An intro to lists
Much like shopping lists in the real world, lists in Mlatu are very useful. It’s the most used data structure and it can be used in a multitude of different ways to model and solve a whole bunch of problems. Lists are SO awesome. In this section we’ll look at the basics of lists and strings (which are lists).
In Mlatu, lists are a homogoenous data structure. It stores several elements of the same type. That means that we can have a list of numbers or a list of characters but we can’t have a list that has a few integers and then a few characters. And now, a list!
$ mlatu script "[4, 8, 15, 16, 23, 42]"
[4, 8, 15, 16, 23, 42]
As you can see, lists are denoted by square brackets and the values in the lists are separated by commas. If we tried a list like [1,2,'a',3,'b','c',4] Mlatu would complain that characters (which are, by the way, denoted as a character between single quotes) are not numbers. Speaking of characters, strings are just lists of characters. "hello" is just syntactic sugar for ['h', 'e', 'l', 'l', 'o']. Because strings are lists, we can use list functions on them, which is really handy.
A common task is putting two lists together. This is done by using the append function.
$ mlatu script "[1,2,3,4] [9,10,11,12] append"
[1,2,3,4,9,10,11,12]
$ mlatu script "\"hello\" \" \" append \"world\""
"hello world"
$ mlatu script "['w', 'o'] ['o', 't'] append"
"woot"
Watch out when repeatedly using the append function on long strings. When you put together two lists (even if you append a single element to a list, for instance: [1,2,3] [4] append), internally, Mlatu has to walk through the whole list of the leftmost parameter of append. That’s not a problem when dealing with lists that aren’t too big. But putting something at the end of a list that’s fifty million entries long is going to take a while. However putting something at the beginning of a list using the cons function is instantaneous.
$ mlatu script "'A' \" SMALL CAT\" cons"
"A SMALL CAT"
$ mlatu script "5 [1,2,3,4,5] cons"
[5,1,2,3,4,5]
Notice how cons takes a number and a list of numbers or a character and a list of characters, whereas append takes two lists. Even if you’re adding an element to the end of a list with append, you have to surround it with square brackets so it becomes a list.
[1,2,3] is actually just syntactic sugar for 1 2 3 nil cons cons cons. nil is an empty list. If we prepend 3 to it, it becomes [3]. If we prepend 2 to that, it becomes [2,3], and so on.
Lists can be compared if the stuff they contain can be compared. When using eq, neq, lt, le, gt, and ge to compare lists, they are compared in lexicographical order. First the heads are compared. If they are equal then the second elements are compared, etc.
$ mlatu script "[3,2,1] [2,1,0] gt"
true
$ mlatu script "[3,2,1] [2,10,100] gt"
true
$ mlatu script "[3,4,2] [3,4] gt"
true
$ mlatu script "[3,4,2] [2,4] gt"
true
$ mlatu script "[3,4,2] [3,4,2] eq"
true
What else can you do with lists? Here are some basic functions that operate on lists.
hd takes a list and returns its head. The head of a list is basically its first element. If the list is empty, hd returns none.
$ mlatu script "[5,4,3,2,1] hd"
5 some
tl takes a list and returns its tail. In other words, it chops off a list’s head. If the list is empty, tl returns none.
$ mlatu script "[5,4,3,2,1] tl"
[4,3,2,1] some
last takes a list and returns its last element. If the list is empty, last returns none.
$ mlatu script "[5,4,3,2,1] last"
1 some
init takes a list and returns everything except its last element. If the list is empty, init returns none.
$ mlatu script "[5,4,3,2,1] init"
[5,4,3,2] some
length takes a list and returns its length, obviously.
$ mlatu script "[5,4,3,2,1] length"
5
empty checks if a list is empty. If it is, it returns true, otherwise it returns false. It’s better to use empty rather than [] eq or nil eq.
$ mlatu script "[1,2,3] empty"
false
$ mlatu script "[] empty"
true
rev reverses a list.
$ mlatu script "reverse [5,4,3,2,1]"
[1,2,3,4,5]
mem takes a thing and a list of things and tells us if that thing is an element of the list.
$ mlatu script "4 [3,4,5,6] mem"
true
$ mlatu script "10 [3,4,5,6] mem"
false