ChaiScript/cheatsheet.md

ChaiScript Versioning

ChaiScript tries to follow the Semantic Versioning scheme. This basically means:

• Major Version Number: API changes / breaking changes
• Minor Version Number: New Features
• Patch Version Number: Minor changes / enhancements

Initializing ChaiScript

chaiscript::ChaiScript chai; // initializes ChaiScript, adding the standard ChaiScript types (map, string, ...)

Note that ChaiScript cannot be used as a global / static object unless it is being compiled with CHAISCRIPT_NO_THREADS.

Adding Things To The Engine

Adding a Function / Method / Member

General

chai.add(chaiscript::fun(&function_name), "function_name");
chai.add(chaiscript::fun(&Class::method_name), "method_name");
chai.add(chaiscript::fun(&Class::member_name), "member_name");

Bound Member Functions

chai.add(chaiscript::fun(&Class::method_name, Class_instance_ptr), "method_name");
chai.add(chaiscript::fun(&Class::member_name, Class_instance_ptr), "member_name");

With Overloads

Preferred

chai.add(chaiscript::fun<ReturnType (ParamType1, ParamType2)>(&function_with_overloads), "function_name");

Alternative

chai.add(chaiscript::fun(static_cast<ReturnType (*)(ParamType1, ParamType2)>(&function_with_overloads)), "function_name");

This overload technique is also used when exposing base members using derived type

struct Base
{
  int data;
};

struct Derived : public Base
{};

chai.add(chaiscript::fun(static_cast<int(Derived::*)>(&Derived::data)), "data");

Lambda

chai.add(
  chaiscript::fun<std::function<std::string (bool)>>(
    [](bool type) {
      if (type) { return "x"; }
      else { return "y"; }
    }), "function_name");

Constructors

chai.add(chaiscript::constructor<MyType ()>(), "MyType");
chai.add(chaiscript::constructor<MyType (const MyType &)>(), "MyType");

Adding Types

It's not strictly necessary to add types, but it helps with many things. Cloning, better errors, etc.

chai.add(chaiscript::user_type<MyClass>(), "MyClass");

Adding Type Conversions

User-defined type conversions are possible, defined in either script or in C++.

ChaiScript Defined Conversions

Function objects (including lambdas) can be used to add type conversions from inside of ChaiScript:

add_type_conversion(type("string"), type("Type_Info"), fun(s) { return type(s); });

C++ Defined Conversions

Invoking a C++ type conversion possible with static_cast

chai.add(chaiscript::type_conversion<T, bool>());

Calling a user-defined type conversion that takes a lambda

chai.add(chaiscript::type_conversion<TestBaseType, Type2>([](const TestBaseType &t_bt) { /* return converted thing */ }));

Class Hierarchies

If you want objects to be convertable between base and derived classes, you must tell ChaiScript about the relationship.

chai.add(chaiscript::base_class<Base, Derived>());

If you have multiple classes in your inheritance graph, you will probably want to tell ChaiScript about all relationships.

chai.add(chaiscript::base_class<Base, Derived>());
chai.add(chaiscript::base_class<Derived, MoreDerived>());
chai.add(chaiscript::base_class<Base, MoreDerived>());

Helpers

A helper function exists for strongly typed and ChaiScript Vector function conversion definition:

chai.add(chaiscript::vector_conversion<std::vector<int>>());

A helper function also exists for strongly typed and ChaiScript Map function conversion definition:

chai.add(chaiscript::map_conversion<std::map<std::string, int>>());

This allows you to pass a ChaiScript function to a function requiring std::vector<int>

Adding Objects

chai.add(chaiscript::var(somevar), "somevar"); // copied in
chai.add(chaiscript::var(std::ref(somevar)), "somevar"); // by reference, shared between C++ and chai
auto shareddouble = std::make_shared<double>(4.3);
chai.add(chaiscript::var(shareddouble), "shareddouble"); // by shared_ptr, shared between c++ and chai
chai.add(chaiscript::const_var(somevar), "somevar"); // copied in and made const
chai.add_global_const(chaiscript::const_var(somevar), "somevar"); // global const. Throws if value is non-const, throws if object exists
chai.add_global(chaiscript::var(somevar), "somevar"); // global non-const, throws if object exists
chai.set_global(chaiscript::var(somevar), "somevar"); // global non-const, overwrites existing object

Adding Namespaces

Namespaces will not be populated until import is called. This saves memory and computing costs if a namespace is not imported into every ChaiScript instance.

chai.register_namespace([](chaiscript::Namespace& math) {
    math["pi"] = chaiscript::const_var(3.14159);
    math["sin"] = chaiscript::var(chaiscript::fun([](const double x) { return sin(x); })); },
    "math");

Import namespace in ChaiScript

import("math")
print(math.pi) // prints 3.14159

Using STL

ChaiScript recognizes many types from STL, but you have to add specific instantiation yourself.

typedef std::vector<std::pair<int, std::string>> data_list;
data_list my_list{ make_pair(0, "Hello"), make_pair(1, "World") };
chai.add(chaiscript::bootstrap::standard_library::vector_type<data_list>("DataList"));
chai.add(chaiscript::bootstrap::standard_library::pair_type<data_list::value_type>("DataElement"));
chai.add(chaiscript::var(&my_list), "data_list");
chai.eval(R"_(
    for(var i=0; i<data_list.size(); ++i)
    {
      print(to_string(data_list[i].first) + " " + data_list[i].second)
    }
  )_");

Executing Script

General

chai.eval("print(\"Hello World\")");
chai.eval(R"(print("Hello World"))");

Unboxing Return Values

Returns values are of the type Boxed_Value which is meant to be opaque to the programmer. Use one of the unboxing methods to access the internal data.

Prefered

chai.eval<double>("5.3 + 2.1"); // returns 7.4 as a C++ double

Alternative

auto v = chai.eval("5.3 + 2.1");
chai.boxed_cast<double>(v); // extracts double value from boxed_value and applies known conversions
chaiscript::boxed_cast<double>(v); // free function version, does not know about conversions

Converting Between Algebraic Types

chaiscript::Boxed_Number(chai.eval("5.3 + 2.1")).get_as<int>(); // works with any number type
// which is equivalent to, but much more automatic than:
static_cast<int>(chai.eval<double>("5.3+2.1")); // this version only works if we know that it's a double

Conversion Caveats

Conversion to std::shared_ptr<T> & is supported for function calls, but if you attempt to keep a reference to a shared_ptr<> you might invoke undefined behavior

// ok this is supported, you can register it with chaiscript engine
void nullify_shared_ptr(std::shared_ptr<int> &t) {
  t = nullptr
}

int main()
{
  // do some stuff and create a chaiscript instance
  std::shared_ptr<int> &ptr = chai.eval<std::shared_ptr<int> &>(somevalue);
  // DO NOT do this. Taking a non-const reference to a shared_ptr is not 
  // supported and causes undefined behavior in the chaiscript engine
}

Sharing Values

double &d = chai.eval("var i = 5.2; i"); // d is now a reference to i in the script
std::shared_ptr<double> d = chai.eval("var i = 5.2; i"); // same result but reference counted

d = 3;
chai.eval("print(i)"); // prints 3

Catching Eval Errors

try {
  chai.eval("2.3 + \"String\"");
} catch (const chaiscript::exception::eval_error &e) {
  std::cout << "Error\n" << e.pretty_print() << '\n';
}

Catching Errors Thrown From Script

try {
  chai.eval("throw(runtime_error(\"error\"))", chaiscript::exception_specification<int, double, float, const std::string &, const std::exception &>());
} catch (const double e) {
} catch (int) {
} catch (float) {
} catch (const std::string &) {
} catch (const std::exception &e) {
  // This is the one that will be called in the specific throw() above
}

Sharing Functions

auto p = chai.eval<std::function<std::string (double)>>("to_string"); 
p(5); // calls chaiscript's 'to_string' function, returning std::string("5")

Note: backtick treats operators as normal functions

auto p = chai.eval<std::function<int (int, int)>>("`+`"); 
p(5, 6); // calls chaiscript's '+' function, returning 11

auto p = chai.eval<std::function<std::string (int, double)>>("fun(x,y) { to_string(x) + to_string(y); }");
p(3,4.2); // evaluates the lambda function, returning the string "34.2" to C++

Language Reference

Variables

var i; // uninitialized variable, can take any value on first assignment;
auto j; // equiv to var

var k = 5; // initialized to 5 (integer)
var l := k; // reference to k
auto &m = k; // reference to k

global g = 5; // creates a global variable. If global already exists, it is not re-added
global g = 2; // global 'g' now equals 2

global g2;
if (g2.is_var_undef()) { g2 = 4; } // only initialize g2 once, if global decl hit more than once

GLOBAL g3; // all upper case version also accepted

Looping

// c-style for loops
for (var i = 0; i < 100; ++i) { print(i); }

// while
while (some_condition()) { /* do something */ }

// ranged for
for (i : [1, 2, 3]) { print(i); }

Each of the loop styles can be broken using the break statement. For example:

while (some_condition()) {
  /* do something */
  if (another_condition()) { break; }
}

Conditionals

if (expression) { }

// C++17-style init-if blocks
// Value of 'statement' is scoped for entire `if` block
if (statement; expression) { }

Switch Statements

var myvalue = 2
switch (myvalue) {
    case (1) {
        print("My Value is 1");
        break;
    }
    case (2) {
        print("My Value is 2");
        break;
    }
    default {
        print("My Value is something else.";
    }
}

Built-in Types

There are a number of built-in types that are part of ChaiScript.

Vectors and Maps

var v = [1,2,3u,4ll,"16", `+`]; // creates vector of heterogenous values
var m = ["a":1, "b":2]; // map of string:value pairs

// Add a value to the vector by value.
v.push_back(123);

// Add an object to the vector by reference.
v.push_back_ref(m);

Numbers

Floating point values default to double type and integers default to int type. All C++ suffixes such as f, ll, u as well as scientific notation are supported

1.0 // double
1.0f // float
1.0l // long double
1 // int
1u // unsigned int
1ul // unsigned long
1ull // unsigned long long

Literals are automatically sized, just as in C++. For example: 10000000000 is > 32bits and the appropriate type is used to hold it on your platform.

Functions

Note that any type of ChaiScript function can be passed freely to C++ and automatically converted into a std::function object.

General

def myfun(x, y) { x + y; } // last statement in body is the return value
def myfun(x, y) { return x + y; } // equiv 

Optionally Typed

def myfun(x, int y) { x + y; } // requires y to be an int

With Guards

def myfun(x, int y) : y > 5 { x - y; } // only called if y > 5

Methods

Methods and functions are mostly equivalent

def string::add(int y) { this + to_string(y); }
def add(string s, int y) { s + to_string(y); } //equiv functionality

// calling new function/method
"a".add(1); // returns a1
add("a", 1); // returns a1, either calling syntax works with either def above

Lambdas

var l = fun(x) { x * 15; }
l(2) // returns 30

var a = 13
var m = fun[a](x) { x * a; } 
m(3); // a was captured (by reference), returns 39

var n = bind(fun(x,y) { x * y; }, _, 10);
n(2); // returns 20 

ChaiScript Defined Types (Classes)

ChaiScript supports user-defined types using the class keyword. Classes can have attributes, constructors, methods, guards, and operator overloads. There is no inheritance between ChaiScript-defined types, but C++ class hierarchies can be exposed (see Class Hierarchies above).

Class Definition (Block Syntax)

Define a type with attributes, a constructor, and methods inside a class block. The keywords var, attr, and auto are interchangeable for declaring attributes.

class Rectangle {
  var width
  var height
  def Rectangle(w, h) { this.width = w; this.height = h; }
  def Rectangle() { this.width = 0; this.height = 0; }
  def area() { this.width * this.height; }
}

var r = Rectangle(3, 4)
print(r.area()) // prints 12

Class Definition (Open Syntax)

Equivalently, attributes and methods can be defined outside a block using the TypeName:: prefix.

attr Circle::radius
def Circle::Circle(r) { this.radius = r; }
def Circle::circumference() { 2.0 * 3.14159 * this.radius; }

Methods can also be added to an existing class after its initial definition:

def Rectangle::perimeter() { 2 * (this.width + this.height); }

Using

var m = Rectangle(5, 10)
print(m.area())       // prints 50 — method call syntax
print(area(m))        // prints 50 — function call syntax (equivalent)

Constructor and Method Guards

Constructors and methods can have guard expressions (after :) that control which overload is selected at call time.

class Clamped {
  var value
  def Clamped(x) : x >= 0 { this.value = x; }
  def Clamped(x) { this.value = 0; }  // fallback when guard fails
}

Clamped(5).value   // 5
Clamped(-3).value  // 0

class Abs {
  var x
  def Abs(v) { this.x = v; }
  def get() : this.x >= 0 { this.x; }
  def get() { -this.x; }
}

Operator Overloading

Operators can be overloaded on user-defined types using backtick-quoted operator names.

class Vec2 {
  var x
  var y
  def Vec2(x, y) { this.x = x; this.y = y; }
  def `+`(other) { Vec2(this.x + other.x, this.y + other.y); }
}

var v = Vec2(1, 2) + Vec2(3, 4)  // v.x == 4, v.y == 6

Operators can also be overloaded as free functions with guards:

def `-`(a, b) : is_type(a, "Vec2") && is_type(b, "Vec2") {
  Vec2(a.x - b.x, a.y - b.y)
}

Cloning Objects

Use clone() to create a deep copy of a ChaiScript-defined object.

var original = Rectangle(10, 20)
var copy = clone(original)
copy.width = 99
print(original.width)  // still 10

Dynamic Objects

All ChaiScript defined types and generic Dynamic_Object support dynamic parameters

var o = Dynamic_Object();
o.f = fun(x) { print(x); }
o.f(3); // prints "3"

Implicit 'this' is allowed:

var o = Dynamic_Object();
o.x = 3;
o.f = fun(y) { print(this.x + y); }
o.f(10); // prints 13

Namespaces

Namespaces in ChaiScript are Dynamic Objects with global scope

namespace("math") // create a new namespace

math.square = fun(x) { x * x } // add a function to the "math" namespace
math.sum_squares = fun(x, y) { math.square(x) + math.square(y) }

print(math.square(4)) // prints 16
print(math.sum_squares(2, 5)) // prints 29

Option Explicit

If you want to disable dynamic parameter definitions, you can set_explicit.

class My_Class {
  def My_Class() {
    this.set_explicit(true);
    this.x = 2; // this would fail with explicit set to true
  }
};

method_missing

A function of the signature method_missing(object, name, param1, param2, param3) will be called if an appropriate method cannot be found

def method_missing(int i, string name, Vector v) {
  print("method_missing(${i}, ${name}), ${v.size()} params");
}

5.bob(1,2,3); // prints "method_missing(5, bob, 3 params)"

method_missing signature can be either 2 parameters or 3 parameters. If the signature contains two parameters it is treated as a property. If the property contains a function then additional parameters are passed to the contained function.

If both a 2 parameter and a 3 parameter signature match, the 3 parameter function always wins.

Context

• __LINE__ Current file line number
• __FILE__ Full path of current file
• __CLASS__ Name of current class
• __FUNC__ Name of current function

Built-in Functions

Evaluation

eval("4 + 5") // dynamically eval script string and returns value of last statement
eval_file("filename") // evals file and returns value of last statement
use("filename") // evals file exactly once and returns value of last statement
                // if the file had already been 'used' nothing happens and undefined is returned

Both use and eval_file search the 'usepaths' passed to the ChaiScript constructor

Reflection and Introspection

ChaiScript provides built-in reflection capabilities for inspecting types, functions, and objects at runtime.

Type Inspection

type_name(x)            // returns the type name of a value as a string
is_type(x, "typename")  // returns true if x is of the named type
type("typename")        // returns a Type_Info object for the named type

// Examples
type_name(1)            // "int"
type_name("hello")      // "string"
is_type(1, "int")       // true
is_type(1, "string")    // false

Object Inspection Methods

Every object in ChaiScript supports these methods:

x.get_type_info()     // returns a Type_Info object for the value
x.is_type("string")   // returns true if x is of the named type
x.is_type(string_type) // returns true if x matches the Type_Info
x.is_var_const()      // returns true if x is immutable
x.is_var_null()       // returns true if x is a null pointer
x.is_var_pointer()    // returns true if x is stored as a pointer
x.is_var_reference()  // returns true if x is stored as a reference
x.is_var_undef()      // returns true if x is undefined

Type_Info

Type_Info objects describe a type. You can get them via type("typename") or x.get_type_info().

var ti = type("int")
ti.name()              // ChaiScript registered name, e.g. "int"
ti.cpp_name()          // mangled C++ type name
ti.cpp_bare_name()     // C++ name without const/pointer/reference
ti.bare_equal(other)   // true if types match ignoring const/ptr/ref
ti.is_type_const()     // true if type is const
ti.is_type_reference() // true if type is a reference
ti.is_type_void()      // true if type is void
ti.is_type_undef()     // true if type is undefined
ti.is_type_pointer()   // true if type is a pointer
ti.is_type_arithmetic() // true if type is arithmetic (int, double, etc.)

Built-in type constants are available: int_type, double_type, string_type, bool_type, Object_type, Function_type, vector_type, map_type.

Function Introspection

Function objects support these introspection methods:

f.get_arity()                // number of parameters (-1 for variadic)
f.get_param_types()          // Vector of Type_Info (first element is return type)
f.get_contained_functions()  // Vector of overloaded functions (empty if not a conglomerate)
f.has_guard()                // true if the function has a guard condition
f.get_guard()                // returns the guard function (throws if none)
f.get_annotation()           // returns the annotation description
f.call([param1, param2])     // call the function with a vector of parameters

// Examples
def my_func(a, b) { return a + b; }
my_func.get_arity()          // 2
my_func.has_guard()          // false

def guarded(x) : x > 0 { return x; }
guarded.has_guard()          // true
guarded.get_guard().get_arity() // 1

// Calling functions dynamically
`+`.call([1, 2])             // 3

System Introspection

get_functions()        // returns a Map of all registered functions (name -> function)
get_objects()          // returns a Map of all scripting objects (name -> value)
function_exists("f")   // returns true if a function named "f" is registered
call_exists(`f`, args) // returns true if f can be called with the given args
dump_system()          // prints all registered functions to stdout
dump_object(x)         // prints information about a value to stdout

// Examples
var funcs = get_functions()
funcs["print"]                  // the print function object
function_exists("print")        // true
call_exists(`+`, 1, 2)          // true

Dynamic_Object Reflection

ChaiScript-defined classes are Dynamic_Objects internally. They support:

obj.get_type_name()    // returns the ChaiScript class name (e.g. "MyClass")
obj.get_attrs()        // returns a Map of all attributes
obj.has_attr("name")   // returns true if the attribute exists
obj.get_attr("name")   // returns the value of the attribute
obj.set_explicit(true) // disables dynamic attribute creation
obj.is_explicit()      // returns true if explicit mode is enabled

// Example
class MyClass {
  var x
  def MyClass() { this.x = 10; }
}
var m = MyClass()
m.get_type_name()      // "MyClass"
m.get_attrs()          // map containing "x" -> 10
type_name(m)           // "Dynamic_Object" (the underlying C++ type)
m.is_type("MyClass")   // true (checks the ChaiScript class name)

JSON

• from_json converts a JSON string into its strongly typed (map, vector, int, double, string) representations
• to_json converts a ChaiScript object (either a Object or one of map, vector, int, double, string) tree into its JSON string representation

Extras

ChaiScript itself does not provide a link to the math functions defined in <cmath>. You can either add them yourself, or use the ChaiScript_Extras helper library. (Which also provides some additional string functions.)