1.4. Expressions

1.4.1. Assignment

exp := derefexp '=' exp
a = 10

For adding new fields to tables there is the “new slot” operator. (see Tables)

exp:= derefexp '<-' exp
tbl.a <- 10
tbl["b"] <- 20

If the slot already exists in the table this behaves like a normal assignment.

1.4.2. Operators ?: Operator

exp := exp_cond '?' exp1 ':' exp2

conditionally evaluate an expression depending on the result of an expression. ?? Null-coalescing operator

exp := exp1 '??' exp2

Conditionally evaluate an expression2 depending on the result of an expression1. Given code is equivalent to:

exp := (exp1 '!=' null) '?' exp1 ':' exp2

C#-like ?? syntax was chosen over Elvis operator ?: which is common in other languages because it is not equivalent to visually similar ternary ? : operator (which checks for falsiness, not null).

It evaluates expressions until the first non-null value (just like || operators for the first true one).

Operator precendence is also follows C# design, so that ?? has lower priority than || ?. and ?[] - Null-propagation operators

exp := value '?.' key
exp := value '?[' key ']'

If key exists, return result of ‘get’ operations, else return null.

let tbl = {bar=123}

tbl.bar // returns 123
tbl.baz // throws an error
tbl?.bar // returns 123
tbl?.baz // returns null
null.bar // throws an error
null?.bar // returns null
tbl?["bar"] // returns 123
tbl?[4567] // returns null

This works for any type (internally done via SQVM::Get(), like an ‘in’ operator), including null. Therefore operator can be chained

let x = tbl?.foo?.bar?.baz?["spam"]

To avoid extra typing, null-propagation operators affect the rest of expression. Otherwise, an expression like


would make no sense because without automatic propagation a null value’s slot could possibly be accessed in runtime. One would have to type ?. everywhere, writing it as


Instead it is done by compiler - once a null-operator is met, it is also assumed for the subsequent ., [] and () operators in an expression.

Note: ‘key’ should not be separated from ‘?.’ or ‘.’ by space[s] or new line. .$ and ?.$ - Type methods access operator

exp := value '.$' key
exp := value '?.$' key

If ‘key’ exists in value’s type built-in methods (default delegates) returns method’s closure, else returns null in case of ‘?.$’ or throws an error if ‘.$’

Note: ‘key’ should not be separated from ‘.$’ and ‘?.$’ by space[s] or new line. Arithmetic

exp:= 'exp' op 'exp'

Quirrel supports the standard arithmetic operators +, -, *, / and %. Other than that is also supports compact operators (+=,-=,*=,/=,%=) and increment and decrement operators(++ and –);:

a += 2;
//is the same as writing
a = a + 2;
//is the same as writing
x = x + 1

All operators work normally with integers and floats; if one operand is an integer and one is a float the result of the expression will be float. The + operator has a special behavior with strings; if one of the operands is a string the operator + will try to convert the other operand to string as well and concatenate both together. For instances and tables, _tostring is invoked. Relational

exp:= 'exp' op 'exp'

Relational operators in Quirrel are : ==, <, <=, <, <=, !=

These operators return true if the expression is false and a value different than true if the expression is true. Internally the VM uses the integer 1 as true but this could change in the future. 3 ways compare

exp:= 'exp' <=> 'exp'

the 3 ways compare operator <=> compares 2 values A and B and returns an integer less than 0 if A < B, 0 if A == B and an integer greater than 0 if A > B. Logical

exp := exp op exp
exp := '!' exp

Logical operators in Quirrel are : &&, ||, !

The operator && (logical and) returns null if its first argument is null, otherwise returns its second argument. The operator || (logical or) returns its first argument if is different than null, otherwise returns the second argument.

The ‘!’ operator will return null if the given value to negate was different than null, or a value different than null if the given value was null. in operator, not in operator

exp:= keyexp 'in' tableexp
exp:= keyexp 'not in' tableexp

Tests the existence of a slot in a table. ‘in’ operator returns true if keyexp is a valid key in tableexp ‘not in’ operator returns true if keyexp is missing in tableexp

let t = {
    foo="I'm foo",
    [123]="I'm not foo"

if("foo" in t) dostuff("yep");
if(123 in t) dostuff();
if(123 not in t) dostuff(); instanceof operator

exp:= instanceexp 'instanceof' classexp

Tests if a class instance is an instance of a certain class. Returns true if instanceexp is an instance of classexp. typeof operator

exp:= 'typeof' exp

returns the type name of a value as string.:

local a={},b="quirrel"
print(typeof a); //will print "table"
print(typeof b); //will print "string" Bitwise Operators

exp:= 'exp' op 'exp'
exp := '~' exp

Quirrel supports the standard C-like bitwise operators &, |, ^, ~, <<, >> plus the unsigned right shift operator >>>. The unsigned right shift works exactly like the normal right shift operator(>>) except for treating the left operand as an unsigned integer, so is not affected by the sign. Those operators only work on integer values; passing of any other operand type to these operators will cause an exception. Operators precedence

-, ~, !, typeof , ++, --


/, *, %

+, -

<<, >>, >>>

<, <=, >, >=, instanceof

==, !=, <=>



&&, in



+=, =, -=, /=, *=, %=

1.4.3. Table Constructor

tslots := ( 'id' '=' exp | '[' exp ']' '=' exp  | 'id' ) [',']
exp := '{' [tslots] '}'

Creates a new table.:

let a = {} //create an empty table

A table constructor can also contain slots declaration; With the syntax:

let a = {
    slot1 = "I'm the slot value"

An alternative syntax can be:

'[' exp1 ']' = exp2 [',']

A new slot with exp1 as key and exp2 as value is created:

let a = {
    [1]="I'm the value"

ES2015-style shorthand table initialization is supported, so the code like below

local x = 123
local y = 345
let tbl = {x=x, y=y}

can also be written as

local x = 123
local y = 345
let tbl = {x, y}

All syntaxes can be mixed:

local x = "bar"
let table=
    function bau(a,b)
        return a+b;

The comma between slots is optional. Table with JSON syntax

Since Squirrel 3.0 is possible to declare a table using JSON syntax(see http://www.wikipedia.org/wiki/JSON).

the following JSON snippet:

let x = {
  "id": 1,
  "name": "Foo",
  "price": 123,
  "tags": ["Bar","Eek"]

is equivalent to the following quirrel code:

let x = {
  id = 1,
  name = "Foo",
  price = 123,
  tags = ["Bar","Eek"]

1.4.4. clone

exp:= 'clone' exp

Clone performs shallow copy of a table, array or class instance (copies all slots in the new object without recursion).

After the new object is ready the “_cloned” meta method is called (see Metamethods).

When a class instance is cloned the constructor is not invoked(initializations must rely on `_cloned` instead

Note: Usage of this operator could be prohibited with #forbid-clone-operator.

1.4.5. Array contructor

exp := '[' [explist] ']'

Creates a new array.:

a <- [] //creates an empty array

Arrays can be initialized with values during the construction:

a <- [1,"string!",[],{}] //creates an array with 4 elements