Lists
A list is an abstract data type used for storing sequential items. They are an important concept in the Lisp family of programming languages, and in functional languages in general. It's more common to use arrays, queues, and stacks for similar tasks in imperative languages.
Contents
Characteristics of a List
The power of lists comes from their definition of head and tail:
The head is the first item in the list; the tail is all remaining items. The tail is itself a list, having its own head and tail:
This can be repeated for every tail:
Exploiting this property allows for operations to be defined on lists using recursion with minimal code.
Minimal Required Functionalities
Function Name* \(\hspace{15mm}\) Provided Functionality new
Creates an empty list. append
Adds an item to the end of the list. prepend
Adds an item to the beginning of the list. head
Returns the item at the beginning of the list. tail
Returns all items except the head. If the list only has 1 item, returns an empty list. is_empty
Returns a bool indicating whether or not the list contains any items. *Exact names are not required. In fact, some functionality may be provided by a given language directly via special syntax.
Sample Python Implementation using a Linked List
Note that Python has a builtin type that it calls list
. It doesn't specifically define the required API to be used as a list ADT (abstract data type), but it does provide builtin syntax for array slicing. Since the tail
function of the list ADT could be considered a specific case of slicing, the argument could be made that the Python list
does fulfill the requirements of the list ADT API in a roundabout fashion.
This sample implements a linked list in an objectoriented style and includes the minimum functionality of a list ADT. The list
class only has a reference to the head node, so the append operation has to walk through all the nodes to find the last node, making it an \(O(n)\) operation.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 

Try out append
and prepend
:
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Now try out head
and tail
:
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Example: Summing all Values in a List
Using the Python implementation from above:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18def sum_list(the_list): if not the_list.is_empty(): return the_list.head() + sum_list(the_list.tail()) else: return 0 some_numbers = List() some_numbers.prepend(0) some_numbers.prepend(0) some_numbers.prepend(0) some_numbers.prepend(0) some_numbers.prepend(0) some_numbers.prepend(0) some_numbers.prepend(0) some_numbers.prepend(0) some_numbers.prepend(0) some_numbers.prepend(0) print(sum_list(some_numbers))
142
Using the Haskell builtin list:
1 2 3 4 5sumList :: (Num a) => [a] > a sumList [] = 0 sumList x = head x + sumList(tail x) main = print(sumList [7,5,4,1,7,7,1,4,3,3])
142
Example: Implement Right Fold
Generalizing upon the previous example, a right fold takes any arbitrary function and cumulatively applies it to the entire list.
Using the Python implementation from above:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25def special_add(x, y): return x + 2 * y # specifically, this is a right fold def fold_list(the_function, the_list): if the_list.tail().is_empty(): return the_list.head() else: return the_function(the_list.head(), fold_list(the_function, the_list.tail())) some_numbers = List() some_numbers.prepend(3) some_numbers.prepend(3) some_numbers.prepend(4) some_numbers.prepend(1) some_numbers.prepend(7) some_numbers.prepend(7) some_numbers.prepend(1) some_numbers.prepend(4) some_numbers.prepend(5) some_numbers.prepend(7) print(fold_list(special_add, some_numbers))
13257
Using the Haskell builtin list:
1 2 3 4 5 6 7 8 9specialAdd :: (Num a) => a > a > a specialAdd x y = x + 2 * y  specifically, this is a right fold foldList :: (a > a > a) > [a] > a foldList f [x] = x foldList f l = f (head l) (foldList f (tail l)) main = print(foldList specialAdd [7,5,4,1,7,7,1,4,3,3])
13257
Try It Yourself: Implement Map
Map
is a function which takes another function and applies it to each item in a list, returning the new list.
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Using only head
and tail
as defined by the List ADT, implement map
.
Use your implementation of map
to apply the function:
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to the list \([32, 57, 16, 8, 38]\).