List

rascal-0.34.0

Synopsis​

An ordered sequence of values.

Description​

A list is a sequence of values with the following properties:

• The list maybe empty.
• The values in the list are ordered.
• The same value may occur more than once.
• The list has a size that is equal to the number of values in the list.
• Each element in a list L has an index. The first element has index 0. The last element has index `size(L)-1`.

Formally, a list can be defined as follows. Given the domains `ELEM` (elements) and `LIST` (lists) and the functions:

``nil :             -> LISTcons: ELEM x LIST -> LISThead: LIST        -> ELEMtail: LIST        -> LIST``

`nil` and `cons` are so-called constructor functions that define the values in `LIST`. They can be paraphrased as:

• The empty list `nil` is an element of `LIST`.
• If `e` is an element of `ELEM` and `l` is an element of LIST, then `cons(e, l)` is also an element in `LIST`.

`head` (take the first element) and `tail` (take the remainder of a list) are defined functions characterized by the axioms:

``head(cons(e, l)) = etail(cons(e, l)) = l``

The cases `head(nil)` and `tail(nil)` are left undefined (and usually correspond to a runtime error in a programming language).

In Rascal, lists are surrounded by brackets `[` and `]` and the elements are separated by commas. Each list has a type of the form `list[T]`, where T is the smallest common type of all list elements. Read the description of lists and their operators and of library functions on lists.

Lists in Daily Life​

• A line of people waiting for the super market checkout or bus stop. credit
• The wagons of a train.
• The Top 100 Music Charts. credit
• Twitter users ordered according to number of followers.
• A to do list.

Lists in computer science​

• The locations in a computer memory.
• The list of processes that use most cpu time.
• The list of procedures that are called by a given procedure.

Lists in Rascal​

• The empty list: `[]`. Its type is `list[void]`.
• A list of integers: `[3, 1, 4]`. Its type is `list[int]`.
• A list of mixed-type values: `[3, "a", 4]`. Its type is `list[value]`.