[Haskell-cafe] The Good, the Bad and the GUI

[Tom Ellis]

On Tue, Aug 12, 2014 at 12:46:05PM +0200, Wojtek Narczyński wrote:
> Continuing my VAT Invoice example, let us say a LineItem that does
> not have a product description (missing value), but it does have all
> the numeric fields filled in.  It is partly erroneous, but it can be
> included in calculation of the total. How would you handle it with
> Either [Error] Invoice style code?

What sort of functionality are you looking for exactly?  What's your
objection to (for example)

    data LineItemGeneral a = LineItem { price :: Price
                                      , quantity :: Quantity
                                      , description :: a }

    type LineItem = LineItemGeneral String
    type LineItemPossiblyIncomplete = LineItemGeneral (Maybe String)
    type LineItemWithoutDescription = LineItemGeneral ()

    totalValue :: LineItemGeneral a -> Value
    totalValue lineItem = price lineItem * quantity lineItem

`totalValue` works for all sorts of line items, whether they have a
description or not.

Tom