Flecs Script

Introduction

Flecs Script is a runtime interpreted DSL for creating entities and components that is optimized for defining scenes, assets and configuration. In a nutshell, Flecs Script is to ECS what HTML/JSX is to a browser.

Some of the features of Flecs Script are:

• Native support for named entities, hierarchies and inheritance
• Assign component values
• Expressions and variables (var + 10)
• Conditionals and loops (if var > 10, for i in [0..10])
• Native integration with templates (procedural assets)

Example

using flecs.meta

struct MaxSpeed {
  value = f32
}

struct Position {
  x = f32
  y = f32
}

prefab SpaceShip {
  MaxSpeed: {value: 100}

  cockpit {
    Position: {x: -10, y: 0}
  }
}

my_spaceship : SpaceShip {
  Position: {x: 10, y: 20}
}

The Basics

This section goes over the basic syntax over Flecs Script.

Entities

An entity is created by specifying an identifier followed by a scope. Example:

my_entity {}

An entity scope can contain components and child entities. The following example shows how to add a child entity:

my_parent {
  my_child {}
}

Note how a scope is also added to the child entity.

To create anonymous entities, leave out the entity name:

{
  my_child {} // named child with anonymous parent
}

Alternatively, the _ placeholder can be used to indicate an anomyous entity:

_ {
  my_child {} // named child with anonymous parent
}

The _ placeholder can be useful in combination with syntax constructs that require an identifier token, such as inheritance:

// anonymous entity that inherits from SpaceShip
_ : SpaceShip { }

Entity names can be specified using a string. This allows for entities with names that contain special characters, like spaces:

"my parent" {
  "my child" {}
}

String names can be combined with string interpolation (see below) to create names that are computed when the script is evaluated:

"USS_$name" {}

By default children are created using the ChildOf hierarchy storage. To select the Parent hierarchy storage, add the tree annotation:

@tree Parent
my_parent {
  my_child {}
}

Tags

A tag can be added to an entity by simply specifying the tag's identifier in an entity scope. Example:

SpaceShip {} // Define SpaceShip tag

my_entity {
  SpaceShip // Add SpaceShip to my_entity
}

Pairs

Pairs are added to entities by adding them to an entity scope, just like tags:

Likes {}
Pizza {}

my_entity {
  (Likes, Pizza)
}

Components

Components are specified like tags, but with an additional value:

my_entity {
  Position: {x: 10, y: 20}
}

For a component to be assignable with a value, it also needs to be described in the reflection framework.

A component can also be added without a value. This will create a default constructed component. Example:

my_entity {
  Position
}

Components can be defined in a script:

using flecs.meta

struct Position {
  x = f32
  y = f32
}

my_entity {
  Position: {x: 10, y: 20}
}

Components can be pairs:

my_entity {
  (Start, Position): {x: 0,  y: 0}
  (Stop,  Position): {x: 10, y: 20}
}

Namespacing

When referring to child entities or components, identifiers need to include the parent path as well as the entity name. Paths are provided as lists of identifiers separated by a dot (.):

Sun.Earth {
  solarsystem.Planet
}

To avoid having to repeatedly type the same paths, use the using statement (see below).

Singletons

To create a singleton component, use $ as the entity identifier:

$ {
  TimeOfDay: { t: 0.5 }
}

Multiple singleton components can be specified in the same scope:

$ {
  TimeOfDay: { t: 0.5 }
  Player: { name: "bob" }
}

Entity kinds

An entity can be created with a "kind", which is a component specified before the entity name. This is similar to adding a tag or component in a scope, but can provide a more natural way to describe things. For example:

SpaceShip my_spaceship {}

This is equivalent to doing:

my_spaceship {
  SpaceShip
}

When using the entity kind syntax, the scope is optional:

SpaceShip my_spaceship // no {}

If the specified kind is a component, a value can be specified between parentheses:

CheckBox my_checkbox(checked: true)

When the entity kind is a component, a value will always be assigned even if none is specified. This is different from component assignments in a scope. Example:

CheckBox my_checkbox(checked: true)

// is equivalent to

my_checkbox {
  CheckBox: {checked: true}
}

CheckBox my_checkbox

// is equivalent to

my_checkbox {
  CheckBox: {}
}

Builtin kinds

Applications can specify the following builtin kinds which provide convenience shortcuts to commonly used features:

prefab SpaceShip

// is equivalent to

Prefab spaceship

prefab SpaceShip {
  slot CockPit
}

// is equivalent to

prefab SpaceShip {
  CockPit {
    (SlotOf, SpaceShip)
  }
}

Inheritance

Scripts can natively specify inheritance relationships between entities, which is useful in particular for prefabs. Example:

prefab SpaceShip {
  MaxSpeed: {value: 100}
}

my_spaceship : SpaceShip {}

The : notation is short for adding an IsA relationship with the relationship syntax:

my_spaceship {
  (IsA, SpaceShip)
}

Relationship hierarchies

By default entity hierarchies are created with the ChildOf relationship. Other relationships can also be used to create hierarchies by combining a pair with a scope. Example:

(IsA, Thing) {
  (IsA, Organism) {
    (IsA, Plant) {
      Tree {}
    }
    (IsA, Animal) {
      Human {}
    }
  }
}

Expressions

Scripts can contain expressions, which allow for computing values from inputs such as component values, template properties and variables. Here are some examples of valid Flecs script expressions:

const x: 10 + 20 * 30
const x: 10 * (20 + 30)
const x: $var * 10
const x: pow($var, 2)
const x: e.parent().name()
const x: Position: {10, 20}
const x: Position: {x: 10, y: 20}

The following sections describe the different features of expressions.

Operators

The following operators are supported in expressions, in order of precedence:

Values

The following table lists the different kinds of values that are supported in expressions:

Initializers

Initializers are values that are used to initialize composite and collection members. Composite values are initialized by initializers that are delimited by {}, while collection initializers are delimited by []. Furthermore, composite initializers can specify which member of the composite value should be initialized. Here are some examples of initializer expressions:

{}
{10, 20}
{x: 10, y: 20}
{{10, 20}, {30, 40}}
{start: {x: 10, y: 20}, stop: {x: 10, y: 20}}
[10, 20, 30]
[{10, 20}, {30, 40}, {50, 60}]
{x += 10, y *= 2}

Initializers must always be assigned to an lvalue of a well defined type. This can either be a typed variable, component assignment, function parameter or in the case of nested initializers, an element of another initializer. For example, this is a valid usage of an initializer:

const x = Position: {10, 20}

while this is an invalid usage of an initializer:

// Invalid, unknown type for initializer
const x: {10, 20}

When assigning variables to elements in a composite initializer, applications can use the following shorthand notation if the variable names are the same as the member name of the element:

// Normal notation
Tree: {color: $color, height: $height}


// Shorthand notation
Tree: {color: $, height: $}

Initializer expressions may contain add assignment (+=) or multiply assignment (*=) operators. These operators allow an initializer to modify an existing value. An example:

e {
  Position: {10, 20}
  Position: {x += 1, y += 2}
}

// e will have Position{11, 22}

This can be especially useful when used in combination with templates (see below):

template Tree {
  prop height = f32: 4

  // Make sure tree doesn't sink through the ground
  Position: {y += height} 
}

e {
  Position: {10, 0}
  Tree: {height: 3}
}

// e will have Position{10, 3}

Match expressions

Match expressions can be used to conditionally assign a value. An example:

const x: = 1

// y will be assigned with value 10
const y: match x {
  1: 10
  2: 20
  3: 30
}

The input to a match expression must be matched by one of its cases. If the input is not matched, script execution will fail. Match expressions can include an "any" case, which is selected when none of the other cases match:

const x: 4

// y will be assigned with value 100
const y: match x {
  1: 10
  2: 20
  3: 30
  _: 100
}

Match expressions can be used to assign components:

e {
  Position: match i {
    1: {10, 20}
    2: {20, 30}
    3: {40, 50}
  }
}

The type of a match expression is derived from the case values. When the case statements in a match contain values of multiple types, the most expressive type is selected. The algorithm for determining the most expressive type is the same as the one used to determine the type for binary expressions. When a match expression contains values with conflicting types, script execution will fail.

New expressions

A new expression is the new keyword followed by an entity statement. New expressions can be used to create entities inside of expressions. The following are examples of valid new expressions:

// Create a new anonymous entity, assign to variable x
const x: new {}

// Create a new anonymous entity with Position component, assign to variable x
const x: new {
  Position: {10, 20}
}

// Create a new entity with name Foo and Position component, assign to variable x
const x: new Foo {
  Position: {10, 20}
}

New expressions can be used anywhere where an expression of an entity type is expected. The following example shows how to use a new expression inside of an initializer:

// Create entity with TrafficLight component which has red, orange and green
// members of type entity.
e {
  TrafficLight: {
    red: new { Color: {255, 0, 0} }
    orange: new { Color: {255, 128, 0} }
    green: new { Color: {0, 255, 0} }
  }
}

The behavior of new expressions is exactly the same as entity statements in that they respect the context in which they are used, such as the current hierarchy scope and with statements:

some_parent {
  // Create new anonymous child of some_parent with Position component, assign 
  // to variable x
  const x: new { Position: {10, 20} }
}

with Position(10, 20) {
  // Create new anonymous entity with Position: {10, 20}
  const x: new { }
}

All features that are supported by entity statements are also available for new expressions, such as the ability to have children:

const x: new {
  Positiion: {10, 20}

  // Child of anonymous entity created by new expression
  child {
    Position: {20, 30}
  }
}

The primary use case for new expressions is to make it possible to create anonymous entities that can be referred to afterwards by a script. Without new expressions this is not possible, as illustrated here:

// Create anonymous entities
{
  Color: {255, 0, 0}
}
{
  Color: {255, 128, 0}
}
{
  Color: {0, 255, 0}
}

e {
  TrafficLight: {
    // Can't refer to anonymous entities here
    red: // ???
    orange: // ???
    green: // ???
  }
}

Without new expressions the only workaround is to use named entities, but this introduces overhead and increases memory footprint. With new expressions the example can be expressed with just anonymous entities:

// Create anonymous entities, store in variables
const red: new {
  Color: {255, 0, 0}
}

const orange: new {
  Color: {255, 128, 0}
}

const green: new {
  Color: {0, 255, 0}
}

e {
  TrafficLight: {
    // Assign variables to members
    red: red
    orange: orange
    green: green
  }
}

A new expression may only create a single entity, so comma operators are not supported.

String interpolation

Flecs script supports interpolated strings, which are strings that can contain expressions. String interpolation supports two forms, where one allows for easy embedding of variables, whereas the other allows for embedding any kind of expression. The following example shows an embedded variable:

const x: "The value of PI is $PI"

The following example shows how to use an expression:

const x: "The circumference of the circle is {2 * $PI * $r}"

To prevent evaluating expressions in an interpolated string, the $ and { characters can be escaped:

const x: "The value of variable \$x is $x"

Types

The type of an expression is determined by the kind of expression, its operands and the context in which the expression is evaluated. The words "type" and "component" can be used interchangeably, as every type in Flecs is a component, and every component is a type. For component types to be used with scripts, they have to be described using the meta reflection addon.

The following sections go over the different kinds of expressions and how their types are derived.

Unary expressions

Unary expressions have a single operand, with the operator preceding it. The following table shows the different unary operators with the expression type:

Binary expressions

Binary expressions have two operands. The following table shows the different binary operators with the expression type. The operand type is the type to which the operands must be castable for it to be a valid expression.

For the operators where the expression type is listed as "other" the type is derived by going through these steps:

• If the types of the operands are equal, the expression type will be the operand type.
• If the types are different:
  • For literal values, find the smallest storage type without losing precision. If operand types are now equal, use that.
  • Find the most expressive type of the two operands (see below)
  • If a cast to the most expressive type does not result in a loss of precision, use that.
  • If the types are both numbers follow these rules in order:
    • If one of the types is a floating point, use f64
    • If one of the types is an integer, use i64
    • If neither, throw a type incompatible error

For equality expressions (using the == or != operators), additional rules are used:

• If one of the operands is a bool, cast the other operand to a bool as well. This ensures that expressions such as 2 == true evaluate to true.
• Equality expressions between floating point numbers are invalid

Type expressiveness is determined by the kind of type and its storage size. The following tables show the expressiveness and storage scores:

The function to determine whether a type is implicitly castable is:

bool implicit_cast_allowed(from, to) {
  if (expressiveness(to) >= expressiveness(from)) {
    return storage(to) >= storage(from);
  } else {
    return false;
  }
}

If either the expressiveness or storage scores are negative, the operand types are not implicitly castable.

Vector operations

If the left operand of a binary expression is of a vector type, the operation will be executed for each of its operands. A vector type is a type that meets the following criteria:

• The type must be a primitive or struct type.
• If the type is a struct type:
  • All members must be of the same type.
  • The member type must be primitive.

For example:

// Valid vector type: all members are of the same primitive type
struct Position {
  float x;
  float y;
  float z;
};

// Not a valid vector type: members are not of a primitive type
struct Line {
  Position start;
  Position stop;
};

// Not a valid vector type: not all members are of the same type
struct Rgba {
  int8_t r;
  int8_t g;
  int8_t b;
  float a;
};

An example of a vector operation:

const p0 = Position: {10, 20, 30}
const p1 = p0 + 1 // {11, 21, 31}

Lvalues

Lvalues are the left side of assignments. There are two kinds of assignments possible in Flecs script:

• Variable initialization
• Initializer initialization

The type of an expression can be influenced by the type of the lvalue it is assigned to. For example, if the lvalue is a variable of type Position, the assigned initializer will also be of type Position:

const p = Position: {10, 20}

Similarly, when an initializer is used inside of an initializer, it obtains the type of the initializer element. In the following example the outer initializer is of type Line, while the inner initializers are of type Point:

const l = Line: {{10, 20}, {30, 40}}

Another notable example where this matters is for enum and bitmask constants. Consider the following example:

const c = Color: Red

Here, Red is a resolvable identifier, even though the fully qualified identifier is Color.Red. However, because the type of the lvalue is of enum type Color, the expression Red will be resolved in the scope of Color.

Functions

Expressions can call functions. Functions in Flecs script can have arguments of any type, and must return a value. The following snippet shows examples of function calls:

const x: sqrt(100)
const x: pow(100, 2)
const x: add({10, 20}, {30, 40})

Currently functions can only be defined outside of scripts by the Flecs Script API. Flecs comes with a set of builtin and math functions. Math functions are defined by the script math addon, which must be explicitly enabled by defining FLECS_SCRIPT_MATH.

A function can be created in code by doing:

ecs_function(world, {
    .name = "sum",
    .return_type = ecs_id(ecs_i64_t),
    .params = {
        { .name = "a", .type = ecs_id(ecs_i64_t) },
        { .name = "b", .type = ecs_id(ecs_i64_t) }
    },
    .callback = sum
});

Function implementations looks like this:

void sum(
    const ecs_function_ctx_t *ctx,
    int32_t argc,
    const ecs_value_t *argv,
    ecs_value_t *result)
{
    int64_t *a = argv[0].ptr;
    int64_t *b = argv[1].ptr;
    *(int64_t*)result->ptr = *a + *b;
}

Methods

Methods are functions that are called on instances of the method's type. The first argument of a method is the instance on which the method is called. The following snippet shows examples of method calls:

const x: v.length()
const x: v1.add(v2)

Just like functions, methods can currently only be defined outside of scripts by using the Flecs Script API.

A method can be created in code by doing:

ecs_method(world, {
    .name = "add",
    .parent = ecs_id(ecs_i64_t), // Add method to i64
    .return_type = ecs_id(ecs_i64_t),
    .params = {
        { .name = "a", .type = ecs_id(ecs_i64_t) }
    },
    .callback = sum
});

Vector functions

Vector functions are functions that accept arguments of a builtin ScriptVectorType type. This allows these functions to accept any type that is a valid vector type (see Vector operations).

Here is a usage example of a vector function:

const red = Rgb: {255, 0, 0}
const blue = Rgb: {0, 0, 255}
const purple: lerp(red, blue, 0.5)

When a vector function is called, all of the arguments provided to parameters of ScriptVectorType must be of the same type. The following code is therefore not valid:

const red = Rgb: {255, 0, 0}
const p = Position: {10, 20, 30}
const red_p: lerp(red, p, 0.5) // Illegal: red and p are of different types

Vector functions are registered like normal functions, but instead of specifying a callback, the application sets vector_callbacks. An example:

ecs_function(world, {
    .name = "lerp",
    .return_type = EcsScriptVectorType,
    .params = {
        { "a", EcsScriptVectorType },
        { "b", EcsScriptVectorType },
        { "t", ecs_id(ecs_f64_t) },
    },
    .vector_callbacks = {
        [EcsF32] = lerp_f32,
        [EcsF64] = lerp_f64
    }
});

The signature for vector functions accepts an additional argument for the number of elements in the vector type:

void lerp_f32(
    const ecs_function_ctx_t *ctx,
    int32_t argc,
    const ecs_value_t *argv,
    ecs_value_t *result,
    int32_t elem_count)
{
    float *a = argv[0].ptr;
    float *b = argv[1].ptr;
    double t = *(double*)argv[2].ptr;
    float *r = result->ptr;
    for (int i = 0; i < elem_count; i ++) {
        r[i] = a[i] + t * (b[i] - a[i]);
    }
}

In the function documentation below the type of vector parameters is written as [].

Builtin functions and constants

The following table lists builtin core functions in the flecs.script.core namespace:

The following table lists builtin methods on the flecs.meta.entity type:

The following table lists doc methods on the flecs.meta.entity type:

To use the doc functions, make sure to use a Flecs build compiled with FLECS_DOC (enabled by default).

The following table lists math functions in the flecs.script.math namespace:

The following table lists the constants in the flecs.script.math namespace:

The following table lists methods of the flecs.script.math.Rng type:

The random number generator can be used like this:

const rng = flecs.script.math.Rng: {}
const x: $rng.f(1.0)

To use the math functions, make sure to use a Flecs build compiled with the FLECS_SCRIPT_MATH addon (disabled by default) and that the module is imported:

ECS_IMPORT(world, FlecsScriptMath);

Templates

Templates are parameterized scripts that can be used to create procedural assets. Templates can be created with the template keyword. Example:

template Square {
  Color: {255, 0, 0}
  Rectangle: {width: 100, height: 100}
}

The script contents of an template are not ran immediately. Instead they are ran whenever an template is instantiated. To instantiate an template, add it as a regular component to an entity:

my_entity {
  Square
}

// is equivalent to

my_entity {
  Color: {255, 0, 0}
  Rectangle: {width: 100, height: 100}
}

Templates are commonly used in combination with the kind syntax:

Square my_entity

Templates can be parameterized with properties. Properties are variables that are exposed as component members. To create a property, use the prop keyword. Example:

template Square {
  prop size: 10
  prop color = Color: {255, 0, 0}

  $color
  Rectangle: {width: size, height: size}
}

Square my_entity(size: 20, color: {38, 25, 13})

Just like const variables, prop variables can explicitly specify a type or implicitly derive their type from the assigned (default) value.

Template scripts can do anything a regular script can do, including creating child entities. The following example shows how to create an template that uses a nested template to create children:

template Tree {
  prop height: 10

  const wood_color = Color: {38, 25, 13}
  const leaves_color = Color: {51, 76, 38}

  const canopy_height: 2
  const trunk_height: $height - $canopy_height
  const trunk_width: 2

  Trunk {
    Position: {0, ($height / 2), 0}
    Rectangle: {$trunk_width, $trunk_height}
    $wood_color
  }

  Canopy {
    const canopy_y: $trunk_height + ($canopy_height / 2)

    Position3: {0, $canopy_y, 0}
    Box: {$canopy_width, $canopy_height}
    $leaves_color
  }
}

template Forest {
  Tree(height: 5) {
    Position: {x: -10}
  }

  Tree(height: 10) {
    Position: {x: 0}
  }

  Tree(height: 7) {
    Position: {x: 10}
  }
}

Forest my_forest

Module statement

The module statement puts all contents of a script in a module. Example:

module components.transform

// Creates components.transform.Position
struct Position {
  x = f32
  y = f32
}

The components.transform entity will be created with the Module tag.

Using statement

The using keyword imports a namespace into the current namespace. Example:

// Without using
flecs.meta.struct Position {
  x = flecs.meta.f32
  y = flecs.meta.f32
}

// With using
using flecs.meta

struct Position {
  x = f32
  y = f32
}

The using keyword only applies to the scope in which it is specified. Example:

// Scope without using
my_engine {
  game.engines.FtlEngine: {active: true}
}

// Scope with using
my_spaceship {
  using game.engines

  FtlEngine: {active: true}
}

A using statement may end with a wildcard (*). This will import all namespaces matching the path. Example:

using flecs.*

struct Position {
  x = f32
  y = f32
}

With statement

When you're building a scene or asset you may find yourself often repeating the same components for multiple entities. To avoid this, a with statement can be used. For example:

with SpaceShip {
  MillenniumFalcon {}
  UssEnterprise {}
  UssVoyager {}
  Rocinante {}
}

This is equivalent to doing:

MillenniumFalcon {
  SpaceShip
}

UssEnterprise {
  SpaceShip
}

UssVoyager {
  SpaceShip
}

Rocinante {
  SpaceShip
}

With statements can contain multiple tags:

with SpaceShip, HasWeapons {
  MillenniumFalcon {}
  UssEnterprise {}
  UssVoyager {}
  Rocinante {}
}

With statements can contain component values, specified between parentheses:

with Color(38, 25, 13) {
  pillar_1 {}
  pillar_2 {}
  pillar_3 {}
}

Variables

Scripts can contain variables, which are useful for often repeated values. Variables are created with the const keyword. Example:

const pi: 3.1415926

my_entity {
  Rotation: {angle: pi}
}

Variables can be combined with expressions:

const pi: 3.1415926
const pi_2: $pi * 2

my_entity {
  Rotation: {angle: pi / 2}
}

In the above examples, the type of the variable is inferred. Variables can also be provided with an explicit type:

const wood = Color: {38, 25, 13}

When the name of a variable clashes with an entity, it can be disambiguated by prefixing the variable name with a $:

const pi: 3.1415926
const pi_2: $pi * 2

pi {
  Rotation: {angle: $pi / 2}
}

Variables can be used in component values as shown in the previous examples, or can be used directly as component. When used like this, the variable name must be prefixed with a $. Example:

const wood = Color: {38, 25, 13}

my_entity {
  $wood
}

// is equivalent to

my_entity {
  Color: {38, 25, 13}
}

Additionally, variables can also be used in combination with with statements. When used like this the variable name must also be prefixed with a $:

const wood = Color: {38, 25, 13}

with $color {
  pillar_1 {}
  pillar_2 {}
  pillar_3 {}
}

Exported variables

Variables can be exported by prefixing a variable declaration with the export keyword. Exported variables can be accessed by the application and from other scripts. The following example shows an exported variable:

// Script 1
export const pi: 3.1415926

This variable can now be accessed from another script:

// Script 2
const pi_2: pi * 2

Exported variables are created as children of the scope in which they are defined:

math {
  export const pi: 3.1415926
}

This will make the variable available to other scripts as math.pi.

The ecs_const_var_init function is used to create exported variables. The following example shows how the same variable can be created from C code:

double pi_value = 3.1415926;

ecs_const_var(world, {
  .name = "pi",
  .parent = ecs_lookup(world, "math"),
  .type = ecs_id(ecs_f64_t),
  .value = &pi_value
});

Exported variables can be used as configuration that is loaded into an application from a script. The following example shows how to load an exported variable from C after it has been defined in a script or has been created with ecs_const_var_init:

ecs_entity_t pi = ecs_lookup(world, "math.pi");
ecs_value_t v = ecs_const_var_get(world, pi);
double *value = v.ptr;
if (value) {
  // Use value
}

The following example shows how exported variables can be used in combination with modules in C++:

struct math {
  inline static double pi;

  math(flecs::world& world) {
    world.script()
      .filename("math.flecs")
      .run();

    world.const_var("pi", pi);
  }
}

// Import module
world.import<math>();


// Use value
double pi_2 = math::pi * 2;

Component values

A script can use the value of a component that is looked up on a specific entity. The following example fetches the width and depth members from the Level component, that is fetched from the Game entity:

grid {
  Grid: { Game[Level].width, Game[Level].depth }
}

To reduce the number of component lookups in a script, the component value can be stored in a variable:

const level: Game[Level]

tiles {
  Grid: { width: $level.width, $level.depth, prefab: Tile }
}

The requested component is stored by value, not by reference. Adding or removing components to the entity will not invalidate the component data. If the requested component does not exist on the entity, script execution will fail.

If statement

Parts of a script can be conditionally executed with an if statement. Example:

const daytime = bool: false

lantern {
  Color: {210, 255, 200}

  if $daytime {
    Emissive: { value: 0 }
  } else {
    Emissive: { value: 1 }
  }
}

If statements can be chained with else if:

const state: 0

traffic_light {
  if $state == 0 {
    Color: {0, 1, 0}
  } else if $state == 1 {
    Color: {0.5, 0.5, 0}
  } else if $state == 1 {
    Color: {1, 0, 0}
  }
}

For statement

Parts of a script can be repeated with a for loop. Example:

for i in 0..10 {
  Lantern() {
    Position: {x: $i * 5}
  }
}

The values specified in the range can be an expression:

for i in 0..$count {
  // ...
}

When creating entities in a for loop, ensure that they are unique or the for loop will overwrite the same entity:

for i in 0..10 {
  // overwrites entity "e" 10 times
  e: { Position: {x: $i * 5} }
}

To avoid this, scripts can either create anonymous entities:

for i in 0..10 {
  // creates 10 anonymous entities
  _ { Position: {x: $i * 5} }
}

Or use a unique string expression for the entity name:

for i in 0..10 {
  // creates entities with names e_0, e_1, ... e_9
  "e_$i" { Position: {x: $i * 5} }
}

Default components

A scope can have a default component, which means entities in that scope can assign values of that component without having to specify the component name.

There are different ways to specify a default component. One way is to use a with statement. Default component values are assigned with the = operator, and don't need a {} surrounding the value. Example:

with Position {
  ent_a = 10, 20
  ent_b = 20, 30
}

Another way a default components are derived is from the entity kind. If an entity is specified with a kind, a DefaultChildComponent component will be looked up on the kind to find the default component for the scope, if any. For example:

// Create a PositionList tag with a DefaultChildComponent
PositionList {
  DefaultChildComponent: {Position}
}

// Derive default component for scope from PositionList
PositionList plist {
  ent_a = 10, 20
  ent_b = 10, 20
  ent_c = 10, 20
}

A common use of default components is when creating structs. struct is a component with member as default child component. Example:

struct Position {
  x = f32
  y = f32
}

// is equivalent to

struct Position {
  member x(f32)
  member y(f32)
}

Note how member is also used as kind for the children. This means that children of x and y derive their default child component from member, which is set to member. This makes it easy to create nested members:

struct Line {
  start {
    x = f32
    y = f32
  }
  stop {
    x = f32
    y = f32
  }
}

// is equivalent to

struct Line {
  member start {
    member x(f32)
    member y(f32)
  }
  member stop {
    member x(f32)
    member y(f32)
  }
}

Semicolon operator

Multiple statements can be combined on a single line when using the semicolon operator. Example:

my_spaceship {
  SpaceShip; HasFtl
}

Comma operator

The comma operator can be used as a shortcut to create multiple entities in a scope. Example:

my_spaceship {
  pilot_a,
  pilot_b,
  pilot_c
}

// is equivalent to

my_spaceship {
  pilot_a {}
  pilot_b {}
  pilot_c {}
}

This allows for a more natural way to describe things like enum types:

enum Color {
  Red,
  Green,
  Blue
}

// is equivalent to

enum Color {
  constant Red
  constant Green
  constant Blue
}

API

This section goes over how to run scripts in an application.

Run once

To run a script once, use the ecs_script_run function. Example:

const char *code = "my_spaceship {}";

if (ecs_script_run(world, "my_script_name", code)) {
  // error
}

Alternatively a script can be ran directly from a file:

if (ecs_script_run_file(world, "my_script.flecs")) {
  // error
}

If a script fails, the entities created by the script will not be automatically deleted. When a script contains templates, script resources will not get cleaned up until the entities associated with the templates are deleted.

Run multiple times

A script can be ran multiple times by using the ecs_script_parse and ecs_script_eval functions. Example:

const char *code = "my_spaceship {}";

ecs_script_t *script = ecs_script_parse(
  world, "my_script_name", code); 
if (!script) {
  // error
}

if (ecs_script_eval(script)) {
  // error
}

// Run again
if (ecs_script_eval(script)) {
}

// Free script resources
ecs_script_free(script);

If a script fails, the entities created by the script will not be automatically deleted. When a script contains templates, script resources will not get cleaned up until the entities associated with the templates are deleted.

Managed script

Managed scripts are scripts that are associated with an entity, and can be ran multiple times. Entities created by a managed script are tagged with the script. When script execution fails, the entities associated with the script will be deleted. Additionally, if after executing the script again an entity is no longer created by the script, it will also be deleted.

To run a managed script, do:

const char *code = "my_spaceship {}";

ecs_entity_t s = ecs_script(world, {
  .code = code
});

if (!s) {
  // error
}

To update the code of a managed script, use the ecs_script_update function:

if (ecs_script_update(world, s, 0, new_code)) {
  // error
}

When a script contains templates, script resources will not get cleaned up until the entities associated with the templates are deleted.