Foreign Function Interface

use crypto
use perspective

use defines an imported function library, which is provided by the application. You must call functions from imported libraries with the fully qualified library name (e.g. crypto::sign()). use can only be used in global scope, and all use statements must appear before other statements in the document.

The runtime model and FFI functions

Because policy may be evaluated more than once or never committed, imported functions should not have side effects and rely only on constant external state. In computer science parlance, they should be “pure”.

In a linear history of commands, each command will only be evaluated once. But because Aranya allows divergent branches of commands that are automatically merged, new commands can appear in the history where they weren’t before, prompting reevaluation of commands that occur later in the braid.

Suppose a command uses FFI to increment a value in an external database. When the command is reevaluated after a merge, that increment will happen again, causing double-counting. Alternatively, if a command is evaluated but not committed there will be over-counting.

If you need to keep track of state in response to commands, you should maintain that state external to Aranya by responding to effects.