by ,. Our goal is to write a function csv that receives a CSV-string and puts all the cells in a two-dimensional list. Example runs follow.

Python interactive session

>>> csv('a,b,c\nd,e\nf,g,h')
Node(Node('a', Node('b', Node('c', Empty()))),
Node(Node('d', Node('e', Empty())),
Node(Node('f', Node('g', Node('h', Empty()))),
Empty())))

GHCi session

ghci> csv "a,b,c\nd,e\nf,g,h"
[["a","b","c"],["d","e"],["f","g","h"]]

Question 4

The formula is incredibly useful and has applications in domains like gamblingprobability and statistics, combinatorics etc. The way to compute is straightforward:

Question 4.1: Factorial

Clearly, being able to compute factorials would make computing more convenient. Therefore, write a function factorial that computes the factorial of a nonnegative integer. Do so in Python and Haskell. Example runs follow.

Python interactive session

>>> factorial(4)
24
>>> factorial(5)
120

GHCi session

ghci> factorial 4
24
ghci> factorial 5
120

Question 4.2: Safe Factorial

Just like we have done in Question 2, our goal is to make our functions safer! Re-define factorial so that we can drop the assumption that the integer is nonnegative. In addition, your function should receive the name of a variable so that more descriptive error messages can be emitted. Use the Either type. Again, do so in Python and Haskell. Example runs follow:

Python interactive session

>>> factorial(4, 'n')
Right(24)
>>> factorial(5, 'k')
Right(120)
>>> factorial(-1, 'n')
Left('n cannot be negative!')
>>> factorial(-1, 'k')
Left('k cannot be negative!')

GHCi session

ghci> factorial 4 "n"
Right 24
ghci> factorial 5 "k"
Right 120
ghci> factorial (-1) "n"
Left "n cannot be negative!"
ghci> factorial (-1) "k"
Left "k cannot be negative!"

Question 4.3: Safe n choose k

Now let us use factorial to define ! Use the formula described at the beginning of the question and our factorial functions to define a function choose that receives integers and and returns . Example runs follow:

Python interactive session

>>> choose(5, 2)
Right(10)
>>> choose(-1, -3)
Left('n cannot be negative!')
>>> choose(1, -3)
Left('k cannot be negative!')
>>> choose(3, 6)
Left('n - k cannot be negative!')

GHCi session

ghci> choose 5 2
Right 10
ghci> choose (-1) (-3)
Left "n cannot be negative!"
ghci> choose 1 (-3)
Left "k cannot be negative!"
ghci> choose 3 6
Left "n - k cannot be negative!"

Question 4.4: n choose k With Validation

Notice that several things could go wrong with ! Instead of using Either, change the implementation of factorial so that it uses the Validation applicative instead. This is so that all the error messages are collected. Your choose function definition should not change, aside from its type. Example runs follow.

Python interactive session

>>> choose(5, 2)
Success(10)
>>> choose(-1, -3)
Failure(['n cannot be negative!', 'k cannot be negative!'])
>>> choose(1, -3)
Failure(['k cannot be negative!'])
>>> choose(3, 6)
Failure(['n - k cannot be negative!'])

GHCi session

ghci> choose 5 2
Success 10
ghci> choose (-1) (-3)
Failure ["n cannot be negative!","k cannot be negative!"]
ghci> choose 1 (-3)
Failure ["k cannot be negative!"]
ghci> choose 3 6
Failure ["n - k cannot be negative!"]

Tip

With the -XApplicativeDo extension, you can actually use do notation on Functors and Applicatives. Give it a try by defining choose using do-notation! For more information on the conditions for when you can use Applicative do-notation, see the GHC Users Guide.

Note

Validation is not included in Haskell’s Prelude. You can use the Validation datatype definition and its supporting typeclass instances as defined in the hands-on portion of Chapter 4.4 (Railway Pattern#Validation).