A 'a t is a Dlist containing elements of type 'a. A Dlist, short for double-ended list, is a purely functional ordered list of elements that can be accessed both from the front and the back.

A name for the datatype, there are different implementations of this interface, you can check this name to identify which implementation you are using.

An empty Dlist, contains no elements.

is_empty t is true iff there are no elements in t.

map f t applies a function to each element of t, creating a new Dlist with the results.

Creates a Dlist containing a single element.

size t returns the number of elements in t. This operation is O(1).

push el t Adds an element to the front of t.

pop el t Removes an element from the front of t.

push_end el t Adds an element to the end of t.

pop_end el t Removes an element from the end of t.

append s1 s2 Creates Dlist that contains all elements of s1 followed by all elements of s2.

of_list elements Creates a Dlist from the elements of the list. The front of the list will become the front ot the Dlist. The first element you pop from the Dlist will be the first element in the list.

Converts Dlist into a string revealing its internal structure. Useful for debugging and testing.

get i s finds element at index i from the front. For example get 0 s is equivalent pop s

to_list s converts a Dlist into a list by popping all elements from the front.