My mistake. I meant to write "can SOMETIMES be entirely on the stack" -- fixed that just now. Functions indeed cannot return closures without heap-allocating t

My mistake. I meant to write "can SOMETIMES be entirely on the stack" -- fixed that just now. Functions indeed cannot return closures without heap-allocating them. (It might be possible in specific obscure cases, but definitely not in general.) However, if you wish to curry a function foo(a,b), you can do that in the caller (instead of at the function definition itself) by writing: let curried_foo = |b| foo(a,b); That is then stack-allocated and can refer to 'a' in the same frame, and can be passed to other functions you might call from there (but not returned upwards, indeed).