This looks like code from the 'I found out about FP, let's apply it everywhere'-stage. For instance, if the iteration over arguments was done with a regular for...in loop, the filename would be visible in its scope and the rest of the code could be simplified due to not having to pass the filename everywhere.

Also, I don't think this demonstrates expressivity well, because the purported functional construct uses unwrap/expect (it panics). The expressivity of Rust allows you to have the whole expression to have Result has its type fairly easily.

(Sorry for ranting a bit, but I think that Not only is this very readable, will tick off people. Which is sad, because it could be changed into something which is more readable but still expressive.)