I have an expr: sea_query::Expr and values: impl Iterator<Item = String> and I just can't construct this query.

I expected to be able to write something like Func::lower(expr).is_in(values).

Ok, FunctionCall doesn't have is_in. I skim through its docs, it's convertible to SimpleExpr. Let's convert it:

use tap::Conv;
Func::lower(expr).conv::<SimpleExpr>().is_in(values)

But it doesn't have is_in either! Apparently, only Expr and ColumnTrait have it. Ok, maybe I need to further convert that SimpleExpr to Expr. Wait, I can't? What do I do now? I tried to resort to raw SQL, but I can't even find a way to convert my expr and values to their SQL string representations. I'm blocked on my task and I'm writing this post.

I want to note that there's a bigger issue here. In the past, I had many other problems with the whole Expr / SimpleExpr distinction, where some functions accept the latter but not the former. E.g., the mentioned Func::lower doesn't accept SimpleExpr, I'm lucky that I have Expr as my input. But why? I don't understand why there are two types. It's confusing, one often lacks features of other, and conversions between them are often unergonomic or impossible. Something needs to be done about this. Please, at least document the difference. Currently, Expr documentation says that it's a

Helper to build a SimpleExpr.

But this seems unclear and unhelpful. Why is the latter "simple", when the former seems like a lower-level building block? Why is the "hepler" a separate type rather than a function? Why aren't all expressions usable in expression contexts, like the one in the title?