Support for if/else expressions in assignments

[seijikun]

Currently, supporting forward declarations of variables ({% let <variablename> %} in askama is a necessity, because assigning values conditionally is hard otherwise.

Would supporting if expressions in variable assignments be an acceptable way to support this?

{% let cond = true %}
{% let value = if cond { 1 } else { 0 } %}

[GuillaumeGomez]

Except for a (big) increase in parser complexity, I don't see any reason why not. Although, if the parser complexity increase is too big, I'd prefer not to do it.

[seijikun]

Ah, but even if the parser complexity wasn't too large, adding this would not solve all cases where forward declarations are in use today :(.

For example this one would still remain:

{% let cond = Some("test") %}
{% let variable %}
{% match cond %}
    {% when Some %}
        {% let variable = 1 %}
    {% when None %}
        {% let variable = 0 %}
{% endmatch %}

[GuillaumeGomez]

Seems like a good argument in favour of not doing it. 😉

[Kijewski]

The problem is if MyStruct { a: "a" } { .. } else { .. }. Rust's parser is context free; no knowledge is needed to tell that this is syntax error: The first brace { already opens the then-body. We would need to carry a flag whether a struct with named fields can be matched at this point.

Actually, I would replicate python's if-else-expression rather than rust's. (We can say that we did it this way because it is valid syntax in jinja! :) )

{{ then_case if condition else otherwise }}