Let's say the user entered:
No, Name, Qty, Price -------------------------------------------- 1. [ ] [99] [10] 2. [Water ] [ ] [10] 3. [Juice ] [ 1] [ ]
The GUI should display total of 990,
Why? Let's be clear about this: given that information, the total is unknown and unknowable, and even if lines 1 and 2 were intended to be the same line, the existence of line 3 means that 990 is almost surely LESS than the correct total, whatever that might be. Supermarkets here don't let you create invoice lines by reference (giving a possibly garbled name) but only by ostension ("I want to buy THIS"). This is why all those bar codes and thingies exist. Libraries here do something similar with books: you show the machine "I want to borrow THIS book" and a guaranteed valid line is created. A GUI clearly *could* be designed to allow badly messed up entries like that, but it is at least questionable whether it *should*. A possible data point: students here are very much used to the instant feedback provided by syntax colouring editors, and few of them would willingly program without such tools. (I'm annoyed by and slowed by this nonsense, but tastes vary.) This is evidence that at least some people strongly prefer instant feedback; by the time they get to the end of a form they know that each and every field is at least plausible taken by itself. and signal four errors: three
missing values (ideally different color of the input fields), and the whole invoice incomplete. The Either [Error] Invoice type does not work, because can either display the errors or calculate total from a correct invoice, never both. And you can't even create LineItem for 2. and 3. Well, maybe you can with laziness, but how would total work then?
That's why I asked in my original post, whether I'd need two types, one for correct complete invoice, and another for the invoice "in statu nascendi". And how to obtain them, lazily, and I mean the person, not the language.
-- Wojtek _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe