On 2017-04-16 13:51, Saurabh Nanda wrote:
Can we use the type-system (or anything else) to enforce the dev to at least _think_ about the blank state?
That is an interesting problem. But I'd say the solution depends highly on your architecture. Tomas' suggestion to use NonEmpty could be one solution, and one you could try to build upon to track more details in the types. Alternatively, you could add a typeclass that just handles the zero-element-case. That way your constraints remind the developer to at least copy-paste a default implementation. showItemTable :: (HasZeroItemPresentation (presenter item)) => presenter item -> [item] -> Table item But maybe there's a much simpler solution: Create a datatype that stores the normal presentation function, a zero-case presentation function, and optionally a one-item presentation function. Now whenever someone creates a new type of item, they need a new presentation adapter. data TablePresenter item = TablePresenter { showZeroItems :: Widget; showItem :: item -> Widget; showSingleItem :: Maybe (item -> Widget) } showItemTable :: TablePresenter item -> [item] -> Table item This should offer safety and code reuse, and it should be easy to integrate because you don't even have to start with types. Why be fancy when the simple solutions work? Cheers, MarLinn