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

[John Lato]

This is starting to look promising.  I think what would be really
interesting would be to abstract out the validation logic so that it could
be presented in a more declarative fashion, something like

    itemPrice *> \item inputPrice ->
        if inputPrice > 0 then return inputPrice
            else fail ("invalid item price for item " ++ show
(item,inputPrice))

    itemTotal *> \item _ -> do
        price <- need $ itemPrice item
        qty <- need $ itemQty item
        return $ price * qty

    itemizedVat *> \item _ -> do
        vatExempt <- need $ vatStatus . client . invoice $ item
        if vatExempt then return 0 else do
            total <- need $ itemTotal item
            rate <- need $ vatRate item
            return $ calcVatForValue total rate

    total *> \invoice _ -> do
        itemTotals <- forM (items invoice) $ \item -> (,) <$> need
(itemTotal item) <*> need (itemizedVat item)
        let (itemVals,vatVals) = unzip itemTotals
            subTotal = sum itemVals
            vatTotal = sum vatVals
        return $ Total { subTotal, vatTotal, fullTotal = subTotal+vatTotal }

Inspired by the shake build system.  Off the top of my head, so there's no
system I know of that implements something like this.  But it might be a
nice way to declare complex validation rules, perhaps?  Error handling
could be handled by individual rules, so we know if there's a valid
itemPrice and itemQty, the itemTotal is valid too.

It might be tricky to implement this exactly as-is, I'm using "itemTotal"
as both a tag to specify a rule match and also a field name.  And typing
some of the input/output stuff might be non-trivial.  Seems like something
that could benefit from a specialized DSL.

John L.

On Thu, Aug 14, 2014 at 5:06 AM, Alberto G. Corona <agocorona at gmail.com>
wrote:

> Formlets ever had cascade validation:
>
> data Item= Item{ name ::String, quantity, price :: Int}
>
> Item <$>
>        <*> inputString `validate` nonEmpty
>        <*> inputInt
>        <*> inputInt
>  `validate` ( \item -> do
>       if theUniverseIsSpanding then do
>            if name item == "carrots" && price=="10" then
>                        fail "we don´t like 10 cent carrots  in an
> expanding universe"
>            else if.....
>
>
> 2014-08-14 11:41 GMT+02:00 Tom Ellis <
> tom-lists-haskell-cafe-2013 at jaguarpaw.co.uk>:
>
> On Thu, Aug 14, 2014 at 11:28:53AM +0200, Wojtek Narczyński wrote:
>> > On 14.08.2014 09:19, Tom Ellis wrote:
>> > >On Wed, Aug 13, 2014 at 05:21:28PM -0700, John Lato wrote:
>> > >>On Wed, Aug 13, 2014 at 4:21 PM, Tom Ellis
>> > >>>     data LineItem = LineItem { name :: Maybe String
>> > >>>                              , quantity :: Maybe Quantity
>> > >>>                              , price :: Maybe Price }
>> > >>Rather than this definition, what about something like:
>> > >>
>> > >>     data LineItemF f = LineItem
>> > >>         { name :: f String
>> > >>         , quantity :: f Quantity
>> > >>         , price :: f Price }
>> > >It seems Wojtek already objected to this approach, though perhaps that
>> > >objection could be overcome
>> >
>> > Hmm, perhaps like this
>> >
>> > LineItemFi = LineItemFi
>> >    { name :: StringFi
>> >    , quantity :: QuantityFi
>> >    , price :: PriceFi }
>> >
>> >
>> > data LineItemUi f = LineItemUi
>> >     { name :: StringUi
>> >     , quantity :: QuantityUi
>> >     , price :: PriceUi }
>>
>> You didn't use f there.
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>
>
>
> --
> Alberto.
>
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