[coldtea]

>However, I believe most of the more popular languages support creation of custom data types?

No, most don't, except if you go into building custom classes.

[dragonwriter]

So, most don't support custom types unless you...use the mechanism they have for defining a custom type?

That seems a very elaborate way to say “No" when the answer is really “Yes”.

[coldtea]

It's a succint way to say "No, not types that will automatically work as primitive types (which normally the variable passed for 0 to 1 would be), and that will work with numeric operators".

Or in other words, a succint way to say "Technically yes, but practically useless, so no".