Related problem

Sometimes when using default, the "default" value is expensive to compute (or produces side-effects), so I'd like it to be computed lazily, only if the input is actually null.

Describe the solution you'd like

I'd like to be able to pass a closure to default instead of a value, and the closure to get run only if the input is null.

To prevent confusing behavior when the value being defaulted is itself a closure, I think this would work best as a flag on default, like --closure(-c) or --lazy(-l).

For example:

> 2 | default -c { print Hi; 3 }
2

> null | default -c { print Hi; 3 }
Hi
3

> { x: 2, y: 4 } | default -c { print Hi; 3 } y
{ x: 2, y: 4 }

> { x: 2 } | default -c { print Hi; 3 } y
Hi
{ x: 2, y: 3 }

Contrast this with the output of regular default:

> 2 | default (print Hi; 3)
Hi
2

Describe alternatives you've considered

Currently, to get this behavior, you need to resort to a clunky if-expression of the form:

if $thing != null { $thing } else { $expensive_computation }

This is especially annoying if $thing is itself a complex expression, as you have to bind it to a variable to avoid duplicating it.

Additional context and details

This feature is very similar to Option::unwrap_or_else from the Rust stdlib.

I'm planning to write the PR for this myself, but wanted to submit the feature request first to get feedback on the usage and API.