FWIW, the read/write distinction really helps me to remember the terms:
For reading, you want to be sure the array you’ve got contains values at least
as specific as you were expecting. So, if you’re expecting Integer, it’s fine
if you get Int. In that case, you might hope that Vector{Int} <:
Vector{Integer}. In other words, reading => covariance, which involves
following type arrows in their true direction.
For writing, you want to be sure the array you’ve got contains values at least
as general as you were expecting. So, if you’re expecting Int, it’s fine if you
get Integer. In that case, you might hope that Vector{Integer} <: Vector{Int}.
In other words, writing => contravariance, which involves going backwards along
type arrows.
If you want to be able to read and write, you need to avoid allowing covariance
and contravariance. When you entirely refuse to look at type arrows, you get
invariance.
— John
On Aug 15, 2014, at 7:58 PM, Brendan O'Connor <[email protected]> wrote:
> On Fri, Aug 15, 2014 at 9:53 PM, John Myles White <[email protected]>
> wrote:
> Hi Brendan,
>
> It looks like you’re hitting Julia’s invariance for the first time:
> http://en.wikipedia.org/wiki/Covariance_and_contravariance_(computer_science)
>
> Oh nice. I could never keep the covariance/contravariance directions
> straight, anyway...
