How to use [unwrap]

The reference page of unwrap has its arguments, constraints and declaration.

This slot is useful if you want to evaluate some expression before the node where it’s referenced is reached. Imagine the following.

.src:Hello World
unwrap:x:+
.dest:x:@.src

In the above example, before the [.dest] node is reached by the Hyperlambda instruction pointer, the value of the [.dest] node will have been “unwrapped” (evaluated), and its value will be “Hello World”. This slot becomes very handy when you invoke lambda objects with expressions, since it allows you to “forward evaluate” said expressions inside your lambda object, before the lambda object is actually executed.

The [unwrap] slot can also optionally be supplied with a [apply-lists] argument as boolean true. By default the [unwrap] slot will throw an exception if you unwrap a node leading to multiple nodes. If you apply lists, it will instead add all the source nodes as destination nodes into the unwrapped node. Consider the following.

.src
   foo1:bar1
   foo2:bar2
unwrap:x:+
   apply-lists:bool:true
.dest:x:@.src/*

If you remove the above [apply-lists] argument or set its value to false, the above code will throw an exception. If you keep the above [apply-lists] argument as is it will result in the following.

.src
   foo1:bar1
   foo2:bar2
unwrap:x:+
   apply-lists:bool:true
.dest
   foo1:bar1
   foo2:bar2