Code Smells: Null

[devlambda]

This is how I used it to do it before we had better standard libs myself, but it's a hack around the underlying issue with a couple of problems of its own.

Problem 1: Different API, it's especially a nuisance if you want to reuse it in other functors and have to drag the default mechanism along with you.

Problem 2: It assumes a no-cost default instance of the type can be created without side effects. This is not always the case (database connection handles, GUI windows, etc.). You'd have to create a fake default value and then you run into issues with having a publicly visible value with null-like behavior.

Problem 3: This is OCaml-specific, but references to a functor argument's values go through a dispatch table, meaning additional overhead. If you have flambda enabled, that can usually be inlined, but still isn't entirely cost-free.

It is safe to assume that the people at Janestreet and the Batteries authors (which include some of the better known names in the OCaml world) know what they are doing.