Add conditional expressions

[Kijewski]

Jinja, being based on Python, allows using conditional expressions:

{{ "then" if condition else "otherwise" }}

will print "then" if condition is truthy, otherwise "otherwise".

This PR adds the same syntax, with a few restrictions, to Askama:

• The condition must evaluate to a bool (or a reference to it), same as in {% if .. %}.
• The else-case can be absent in Jinja, then behaves like else None. For Jinja that makes sense. It renders {{ None }} like an empty string. Askama does not do that: Option<T> can not be rendered for any T; we don't unwrap automatically. So, without automatic unwrapping, I don't see a case when you would want to omit the else-case.

Todo:

• Update book

[Kijewski]

Oops:

$ type cargo
cargo ist ein Alias von »RUSTC_BOOTSTRAP=1 cargo«.

I did not quite get #525 before. :) Wow, that's great!

[Kijewski]

@GuillaumeGomez, so what do you think of this feature? Documentation and UI tests are missing, but the functionality is all there. If you give me a ":+1:" on the feature, I'd add the missing parts.

[GuillaumeGomez]

I'm not convinced. The syntax is really different compared to the rest. Adding conditional expressions inside the "expression tag" seems a bit too much imo.

I did not quite get #525 before. :) Wow, that's great!

It's just the first step. I'm working on the second step currently and hopefully it'll crazily improve nightly outputs, even if you write stuff like:

#[derive(Template)]
// And other attributes
#[template(src = "{{ x.y }}")]
struct X {
    x: u32,
}

It'll be able to show directly that the x.y part is wrong.

[Kijewski]

Actually, I use is pattern all the time in jinja templates. Simple things like {{ "logoff" if user else "login" }}. Of course, you can always write {% if user %}logoff{% else %}login{% endif %}, but it's longer and at least to me it's harder on the eyes.

(At least for templating, I like python's "infix if" better that rust's "prefix if", even without the fact that it's easier to parse.)

[GuillaumeGomez]

Hum... Well I don't really a strong argument against it either. Is there syntax conflict possibility? That's my main worry here.

[Kijewski]

Only with {% for elem in iterator if condition %}. In Jinja the if is parsed as prefix to the condition. If you want to use an if-expression in the iterator-expression, then you have to surround it with parentheses: {% for elem in (iterator if condition) %}

That's why I introduced Expr::parse_no_if_else(), so it's the same in Askama.

[GuillaumeGomez]

This is absolutely horrifying for my brain to parse. 🤣

[Kijewski]

What about this formatting? :)

{{
   a if b else
   c if d else
   e
}}

[GuillaumeGomez]

That doesn't help much, I'm too used to blocks. But it's fine, don't worry. One question though, does this work:

{{ a if b else (c if d else e) }}

?

[Kijewski]

Yes, that does work.

[GuillaumeGomez]

Thinking some more, does it mean we can have stuff like:

{{ a(x if y else z) }}
{% if (a if x else b) %}

?

[Kijewski]

Yes! Both examples would work.

[GuillaumeGomez]

I'm very afraid of opening this door. 🤣

[GuillaumeGomez]

Anyway, I don't any valid argument against this feature, so if you have a use for it and want to add it, then let's go. Please ping me once PR is ready for review with tests and docs. :)