[topspin]

>> Because Rust doesn't have function overloading

That is the insight I needed. Thank you.

Rust seems novel in that despite having powerful abstractions and a rigorous type system it does not support overloaded functions. I gather from some of the discussions about it that this design decision greatly simplifies the compiler implementation; supporting function overloading would necessitate answering several very tough questions such as whether a function can be overloaded on argument lifetime.

So the Rust standard library has established conventions (in this case 'unwrap', 'else' and or 'combined' in various ways) to deal with the permutations that naturally emerge given the lack of function overloading. It's important understand this and inculcate these conventions, particularly when designing interfaces for use by others.

[masklinn]

> Rust seems novel in that despite having powerful abstractions and a rigorous type system it does not support overloaded functions.

Neither Haskell nor ML (incl. its children) have function overloading.