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Sentinel Language - Functions | Sentinel

Functions allow you to create reusable code to perform computations.

Below is a trivial example function that doubles a number and shows the main rule using the function:

Press "Run" to get policy output

Functions may contain any expressions, conditionals, and loops. They must return a value at the end of their execution. If no reasonable return value exists, the function should return undefined.

NOTE: Named functions must be created within the package scope.

Named functions are declared using the func keyword as its own statement. They provide a safe method of creating functions and have additional restrictions that do not apply to anonymous functions.

Firstly, named functions cannot be re-assigned, and also cannot use a name that is already used elsewhere. For instance, the below example will error due to the attempt to reassign the named function identifier to a new value:

func sum(a, b) {
  return a + b
}

sum = 4

Additionally, the following will error due to the named function attempting to make use of an already assigned identifier:

sum = 4

func sum(a, b) {
  return a + b
}

Named functions are helpful for policy authors to declare critical functions whose value or implementation should not be changed.

An anonymous function is created by assigning a variable to a func. The variable can be reassigned at any time including to different value types. Anonymous functions are helpful for use cases like closures, where a function can return another function.

func makeAdder(a) {
  return func(b) {
    return a + b
  }
}

A function can have zero or more parameters. These parameters are specified within parentheses () separated by commas. If a function has no parameters, the parentheses must still be present.

A function must terminate with a return statement to return a value back to the caller. Only a single value can be returned. The type of a return can vary between return statements.

Anonymous function example:

add1 = func(x) { return x + 1 }

Named function example:

func add1(x) {
  return x + 1
}

Both examples create a function that adds 1 to the parameter x.

Functions are called by accessing their name followed by parentheses with a list of comma-separated values for the parameters. If the function takes no parameters, an empty set of parentheses must still be used to denote a function call.

To call the function in the example above:

The body of a func creates a new scope.

The parent scope is the scope in which the function is created. This allows for the creation of functions within functions that can access their outer scopes.

Example:

f = func() {
    a = 42
    print(a) // 42
    return undefined
}

print(a) // undefined

a = 18
f = func() {
    a = 42
    return undefined
}

print(a) // 18

And below is an example of creating a function in a function which uses outer values:

f = func() {
    a = 42
    double = func() { return a * 2 }
    return double()
}

print(f()) // 84

A more complex example below shows how scoping works when passing functions around as arguments or results:

f = func() {
    a = 42
    double = func() { return a * 2 }
    return double
}

double = f()
print(double()) // 84

The parameters to a function are passed by value, but not deep copied. This means that elements of collections can still be modified but the root value cannot be changed.

Example:

f = func(x) {
    x = "value"
    return x
}

x = "outside"
f(x)
print(x) // "outside"

f = func(x) {
    append(x, "value")
    return x
}

x = []
f(x)
print(x) // ["value"]

A function is allowed to call other functions, including itself. This can be used to implement recursion. For example, the fibonacci sequence is implemented below:

fib = func(x) {
    if x <= 0 {
        return undefined
    }

    if x == 1 {
        return 1
    } else {
        return x + fib(x - 1)
    }
}

Note that this example also shows using undefined as a return value in cases where a function has undefined behavior.

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