Exact, fallible destructuring

[musjj]

There's currently no way to assert if the structure of an object exactly matches a pattern, even with the alternate destructuring ?//:

$ jq -r '. as [$foo] | $foo' <<<'["foo"]'
foo

$ jq -r '. as [$foo] | $foo' <<<'["foo", "bar"]'
foo

$ jq -r '. as [$foo,$bar] ?// [$bar] | $bar' <<<'["foo"]'
null

It would be helpful if there's a destructuring operator that will throw an error if there are no exact matches. Perhaps it could look something like:

$ jq -r '. is [$foo] | $foo' <<<'["foo", "bar"]'
jq: error (at <stdin>:1): Cannot destructure array with pattern [$foo]

$ jq -r '. is [$foo,$bar] ?// [$bar] | $bar' <<<'["foo"]'
foo

$ jq -r '. is [$foo,$bar] ?// [$bar] | $bar' <<<'["foo","bar","baz"]'
jq: error (at <stdin>:1): Cannot destructure array with pattern [$bar]

[nicowilliams]

It would be helpful if there's a destructuring operator that will throw an error if there are no exact matches. Perhaps it could look something like:

In general it'd be nice if we had a `!` operator to do this, not just in destructuring but also things like array and object indexing. `.a?` is `empty` if `.` does not have an `"a"` key, and `.a!` should `error` if if `.` does not have an `"a"` key. And `any_expression_here!` might `error` if the expression is `empty`, say.