[yagoham]

The Nickel interpreter is intended to offer a mechanism to make it possible to extend it to add "effects", which is really just a pompous name for an external call. The idea is that, if you want to integrate it with Terraform for example, you would want to have a "getIp" effect to retrieve the IP of a machine once it has been evaluated. So you implement your external handler (say in Rust or whatever), and then you can call "getIp" from a Nickel program. Currently, we see no reason Nickel would ship with any effect by default. These are really just for extension purpose. Such additional effects would be required to be commutative in order not to hurt parallelizability, but you can't enforce that mechanically, so you'll have to trust the implementer.

[throwaway894345]

This makes sense--allowing or preventing a particular instance of the interpreter to make side effects is perfectly reasonable. It would be concerning if all instances of the interpreter allowed client programs to make side effects.