nominalwhen

nominalwhen is a term used in certain programming contexts, particularly in functional programming languages like Haskell, to describe a type or construct that carries no meaningful information. It's a way of representing the presence of something without needing to store any associated data. Think of it as a placeholder or a signal that an event has occurred or a condition has been met, but the actual value of that occurrence is irrelevant.

The primary purpose of nominalwhen is to aid in type safety and to distinguish between different actions or states that might otherwise have the same underlying type. For example, in a program dealing with user authentication, you might have different actions like "UserLoggedIn" and "UserLoggedOut". Even though both actions might conceptually be represented by a simple "success" or "done" signal, using nominalwhen allows the compiler to ensure that you are handling the correct type of login or logout event.

In essence, nominalwhen types are zero-width types. They occupy no memory at runtime because they don't store any data. Their value lies entirely in their type name and the compiler's ability to differentiate between distinct nominalwhen types. This can lead to more robust and expressive code by making the intent and context of operations explicit at the type level.