7. Data.List

Welcome to the Stone Age, where we forget all tools and write functions using recursion, pattern matching, guards and conditionals.

For each of the following functions, write two check-expects (examples) and then write the function.

Group A: Useful Daily

• myIntersperse: takes in an element and a list, and puts the element between every pair of the list.

    myIntersperse 6 [1,2,9,10] == [1,6,2,6,9,6,10]
  

• myIntercalate: like intersperse, but for lists. Takes a list and a list of lists, and puts the first list in between each pair of lists from the second input. It “flattens” (concatenates) the result, removing one level of list. Note: String is [Char], so this applies to putting strings in between a list of words.

    myIntercalate "_GO_" ["Speed","Racer","Car"] == "Speed_GO_Racer_GO_Car"
  

Group B

• myAnd takes a list of booleans and returns True only when they are all True. In the empty list, “all” of the entries are True. (Try to find one that isn’t!)

• myOr takes a list of booleans and returns True if any item in the list is True. In the empty list, there aren’t any items that are True.

• myAll takes two arguments: a function that produces a boolean, and a list of items. It gives True if applying the function to every item in the list results in a list with all entries True.

• myAny: just like myAll but it gives True if any result is True.

• myConcat takes a list of lists and concatenates them together (removing one level of nesting).

    myConcat [['b','i','r','d'],[' '],['d','o','g']] == ['b','i','r','d',' ','d','o','g']
  

• myIterate :: (a->a) -> a -> [a] takes in a function f and an item x, and returns an infinite list created by repeatedly applying f:

    [x, f x, f (f x), f (f (f x)), ...]
  

• myConcatMap applies a function to items in a list and then concatenates all of the results.

Group C

Making groups. We are going to continue to write “my” versions of all of these. Consult Chapter 7 in the book for examples of each of these.

• splitAt
• myTakeWhile
• myDropWhile
• span
• break (skip 2023)
• partition (skip 2023)

Group D

We are going to write “my” versions of all of these.

• find
• elemIndex (this is not the primary way to find something!!)
• elemIndices(skip 2023)
• findIndex (skip 2023)
• findIndices (skip 2023)
• Concisely state a similarity and a difference between elemIndices and findIndices. (skip 2023)

More

• Read the paragraph about “generic equivalents” that appears after insert and before the “deleteBy” paragraph.

Rules for the Stone Age

Banned Functions

• fold
• length
• head, tail
• fst, snd
• map
• elem
• elemIndices (yikes!!)
• if-then-else (there are clearer ways to write choices)
• concat (until you write it)
• concatMap (!!)

Discouraged Functions

• guards are OK but there are usually better ways

Encouraged

• patterns
• recursion
• list comprehensions