Correct. True functional-style HOFs require passing around arbitrarily-sized heap closures, which doesn't work with rust's memory model. Rust lets you Box closu

Correct. True functional-style HOFs require passing around arbitrarily-sized heap closures, which doesn't work with rust's memory model. Rust lets you Box closures, but it's very inconvenient compared to treating them as first-class values, so I didn't count it.