Improve boolean support in query

[josevalim]

The goal is to support this:

DF.mutate(foo: select(a == b, c, 0))

As well as multi-clause:

DF.mutate(foo: select do
  a == b -> c
  d < e -> f
  true -> 0
end)

We chose select do instead of cond because, opposite to cond which only evaluates the clauses that matches, queries always evaluates all clauses in order to build the expression.

In order to support this, we will need:

1. To allow any value as the second and third arguments of select (the first argument will always be a list). In this case, it should be enough to call from_list([...]) on arguments that are not lists
2. Provide and/2, or/2, and not/1
3. Implement and document select/1 macro defined above

The reason why we decided to handle this at the query level is because Series.and(series, false) does not really have a purpose and we should keep and/2 and in Explorer.Series remain strict about expecting series. We may want to lift this restriction in the future but, at a first glance, it sounds reasonable.

[josevalim]

@sasikumar87 is it correct to say your PR does the third item in the list above? I also think the first item is already done (but not the second).

[sasikumar87]

@josevalim yes, third item in the list above.

[josevalim]

@sasikumar87 do you want to send a PR for the second item?

The idea is to introduce functions for and, or, and not. and/2 would look like this:

def left and right when Kernel.and(is_boolean(left), is_boolean(right)), do: Kernel.and(left, right)
def left and right, do: Explorer.Series.and(boolean!(left), boolean!(right))

defp boolean!(%Series{dtype: :boolean} = series) do
  series
end

defp boolean!(other) do
  raise ArgumentError, "boolean operators require either a boolean (true/false) or a boolean series, got: #{inspect(other)}"
end

It is untested but that's the general idea. :)

[sasikumar87]

@josevalim will do

[sasikumar87]

@josevalim to confirm for and/2, we should also support one of them being a boolean and other being series, right?

[josevalim]

Ah yes, I forgot a clause in boolean! That accepts a Boolean and converts it a series!