[pgwhalen]

Usually I balk at the idea of of someone describing a language feature as “first class” because it seems to wishy washy. But in this thread you’ve shown me that maybe the best definition is through “proof by negation,” by patiently responding to arguments and demonstrating why a certain usage and the ensuing restriction around it means it is not first class. Bravo!

[naasking]

I agree the term is often abused, but I think the wikipedia page actually does a decent job of making the notion somewhat precise, along the lines I've been arguing here:

https://en.wikipedia.org/wiki/First-class_citizen

If you want to see what queries as first-class values looks like, LINQ in .NET is pretty close. I can actually write a series of queries that build on and compose with each other, like this:

    IQueryable<Person> RunQuery(int userSelection)
    {
        var first = from x in People
                    select x;
        var second = userSelection == 1
            ? from x in first where x.Birthday > '2000-01-01' select x
            : from x in first where x.Name.Contains("Jane") select x;
        return DumbJoin(first, second);
    }

    IQueryable<Person> DumbJoin(IQueryable<Person> first, IQueryable<second>)
    {
        return from x in second
               join y in first on y.Role equals x.Role into g
               select g;
    }

This query is nonsense, but it just shows you what composition really looks like when queries are first-class values. I wish raw SQL were like this!