add way to filter (select?) items in a list

[suaviloquence]

I don't want to add first-class functions, so at most filtering based on a property or property equality (with a similar syntax to CSS (pseudo-)selectors?). What would this look like?

[suaviloquence]

with a = [1, 2, 3, Null, "abc", 4]:

just-ints: a [type="int"]

• css attribute syntax
• bracket syntax semantically hints a must be an array and is also an int
• support equality [type=int] and presence [non-null]
• what would boolean logic (and/or/not) look like?
• would have to have ways to expose properties, like filters
  • another trait with a Fn(PValue<'_>, rhs: Option<EValue<'_>>) -> Result<bool>, another proc-macro probably
• arbitrary? complexity with inlines
• what magnitude of properties would we have to write?

just-ints: a [item: $item | type() | eq(to: "int")]

• boolean-lambda
• python-like lambda syntax
• needs boolean as a first-class value (or C nonzero int logic, but we're better than that)
• dangerously close to first-class function support
• arbitrarily expressive power with boolean filters and, or, and not
  • but typically-infix operators look quite ugly in pipeline-filter syntax with named arguments
• this is probably the better idea anyway.

[suaviloquence]

in a68d978