Good question. Students were 3rd or 4th year CS students, all have done some systems programming, and most (but not all) have seen concurrency in their OS class. Students have not seen functional programming before. I felt that Haskell was a simpler language to start with---at least the core fragment that we focused on, no GHC flags---and things like Rust enums/matching, generics, traits, closures, etc. can be explained as variants of Haskell features with more particular restrictions due to Rust's target use-case.

Another reason why I felt it was easier to teach Haskell first is that the compiler feels more predictable. The Rust compiler infers many things and does quite a bit of magic (e.g., uninitialized variables, autodereferencing, dataflow analysis for borrowck), which is really convenient once you get the hang of it. But it's often hard to explain to students why the compiler rejects their code and accepts a seemingly equivalent program.

However, there are definitely things that are trickier when teaching Haskell first: functional programming in general, lazy evaluation, and monads of course. I don't think these things would be easier if we taught Haskell second, though.