Passing column values through custom TypeScript functions ...

const hz = db.$with('sq').as(
    db
      .select({
        alt: sql<number>`cast(${
          convertEquatorialToHorizontal(datetime, observer, {
            ra: observations.ra,
            dec: observations.dec
          }).alt
        } as float)`.as('alt')
      })
      .from(observations)
  )