Lists in Sass are immutable; all list functions return a new list rather than updating the existing list in-place.
All list functions work for maps as well, treating them as lists of pairs.
length($list) : Returns the length of a list.
nth($list, $n): Returns a specific item in a list.
set-nth($list, $n, $value): Replaces the nth item in a list.
join($list1, $list2, [$separator, $bracketed]) : Joins together two lists into one.
append($list1, $val, [$separator]) : Appends a single value onto the end of a list.
zip($lists...) : Combines several lists into a single multidimensional list.
index($list, $value): Returns the position of a value within a list.
list-separator($list) : Returns the separator of a list.
is-bracketed($list) : Returns whether a list has square brackets.