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# Lists
Limbo supports lists of arbitrary types as a basic construct.
## Source
### lists.b:16,20
Three lists are declared.
The `len` statement can be used on a list to return the number of elements in the respective list. Note that the list `persons` is a list of tuples whose pairing is that of a string and an integer.
### lists.b:22,28
The lists `names` and `ages` are both populated.
The `::` operator functions on lists by prepending the given element before the head of the current list. Note the interaction between successive calls to the `::` operator prior to assignment and how it affects order.
### lists.b:30,46
The list `persons` is built by making copies of the `names` and `ages` lists as `n` and `a`, respectively.
The `hd` statement returns the head of the list to its right.
The `tl` statement returns the tail, that is the list bar the head element, of the list to its right.
An empty list is equivocal to `nil`. That is, a nil list is an empty list.
Note: The effects upon temporary copies of `names` and `ages` do not affect the original lists they are copies of, as the temporary copies are copies, rather than references to the original lists.
## Demo
; limbo lists.b
; lists
Lens: 0, 0, 0
Lens: 3, 3, 0
Persons:
Spike: 27
Jet: 36
Ed: 13
Tmp lens: 0, 0
Lens: 3, 3, 0
;
## Exercises
- Try reversing a list.
- Try interleaving two lists.
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