Make `default` a thunk?

[bbc2]

The fact that default is a value makes it static. In some cases, like generating random values and incrementing IDs, it would be practical to customize default so that it can return a different value for a given factory function (I guess it could even be the default behavior, but it's not my issue here).

This can be worked around by creating factory functions manually but I think this defeats the purpose of this PPX.

So, would it make sense to be able to define default as function?

let default () =
  ...

[leamingrad]

That sounds sensible to me, I would be in favour of this change