[Haskell-cafe] Ensuring that blank-state has been handled in UIs?

[Saurabh Nanda]

Hi Everyone,

In most user-interfaces we need special-treatment for blank states
(zero-item state). For example:

* In case there are no customers, don't show an empty table, instead show
the message "You seem to have no customers, why not start by creating
one..."
* In case there are no bookings, don't show an empty table, instead show
the message, "No bookings here. Want to create one?"
* In case there are no search results, don't show an empty table, instead
show the message, "No items found. Undo your last filter?"

In some cases, the one-item case also needs special treatment, but I'm
unable to come with relatable examples at the moment.

A lot of times the dev simply forgets about the blank state, and it is
caught during QA, which results in a quick-fix on the following lines:

    if length item > 0 then displayTable else showBlankState
    if Data.Map.Strict.size map > 0 then displayTable else showBlankState
    if Data.Set.Ordered.size set > 0 then displayTable else showBlankState

Is there a way to prevent this bug from the get-go? Can we use the
type-system (or anything else) to enforce the dev to at least _think_ about
the blank state? Obviously, the type system can't help us with _what_ needs
to be done with the blank state, but at least it can _remind_ the dev at
compile-time about handling blank states.

-- Saurabh.
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