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
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