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.
Features of PICO-8 Lua[]
The following are the major language features of Lua as implemented and extended 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 ]]
You can quickly toggle between active and inactive multi-line comments by adding an extra hyphen. This allows for quickly switching on and off different portions of the code (for example, to help with debugging).
Be sure that the closing set of brackets has their own double-hyphen so that you aren't left with hanging brackets when using this trick.
--[[
code that i don't
want to be active
--]]
---[[
code that is
now active
--]]
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, binary0b1000000000000000.0000000000000000 - Maximum value: hex
0x7fff.ffff, decimal32767.9999847412109375, binary0b0111111111111111.1111111111111111 - Number literals can use decimal (
17.25), hexadecimal (0x11.4), or binary (0b10001.01). - 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.
- 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
- Minimum value: hex
- Booleans (
trueandfalse) - 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?").
- String literals can use single quotes (
- 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
- Sequences, indexed from 1:
As in Lua, all composite and custom data types are based on tables.
Arithmetic operators[]
PICO-8 supports these arithmetic operators:
- negation:
-a - addition:
a + b - subtraction:
a - b - multiplication:
a * b - division:
a / b - division + floor:
a \ b - modulo:
a % b - exponentiation:
a ^ b
Arithmetic operators take numbers as arguments and return a number.
Note: The integer division operator is similar to div operators in other languages. It performs regular division and then floors the result to produce an integer. It is faster than evaluating the equivalent flr(a / b) expression.
Bitwise operators[]
PICO-8 supports these bitwise operators:
- not:
~a - or:
a | b - and:
a & b - xor:
a ^^ b. Lua synonym:a ~ b - shift left:
a << b - arithmetic shift right:
a >> b - logical shift right:
a >>> b - rotate left:
a <<> b - rotate right:
a >>< b
Bitwise operators take numbers as arguments and return a number.
Note: Arithmetic shifts maintain sign by filling empty bits with the top bit of the shifted value, while logical shifts fill empty bits with 0:
> v = -2 > print(tostr(v, true)) -- tostr(v, true) converts v to a hex string 0xfffe.0000 > print(v >> 1) -1 > print(tostr(v >> 1, true)) 0xffff.0000 > print(v >>> 1) 32767 > print(tostr(v >>> 1, true)) 0x7fff.0000
Memory operators[]
PICO-8 provides shorthand for accessing memory directly, offering three prefix operators that mirror the functionality of peek(), peek2(), and peek4():
- peek:
@address - peek2:
%address - peek4:
$address
Using these operators appears to be significantly faster than calling the corresponding peek() functions.
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 [wikipedia:?: ?: "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 value types 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[]
In Lua, assignment places a value in a variable or a table element, like so:
a = b
t.a = b
PICO-8 also has assignment operators, where an operation is done on the destination variable and the result is assigned back to it. Similar operators are found in languages like C/C++/Java/etc.
This is a list of PICO-8 assignment operators, and the expressions each operator is shorthand for:
a += b -- a = a + b
a -= b -- a = a - b
a *= b -- a = a * b
a /= b -- a = a / b
a \= b -- a = a \ b
a %= b -- a = a % b
a ^= b -- a = a ^ b
a ..= b -- a = a .. b
a |= b -- a = a | b
a &= b -- a = a & b
a ^^= b -- a = a ^^ b or a = a ~ b
a <<= b -- a = a << b
a >>= b -- a = a >> b
a >>>= b -- a = a >>> b
a <<>= b -- a = a <<> b
a >><= b -- a = a >>< b
It's worth noting that there is not a ~= assignment operator to match the 0.2.5 addition of the ~ bitwise-xor operator. This is likely due to difficulties in having PICO-8's Lua regex-based pre-processor, which silently converts these assignment operators into vanilla lua assignments, successfully recognize the difference between the existing ~= inequality operator and such an assignment operator.
Operator priorities[]
In math and programming, an expression is evaluated using a certain order of operations. One operator will take priority over another based on its precedence value.
This is the full list of operators, ordered from first-evaluated to last-evaluated:
| Precedence | Operator | Description | Associativity |
|---|---|---|---|
| 1 | a(…)
|
function call table lookup member access |
left-to-right |
| 2 | a ^ b | exponent/power | right-to-left |
| 3 | -a
|
negate bitwise not logical not peek peek2 peek4 |
right-to-left |
| 4 | a * b
|
multiply divide modulo divide + floor |
left-to-right |
| 5 | a + b
|
add subtract |
left-to-right |
| 6 | a .. b
|
concatenate | right-to-left |
| 7 | a << b
|
shift left arithmetic shift right logical shift right rotate left rotate right |
left-to-right |
| 8 | a & b
|
bitwise and | left-to-right |
| 9 | a ~ b, a ^^ b
|
bitwise xor | left-to-right |
| 10 | a | b
|
bitwise or | left-to-right |
| 11 | a == b
|
equal not equal less than less than or equal greater than greater than or equal |
left-to-right |
| 12 | a and b
|
logical and | left-to-right |
| 13 | a or b
|
logical or | left-to-right |
Note that associativity dictates which order operations are done when more than one in the same group appear in sequence. For instance, multiply and divide are in the same group, and it's possible to test their associativity by adding explicit parentheses to force the order of operations:
> ?4/2*8 -- this is what lua does implicitly 16 > ?4/(2*8) -- let's try forcing the right operation first 0.25 > ?(4/2)*8 -- ok, that didn't match, so try the left 16
Forcing the left operator to happen first matches what happens when there are no parentheses, so this group of operators is left-to-right associative. Similarly, the opposite would be true for right-to-left associativity.
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. Inside of PICO-8 this may be referenced by using Ctrl+P to switch to the "puny" font and entering its name in lowercase (uppercase and lowercase are swapped on PICO-8). Be warned: this is an undocumented feature; it is not guaranteed to survive until version 1.0, and what is found in _ENV is not guaranteed to be the same from one version of PICO-8 to the next.
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 the 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(v^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
PICO-8 extends standard Lua with an abbreviated, single-line while statement, differentiated by having parentheses around the condition and no do or end keywords. Some examples:
-- same as the example above, but very terse
x=0 while(x<5) print(x) x+=1
-- stop and wait synchronously for any button to be pressed
while(btn() == 0) flip()
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 Cutscenes and Coroutines for a complete description and example of using coroutines for driving animations.
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