moonscript/docs/index.md

MoonScript

MoonScript compiles to Lua

Assignment

Unlike Lua, there is no local keyword. All assignments to names that are not already defined will be declared as local to the scope of that declaration. If you wish to create a global variable it must be done using the export keyword.

hello = "world"
a,b,c = 1, 2, 3

Update Assignment

+=, -=, /=, *=, %= operators have been added for updating a value by a certain amount have been added. They are aliases for their expanded equivalents.

x += 10

Is the same as:

x = x + 10

Comments

Like Lua, comments start with -- and continue to the end of the line.

Literals & Operators

MoonScript supports all the same primitive literals as Lua and uses the same syntax. This applies to numbers, strings, booleans, and nil.

MoonScript also supports all the same binary and unary operators. != is also added as an alias of ~=.

Function Literals

All functions are created using a function expression. A simple function is denoted using the arrow: ->

my_function = ->
my_function() -- does nothing

The body of the function can either by one statement placed directly after the arrow, or it can be a series of statements indented on the following line:

func_a = -> print "hello world"

func_b = ->
  value = 100
  print "The value:", value

If a function has no arguments, it can be called using the ! operator, instead of empty parentheses.

We can call the two functions above like so:

func_a! -- equivalent to `func_a()`
func_b!

Functions with arguments can be created by preceding the arrow with a list of argument names in parentheses:

sum = (x, y) -> print "sum", x + y

Functions can be called by listing the values of the arguments after the name of the variable where the function is stored:

sum 10, 20

Functions will coerce the last statement in their body into a return statement, giving you implicit return:

sum = (x, y) -> x + y
print "The sum is ", sum 10, 20

Of course if you wanted to explicitly return, you can use the return keyword.

sum = (x, y) -> return x + y

In order to avoid ambiguity in when calling functions, parentheses can be used to surround the arguments. This is required here in order to make sure the right arguments get sent to the right functions.

print "sum 1:", sum(10, 20), "sum 1:", sum(30, 40)

The following are equivalent:

print "the value is", sum 10, get_number "decimal", "1 thousand"

print("the value is", sum(10, get_number("decimal", "1 thousand")))

Fat Arrows

Because it is an idiom in Lua to send the object as the first argument when calling a method, a special syntax is provided for functions which automatically includes this self argument.

func = (num) => self.value + num

Is the same as:

func = (self, num) -> self.value + num

Table Literals

Like in Lua, tables are delimited in curly braces.

some_values = { 1, 2, 3, 4 }

Unlike Lua, assigning a value to a key in a table is done with : (instead of =).

some_values = {
  name: "Bill",
  age: 200,
  ["favorite food"]: "rice"
}

The curly braces can be left off if a single table is being assigned.

profile = 
  height: "4 feet",
  shoe_size: 13,
  favorite_foods: {"ice cream", "donuts"}

Newlines can be used to delimit values instead of a comma (or both):

values = {
  1,2,3,4
  5,6,7,8
  name: "superman"
  occupation: "crime fighting"
}

Table Comprehensions

Table comprehensions provide a quick way to iterate over a table's values while applying a statement and accumulating the result.

The following creates a copy of the items table but with all the values doubled.

items = { 1, 2, 3, 4  5}
doubled = [item * 2 for i, item in ipairs items]

The items included in the new table can be restricted with a when clause:

slice = [item in i, item in ipairs items when i > 1 and i < 3]

Because it is common to iterate over the values of a numerically indexed table, an * operator is introduced. The doubled example can be rewritten as:

doubled = [item for item in *items]

The for and when clauses can be chained as much as desired. The only requirement on a comprehension is that there is at least one for clause.

Using multiple for clauses is the same as using nested loops:

x_coords = {4, 5, 6, 7}
y_coords = {9, 2, 3}

pairs = [{x,y} for x in *x_coords for y in *y_coords]

For Loop

There are two for loop forms, just line in Lua. A numeric one and a generic one:

for i = 10, 20
  print i

for k = 1,15,2 -- an optional step provided
  print k 

for key, value in pairs object
    print key, value

The slicing and * operators can be used, just like with table comprehensions:

for item in *items[2:4]
  print item

A shorter syntax is also available for all variations when the body is only a single line:

for item in *items do print item

for j = 1,10,3 do print j

A for loop can also be used an expression. The last statement in the body of the for loop is coerced into an expression and appended to an accumulating table if the value of that expression is not nil.

Doubling every even number:

doubled_evens = for i=1,20
  if i % 2 == 0
    i * 2
  else
    i

Filtering out odd numbers:

my_numbers = {1,2,3,4,5,6}
odds = for x in *my_numbers
  if x % 2 == 1 then x

For loops at the end of a function body are not accumulated into a table for a return value (Instead the function will return nil). Either an explicit return statement can be used, or the loop can be converted into a list comprehension.

func_a -> for i=1,10 do i
func_b -> return for i=1,10 do i

print func_a! -- prints nil
print func_b! -- prints table object

This is done to avoid the needless creation of tables for functions that don't need to return the results of the loop.

While Loop

The while loop also comes in two variations:

i = 10
while i > 0
  print i
  i -= 1

while running == true do my_function!

Like for loops, the while loop can also be used an expression. Additionally, for a function to return the accumlated value of a while loop, the statement must be explicitly returned.

Conditionals

have_coins = false
if have_coins
  print "I have coins"
else
  print "I don't have coins"

A short syntax for single statements can also be used:

have_coins = false
if have_coins then print "I have coins" else print "I don't have coins"

Because if statements can be used as expressions, this can able be written as:

have_coins = false
print if have_coins then "I have coins" else "I don't have coints"

Conditionals can also be used in return statements and assignments:

is_tall = (name) ->
  if name == "Rob"
    true
  else
    false

message = if is_tall "Rob"
  "I am very tall"
else
  "I am not so tall"

print message -- prints: I am very tall

Line Decorators

For convenience, the for loop and if statement can be applied to single statements at the end of the line:

print "hello world" if name == "Rob"

This is equivalent to:

if name == "rob"
  print "hello world"

And with basic loops:

print "item: ", item for item in *items

Object Oriented Programming

A simple class:

class Inventory
  new: =>
    @items = {}

  add_item: (name) =>
    if @items[name]
      @items[name] += 1
    else
      @items[name] = 1

A class is declared with a class statement followed by a table-like declaration where all of the methods and properties are listed.

The new property is special in that it will become the constructor.

Notice how all the methods in the class use the fat arrow function syntax. When calling methods on a instance, the instance itself is sent in as the first argument. The fat arrow handles the creation of a self variable.

The @ prefix on a variable name is shorthand for self.. @items becomes self.items.

Creating an instance of the class is done by calling the name of the class as a function.

inv = Inventory!
inv\add_item "t-shirt"
inv\add_item "pants"

Because the instance of the class needs to be sent to the methods when they are called, the '' operator is used.

All properties of a class are shared among the instances. This is fine for functions, but for other types of objects, undesired results may occur:

class Person
  clothes: {}
  give_item: (name) =>
    table.insert @clothes, name

a = Person!
b = Person!

a\give_item "pants"
b\give_item "shirt"

print item for item in *a.clothes -- will print both pants and shirt

Inheritance

The extends keyword can be used in a class declaration to inherit the properties and methods from another class.

class BackPack extends Inventory
  size: 10
  add_item: (name) =>
    if #@items > size then error "backpack is full"
    super name

Here we extend our Inventory class, and limit the amount of items it can carry. The super keyword can be called as a function to call the function of the same name in the super class. It can also be accessed like an object in order to retrieve values in the parent class that might have been shadowed by the child class.

Types

Every instance of a class carries its type with it. This is stored in the special __class property. This property holds the class object. The class object is what we call to build a new instance. We can also index the class object to retrieve class methods and properties.

b = BackPack!
assert b.__class == BackPack

print BackPack.size -- prints 10

Export Statement

Because, by default, all assignments to variables that are not lexically visible will be declared as local, special syntax is required to declare a variable globally.

The export keyword makes it so any following assignments to the specified names will not be assigned locally.

export var-name [, var-name2, ...]

This is especially useful when declaring what will be externally visible in a module:

-- my_module.moon
module "my_module", package.seeall
export print_result

length = (x, y) -> math.sqrt x*x + y*y

print_result = (x, y) ->
  print "Length is ", length x, y

-- main.moon
require "my_module"

my_module.print_result 4, 5 -- prints the result

print my_module.length 6, 7 -- errors, `length` not visible

Import Statement

Often you want to bring some values from a table into the current scope as local variables by their name. The import statement lets us accomplish this:

import insert from table

The multiple names can be given, each separated by a comma:

import C, Ct, Cmt from lpeg

Sometimes a function requires that the table be sent in as the first argument (when using the \ syntax). As a shortcut, we can prefix the name with a \ to bind it to that table:

-- some object
my_module = 
    state: 100 
    add: (value) =>
        self.state + value

import \add from my_module

print add(22) -- equivalent to calling my_module:get(22)

With Statement

A common pattern involving the creation of an object is calling a series of functions and setting a series of properties immediately after creating it.

This results in repeating the name of the object multiple times in code, adding unnecessary noise. A common solution to this is to pass a table in as an argument which contains a collection of keys and values to overwrite. The downside to this is that the constructor of this object must support this form.

The with block helps to alleviate this. It lets us use a bare function and index syntax in order to work with the object:

with Person!
  .name = "Oswald"
  \add_relative my_dad
  \save!
  print .name

Is equivalent to:

_person = Person!
_person.name = "Oswald"
_person\add_relative my_dad
_person\save!
print _person.name

This is more expressive than trying to create multiple constructors to handle unique instances of initializing an object.

The with statement can also be used as an expression which returns the newly created object.

file = with File "favorite_foods.txt"
  \set_encoding "utf8"

Or...

create_person = (name,  relatives) ->
  with Person!
    .name = name
    \add_relative for relative in *relatives

me = create_person "Leaf", {dad, mother, sister}

The Using Clause; Controlling Destructive Assignment

This isn't implemented yet

While lexical scoping can be a great help in reducing the complexity of the code we write, things can get unwieldy as the code size increases. Consider the following snippet:

i = 100

-- many lines of code...

my_func = ->
    i = 10
    while i > 0
        print i
        i -= 1

my_func()

print i -- will print 0

In my_func, we've overwritten the value of i mistakenly. In this example it is quite obvious, but consider a large, or foreign code base where it isn't clear what names have already been declared.

It would be helpful to say which variables from the enclosing scope we intend on change, in order to prevent us from changing others by accident.

The using keyword lets us do that. using nil makes sure that no closed variables are overwritten in assignment. The using clause is placed after the argument list in a function, or in place of it if there are no arguments.

i = 100

my_func = (using nil) ->
    i = "hello" -- a new local variable is created here

my_func()
print i -- prints 100, i is unaffected

Multiple names can be separated by commas. Closure values can still be accessed, they just cant be modified:

tmp = 1213
i, k = 100, 50

my_func = (add using k,i) ->
    tmp = tmp + add -- a new local tmp is created 
    i += tmp
    k += tmp

my_func(22)
print i,k -- these have been updated