How to use [whitelist]

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

This slot temporarily within the given scope changes the available slots, allowing you to declare a block of lambda, where only a sub-set of your vocabulary is available for some piece of code to invoke. This allows you to relatively securely allow some partially untrusted source to pass in a piece of Hyperlambda, for then to allow it to evaluate its own Hyperlambda. The slot takes two arguments.

• [vocabulary] - Whitelisted slots
• [.lambda] - Lambda object to evaluate for the given scope

.result
whitelist

   vocabulary
      set-value
      return

   .lambda

      // Inside of this [.lambda] object, we can only invoke [set-value], and no other slots!
      set-value:x:@.result
         .:foo

      // Notice, the next line will throw an exception if you remove its "." character,
      // because [add] is not whitelisted in our above [vocabulary] declaration!
      .add:x:@.result
         .
            foo:bar
      return
         result:success

For security reasons the [whitelist] invocation’s [.lambda] object is immutable, and the caller cannot access nodes outside of the [.lambda] object itself, which prohibits the caller to modify, and/or read nodes from outside of its [whitelist] invocation. In addition a [whitelist] invocation creates its own result stack object, allowing the [whitelist] invocation to return values and nodes to the caller using for instance the [return] slot. If you execute the above Hyperlambda you can see how this semantically works by realizing how the above [.result] node never has its value actually changed, because our invocation to [set-value] inside our whitelist invocation yields a “null node-set result”. Hence, semantically the [whitelist] slot works the same way signaling a dynamic slot works in Hyperlambda, in that the invocation treats its [.lambda] object as if it was a dynamic slot, isolating it from the rest of our code, allowing the lambda object to return values and nodes to the caller.