Query documentation could use an example on how to compare column with dynamic value

[moll]

Hey!

Stumbled upon another slightly unclear thing — how to filter against a value from a variable. Turns out all of the filter_ examples in http://tathougies.github.io/beam/user-guide/queries/basic/ use hard-coded values that I guess work due to overloaded strings. When one plugs in a variable, for example, of type String to either side, they get a type mismatch. Because ==. is fairly generic, the error is somewhat backwards — not pushing the String towards QExpr, but vice versa. As I remembered seeing Beam.val_ when playing with insertExpressions, I thought to try that.

I think it'd be valuable to newcomers if one such example were documented, too. The overloaded string or Num variants do for nice examples, but don't address the complexity that later surfaces again.

[moll]

Ah, I did find val_ mentioned in http://tathougies.github.io/beam/user-guide/expressions/, but unfortunately it's information scent wasn't particularly good when working in the context of queries to know to look there. I know it'd be preferable if everyone read everything, but it'd probably do good to repeat things. Looking at it, the expressions documentation seems more suited to live inside the query section. "Basic queries" and "More complex SELECTs" could also be unified. A single WHERE clause is basic, yet under this classification it's a "complex" one. I don't think there are any two people that would classify various constructs the same way, so why split them up. Skimming up and down and Ctrl-Fing works better than clicking around the sidebar.