The query team has decided on the following semantics and syntax:

Name: $first/last

Evaluates to the value at the first (or last) position in the passed in array. Errors if the passed in value is not an array and not nullish. Evaluates to missing if the array is empty, which is consistent with $arrayElemAt's behavior. Evaluates to null if input is nullish.

Syntax:

{ $first: <Expression> }