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Daniel Kang edited this page Jan 21, 2019 · 10 revisions

Tengo Objects

All object types in Tengo implement Object interface. The following runtime types implement Object interface too.

Object Iterface

TypeName() string

TypeName method should return the name of the type. Type names are not directly used by the runtime (except when it reports a run-time error), but, it is generally a good idea to keep it short but distinguishable from other types.

String() string

String method should return a string representation of the underlying value. The value returned by String method will be used whenever string formatting for the value is required, most commonly when being converted into String value.

BinaryOp(op token.Token, rhs Object) (res Object, err error)

In Tengo, a type can overload binary operators (+, -, *, /, %, &, |, ^, &^, >>, <<, >, >=; note that < and <= operators are not overloadable as they're simply implemented by switching left-hand side and right-hand side of >/>= operator) by implementing BinaryOp method. BinaryOp method takes the operator op and the right-hand side object rhs, and, should return a resulting value res.

Error value vs runtime error

If BinaryOp method returns an error err (the second return value), it will be treated as a run-time error, which will halt the execution (VM.Run() error) and will return the error to the user. All runtime type implementations, for example, will return an ErrInvalidOperator error when the given operator is not supported by the type.

Alternatively the method can return an Error value as its result res (the first return value), which will not halt the runtime and will be treated like any other values. As a dynamically typed language, the receiver (another expression or statement) can determine how to translate Error value returned from binary operator expression.

IsFalsy() bool

IsFalsy method should return true if the underlying value is considered to be falsy.

Equals(o Object) bool

Equals method should return true if the underlying value is considered to be equal to the underlying value of another object o. When comparing values of different types, the runtime does not guarantee or force anything, but, it's generally a good idea to make the result consistent. For example, a custom integer type may return true when comparing against String value, but, it should return the same result for the same inputs.

Copy() Object

Copy method should a new copy of the same object. All primitive and composite value types implement this method to return a deep-copy of the value, which is recommended for other user types (as copy builtin function uses this Copy method), but, it's not a strict requirement by the runtime.

User Types

Basically Tengo runtime treats and manages both the runtime types and user types exactly the same way as long as they implement Object interface. You can add values of the custom user types (via either Script.Add method or by directly manipulating the symbol table and the global variables), and, use them directly in Tengo code.

Here's an example user type, Time:

type Time struct {
	Value time.Time
}

func (t *Time) TypeName() string {
	return "time"
}

func (t *Time) String() string {
	return t.Value.Format(time.RFC3339)
}

func (t *Time) BinaryOp(op token.Token, rhs objects.Object) (objects.Object, error) {
	switch rhs := rhs.(type) {
	case *Time:
		switch op {
		case token.Sub:
			return &objects.Int{
				Value: t.Value.Sub(rhs.Value).Nanoseconds(),
			}, nil
		}
	case *objects.Int:
		switch op {
		case token.Add:
			return &Time{
				Value: t.Value.Add(time.Duration(rhs.Value)),
			}, nil
		case token.Sub:
			return &Time{
				Value: t.Value.Add(-time.Duration(rhs.Value)),
			}, nil
		}
	}

	return nil, objects.ErrInvalidOperator
}

func (t *Time) IsFalsy() bool {
	return t.Value.IsZero()
}

func (t *Time) Equals(o objects.Object) bool {
	if o, ok := o.(*Time); ok {
		return t.Value.Equal(o.Value)
	}

	return false
}

func (t *Time) Copy() objects.Object {
	return &Time{Value: t.Value}
}

Now the Tengo runtime recognizes Time type, and, any Time values can be used directly in the Tengo code:

s := script.New([]byte(`
	a := currentTime + 10000  // Time + Int = Time
	b := a - currentTime      // Time - Time = Int
`))

// add Time value 'currentTime'
err := s.Add("currentTime", &Time{Value: time.Now()}) 
if err != nil {
	panic(err)
}

c, err := s.Run()
if err != nil {
	panic(err)
}

fmt.Println(c.Get("b")) // "10000"

Callable Types

type Callable interface {
	Call(args ...Object) (ret Object, err error)
}

Any types that implement Callable interface (in addition to Object interface), values of such types can be used as if they are functions.

To make Time a callable value, add Call method to the previous implementation:

func (t *Time) Call(args ...objects.Object) (ret objects.Object, err error) {
	if len(args) != 1 {
		return nil, objects.ErrWrongNumArguments
	}

	format, ok := objects.ToString(args[0])
	if !ok {
		return nil, objects.ErrInvalidTypeConversion
	}

	return &objects.String{Value: t.Value.Format(format)}, nil
}

Now Time values can be "called" like this:

s := script.New([]byte(`
	a := currentTime + 10000  // Time + Int = Time
	b := a("15:04:05")        // call 'a'
`))

// add Time value 'currentTime'
err := s.Add("currentTime", &Time{Value: time.Now()}) 
if err != nil {
	panic(err)
}

c, err := s.Run()
if err != nil {
	panic(err)
}

fmt.Println(c.Get("b")) // something like "21:15:27"

Scripts and Script Variables

...

Compiler and VM

...

Sandbox Environments

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