[hibikir]

There are a few usecases for this in some languages, where your functions might as well be class extensions. But you need a huge standard library with non-competitive types, or you end up with deep dependency stacks.

You also run into trouble if your language has side effects (ie, almost all of them). A leftPad that launches a fiber to mine cryptocurrency or sends an http call that fires nuclear missiles can still pass tests. It's hard to guarantee hygiene via tests alone.

[usernametaken29]

The person I was discussing this with was indeed working on lambda calculus and provability of side effects. At least in theory it is possible to account for all classes of side effects and remove any risk. Also, what other people have said here, functions can be immutable artefacts with a fixed hash (just their own content) which mitigates a whole cluster of supply chain issues at once