Applicative combinators instead of construct!

[mausch]

Hi and thanks for this library!
I'm relatively new to Rust and being a fan of optparse-applicative I was happy to find bpaf.
However I'm having issues with the constructor! macro, it's a bit too magicky for me as a Rust newbie...
I can't seem to find any combinators to build the ParseCon (I think?), is this possible at all without using the macro?
I know that currying isn't idiomatic in Rust but maybe bpaf already has some sort of lift2 / apply functions somewhere?

[pacak]

construct! macro works in two different modes: alternative and applicative combinations. For applicative you put parsers for all the variants into a set of square brackets and construct! combines them giving preference to one that consumes earlier item from the command line or one that succeds first in a list if they consume the same item. For applicative the easiest way to think about it is that instead of parsers (say a: impl Parser<A> and b: impl Parser<B>) you have variables (a: A and b: B) and you simply construct the result: Foo { a, b }. construct!(Foo { a, b }) does the same but with Parsers, giving you impl Parser<Foo> back. A tiny bit of magic allows you to use () to specify that a is not a Parser by itself, but instead a function that produces one. As a beginner you might want to avoid using that bit of magic.

Rust doesn't play nicely with currying unfortunately: no higher order types, no curried type applications, lots of things are not Copy. I had some combinators exposed in earlier versions but decided against it since it was hard to use.