Programming Flix

Boxing and Unboxing

Unlike Java, Flix never performs implicit boxing or unboxing of values.

We believe auto boxing is a design flaw and do not plan to support it. Hence, primitive values must be manually boxed and unboxed.

Boxing

The following example shows how to box a primitive integer:

def f(x: Int32): String \ IO = 
    let i = Box.box(x); // Integer
    i.toString()

Here the call to Box.box(x) returns an Integer object. Since i is an object, we can call toString on it. Boxing is a pure operation, but calling toString has the IO effect.

Unboxing

The following example shows how to unbox two Java Integer objects:

import java.lang.Integer

def sum(x: Integer, y: Integer): Int32 = 
    Box.unbox(x) + Box.unbox(y)

Here the call to Box.unbox returns an Int32 primitive value.

Unboxing is a pure operation.