I like visuals so I made a visual to represent @LostKobrakai’s awesome explanation:

Screen Shot 2024-02-17 at 6.44.04 AM793×398 16.4 KB

And one thing that sometimes confuses (me at least) is that sometimes the “Outside data” and “Runtime data” have the same representation, for example for Ecto.UUID. Seeing it in this visual form makes the reason behind that easier for me to understand/remember:

Screen Shot 2024-02-17 at 6.46.06 AM1223×483 24.3 KB

Already merged:

I’m not sure where to put the checks, but at least I can tell you that you only need to iterate once, roughly like this:

def valid_custom_type?(list) do
  list
  |> Enum.reduce_while(0, fn
    _d, l when l > 127 ->
      {:halt, :invalid}
    d, _l when d not in 0..2 ->
      {:halt, :invalid}
    _d, l ->
      {:cont, l + 1}
  end)
  |> is_integer()
end

This will iterate not more than once and not more than the first 128 digits of the list.

Due to the early return it should not become a bottleneck that soon.

That’s a great explanation, and from what I gather, for the question of “Should we be ok with doing the same work twice, once in cast and once in dump” the answer is “yes”.

And since outside data can also be formatted consistently with runtime data, would it be more correct to say that

cast goes from anything to runtime data
dump goes from anything to database column data

With this in mind, it’s almost always ok to actually call cast function from your dump function, does this sound right? (I know it depends, but I’ve noticed I could do that in most custom types I wrote).

Omg, can this answer please be included in the official Elixir docs? Maybe as a cheatsheet. I’ve never read a better explanation than this one!