File operations are a good candidate for testing with side effects since they ship with every OS and are not very expensive in a tmpfs, but you don't have to let it perform side effects. You could pass the eval function a delegate which it calls methods on to perform side effects and pass in a mocked delegate during testing.

If it was easier to interpret byte codes, nobody would use a tree-walk interpreter. There’s no performance reason to use a tree-walk interpreter. They all do it because it’s easy. You basically already have the expression in tree form, regardless of where you end up. So, stop processing the tree and just interpret it.

> If it was easier to interpret byte codes

I’m not claiming anything, certainly not that; I’m questioning whether the additional complexity of designing a bytecode VM is worth it because it makes testing easier.