Lua

Lua is a programming language designed primarily for embedded systems. It is popular in the video game industry as a language that can be embedded in a larger game engine.

PICO-8 implements a subset of Lua for writing game cartridges. Because it is a subset, not all features of Lua are supported. Most notably, PICO-8 does not include the Lua standard library, and instead provides a proprietary collection of global functions.

• Official Lua documentation
• Lua (Wikipedia)

Features of PICO-8 Lua

The following are the major language features of Lua as implemented by PICO-8.

Comments

PICO-8 code comments are preceded by two hyphens (--) and go to the end of the line. They also support a multi-line syntax using double-brackets (--[[ ... ]]).

-- one-line comment
--[[ multi-line
comment ]]

Values

PICO-8 supports these fundamental types of values:

• Numbers, in the range supported by signed 16.16 bit fixed point:
  • Minimum value: hex 0x8000.0000, decimal -32768.0
  • Maximum value: hex 0x7fff.ffff, decimal 32767.9999847412109375
  • Number literals can use decimal (17.25) or hexadecimal (0x11.4).
  • Things to be aware of:
    • PICO-8 prints and converts numbers to decimal strings by rounding off to four decimal places. This has the side effect that the maximum number rounds up to 32768.0, which is actually not a valid PICO-8 number. It also means that the tiniest negative fraction below 0 displays as the somewhat-unintuitive -0.
    • This range is narrow in comparison to modern platforms. Be aware of situations where you might overflow, e.g. calculating the distance between corners of the screen requires calculating 128*128+128*128, which will overflow and wrap to -32768.
• Booleans (true and false)
• Strings
  • String literals can use single quotes ('hello') or double quotes ("hello"), and the quote character can appear in the string escaped with a leading backslash ("you said \"hello\", yes?").
• Nil (nil)
• Functions (see below)
• Coroutines (see below)
• Tables
  • Sequences, indexed from 1: x = {2, 3, 5, 7, 11}; x[1] == 2
  • Mappings: x = {a=1, b=2, c=3}; x.a == x['a'] == 1
  • Unset indexes evaluate to nil: x[0] == nil

As in Lua, all composite and custom data types are based on tables.

Arithmetic operators

PICO-8 supports these arithmetic operators:

• addition: a + b
• subtraction: a - b
• multiplication: a * b
• division: a / b
• modulo: a % b
• exponentiation: a ^ b

Arithmetic operators take numbers as arguments and return a number.

Relational operators

PICO-8 supports these relational operators:

• less than: a < b
• greater than: a > b
• less than or equal to: a <= b
• greater than or equal to: a >= b
• equal: a == b
• not equal: a ~= b; PICO-8 synonym: a != b

Relational operators take numbers as arguments and return either true or false.

Logical operators

PICO-8 supports the following logical operators:

• and: a and b
• or: a or b
• not: not a

In logical expressions, both false and nil are treated as false, and all other values are treated as true.

Logical expressions "short circuit," which is to say they stop evaluating expressions left to right as soon as the value of the expression is known. For example, "is_alive() and can_shoot()" will first call is_alive(), and will only call can_shoot() afterwards if is_alive() returns true.

"Is there logical xor operator?"

If your operands are guaranteed to be boolean values, i.e. only true or false, then the == operator does what an xor operator does. If they are not guaranteed, then an ugly-but-functional alternative is not b == not c.

"Is there a ternary operator?"

A useful equivalent of the choice-making ?: "ternary operator" from the C-family languages is a and b or c, which evaluates to b if a is true, or c if a is false. One important and useful aspect of a ternary operator is that it uses the short-circuiting feature of logical operators to avoid invoking unwanted side effects or costs. There is one caveat: if b is false or nil, the expression will fall through and choose c.

Much more detail and additional approaches can be found in the official Lua documentation. Among them is the suggestion to use a ternary function, and it is good to note here that a ternary function uses the same number of PICO-8 tokens per invocation as the logical-operator approach, while also removing the need for isolating complex expressions with parentheses, but with the downsides that short-circuiting is lost and performance is a bit poorer.

String operators

PICO-8 supports the string concatenation operator: a..b

a must be a string. b can be a string or a number (which is coerced into a string).

No other type of value can be concatenated with a string. You can use the tostr() function to convert other values to strings:

print("a > b: "..tostr(a > b))

You can determine the length of a string using the sequence length operator (#):

x = "hello there"
print(#x)            -- 11

See also sub().

Assignment operators

As in Lua, you can assign a value to a variable or table slot

myvar = value

PICO-8 adds shortcut assignment operators for the arithmetic operators, often found in other languages like C:

myvar += value       -- myvar = myvar + value
myvar -= value       -- myvar = myvar - value
myvar *= value       -- myvar = myvar * value
myvar /= value       -- myvar = myvar / value
myvar %= value       -- myvar = myvar % value

(There are no ^= or ..= assignment operators.)

Variables

PICO-8 supports global variables accessible to the entire program, and local variables accessible only within the function where they are declared. If a variable is not declared as local, then it is global.

-- a global variable
player_pos = {20, 60}

function move_player(newx, newy)
  player_pos = {newx, newy}
end

function circumference(r)
  -- a local variable
  local pi = 3.14
  return 2 * pi * r
end

Caution: A mistyped variable name is often interpreted as a global variable with no value assigned, which evaluates to nil. Most uses of unexpectedly nil values result in a runtime error, but these are not always easy to find.

Note that, unlike vanilla Lua, there is no access to the hidden _G table containing all global values.

However, the table of all global values can still be accessed via the _ENV variable. (Be warned - this is undocumented, though an integral part of the lua language. It can also only be typed in via an external editor due to the caps).

Functions

A function is a collection of statements that can be executed by calling it. The behavior of a function can be parameterized, and a function may return a value as a result.

function distance(x1, y1, x2, y2)
  return sqrt((x2 - x1)^2 + (y2 - y1)^2)
end

PICO-8 includes many built-in global functions, such as sqrt() in this example. See APIReference.

If control reaches a return statement, then the function exits. If return includes a value, then the function call evaluates to that value. If control reaches the end of the function's statement block without seeing a return statement, then the function returns nil.

A function in Lua is a "first class" value, just like other values. A named function in the outermost block is equivalent to a global variable whose value is a function. A named function can also appear inside another function, which is equivalent to a local variable.

A function can omit the name if it is called right away or otherwise used as a value (an "anonymous function").

x = {2, 3, 5, 7, 11}
foreach(x, function(v) print(x^2) end)

Parentheses can be omitted when there is only one argument and it is a table or a literal string:

add_particle{type=snow, x=rnd(128), y=0, c=7}
print"hello, winter!"

Lua functions are lexically scoped.

Conditional statements

The Lua if statement takes a condition and a block of statements, and executes the statements only if the condition is true:

if score >= 1000 then
  print("you win!")
  score = 0
end

An if statement can include any number of elseif sections to test multiple conditions until one is found true, and an optional final else section to evaluate if none of the conditions were true. The general form is:

if cond1 then
  ...
elseif cond2 then
  ...
else
  ...
end

PICO-8 extends standard Lua with an abbreviated, single-line if statement, differentiated by having parentheses around the condition and no then or end keywords. You may use else, but it must be on the same line. There is no support for elseif. Some examples:

if (cond1) print("cond1")

if (cond2) print("cond2") else print("not cond2")

A common misconception is that you must compare a boolean variable to true or false in an if statement, but a condition is a boolean, so you can drop any boolean variable straight into an if statement:

-- these two blocks are functionally identical

if cond == true then
  print("cond")
elseif cond == false then
  print("not cond")
end

if cond then
  print("cond")
else
  print("not cond")
end

The only time you might need to compare a boolean variable to a value is when it may not be initialized yet. When variables are uninitialized, their value will be nil. When a nil value is used as a conditional expression, it is treated the same as false. Thus, if you have a boolean variable that may not be set yet, you should check for nil and initialize it before testing it as a boolean.

while and repeat loops

The while statement executes a block of statements repeatedly as long as a given conditional expression is true:

x = 0
while x < 5 do
  print(x)
  x += 1
end

The repeat statement executes a block of statements repeatedly until a given conditional expression is true:

x = 0
repeat
  print(x)
  x += 1
until x > 4

while does not execute its block if the condition is already false. repeat always executes its block at least once, then tests the condition.

The break statement anywhere in a loop terminates the loop immediately without testing the condition. If the loop is nested inside another loop, only the innermost loop is terminated.

for loops

There are two kinds of for loops in Lua. The numeric for loop traverses a numeric sequence from start to end, using an optional "stride" (with a default stride of 1).

-- draws 16 color bars
for c=0,15 do
  rectfill(c*8, 0, c*8+7, 127, c)
end

-- prints 1, 3, 5, 7, 9
for i=1,10,2 do
  print(i)
end

The other kind of for loop is the "generic for." In PICO-8, this is most commonly used with the all() and pairs() built-ins for traversing tables:

tbl = {2, 3, 5, 7, 11}
for v in all(tbl) do
  print(v)
end

tbl = {a=1, b=2, c=3}
for k,v in pairs(tbl) do
  print(k.."="..v)
end

The generic for statement expects the "in" expression to return a special kind of value called an iterator. The built-ins all() and pairs() return iterators. For information on how to create your own iterators, see Iterators and the Generic for in the Lua manual.

As with while and repeat, you can use the break statement to terminate a for loop immediately.

Tables

Tables are the primary composite data structure in Lua. They are used as containers, especially sequences (like lists or arrays) and mappings (also known as dictionaries or hashtables). Tables can be used as general purpose objects, and when combined with metatables (see below) can implement object-oriented concepts such as inheritance.

See Tables for a complete introduction to using tables in PICO-8.

Methods

A method is a function that is stored as a value in a table. Lua has special syntax for defining and calling methods that cause the table itself to be passed to the method as a parameter named self.

ball = {
  xpos = 60,
  ypos = 60
}

function ball:move(newx, newy)
  self.xpos = newx
  self.ypos = newy
end

print(ball.xpos)           -- 60

ball:move(100, 120)
print(ball.xpos)           -- 100

In both the definition and the method call, using the colon (:) instead of the dot (.) implies the self behavior. If you define the method as a simple property of the table, you must remember to explicitly mention the self argument. This is equivalent:

ball = {
  xpos = 60,
  ypos = 60,

  -- without the colon syntax, must mention self argument explicitly
  move = function(self, newx, newy)
    self.xpos = newx
    self.ypos = newy
  end
}

-- using the colon, ball is passed as self automatically
ball:move(100, 120)

-- using the dot, must pass self explicitly
ball.move(ball, 100, 120)

Metatables

The metatable for a table defines the behavior of using the table as a value with Lua operators. You can customize a table's metatable to specify custom operator behaviors.

The most common use of metatables is to implement object-oriented inheritance by redefining the __index operator. The new definition tells Lua to check for a given property on a parent prototype object if the property is not set on the current object.

function myclass:new(o)
  o = o or {}
  setmetatable(o, self)
  self.__index = self
  return o
end

PICO-8 supports the setmetatable(), getmetatable(), rawset(), rawget(), rawlen(), and rawequals() built-ins. It does not support any other related Lua functions.

See setmetatable() for more information, links to references, and a more complete example.

Coroutines

A coroutine is a special kind of function that can yield control back to the caller without completely exiting. The caller can then resume the coroutine as many times as needed until the function exits.

PICO-8 supports coroutines using built-in global functions instead of Lua's coroutine library. See cocreate(), coresume(), costatus(), and yield().

See cocreate() for more information, links to references, and a complete example.

Differences from Lua

PICO-8 does not include the Lua standard library. See Math for a description of the PICO-8 mathematical functions. See APIReference for a complete list of PICO-8 built-in functions.

josefnpat wrote a list of technical differences between PICO-8's Lua and the official Lua 5.2.

• List of differences: https://gist.github.com/josefnpat/bfe4aaa5bbb44f572cd0
• Discussion: http://www.lexaloffle.com/bbs/?tid=2611