(Apologies for any duplicates... for some reason the original's reply address is haskell-cafe@googlegroups.com rather than haskell-cafe@haskell.org, and it sent a rejection notice for my reply.)

If I understand your model right, the Item is just a count, and it's the Basket's 'name' field which actually says what the item is? Assuming I have that right, check this out:

-- A basket is the association of an item with a count.

data Basket = Basket {item :: String, count :: Int}

-- Open and closed baskets are just baskets with different behavior.

newtype OpenBasket = OpenBasket {getOpenBasket :: Basket}

newtype ClosedBasket = ClosedBasket {getClosedBasket :: Basket}

-- A cart can contain a mix of both basket types.

type CartBasket = Either OpenBasket ClosedBasket

data Cart = List CartBasket

-- We'll want to conveniently inspect CartBaskets.

getCartBasket :: CartBasket -> Basket

getCartBasket = either getOpenBasket getClosedBasket

-- This function accepts only OpenBaskets. No need for a runtime check.

addToOpenBasket :: OpenBasket -> OpenBasket

addToOpenBasket (OpenBasket b@(Basket _ i))

= OpenBasket (b {count = i+1})

-- First-class functions let you keep this safety all the way to the top...

data UserControl a = UserControl {label :: String, behavior :: a -> a}

-- Only allow the user to see valid behaviors!

getAddToCartBasketUserControl :: CartBasket -> UserControl CartBasket

getAddToCartBasketUserControl =

let f = either (Left . addToOpenBasket) Right

in either (const (UserControl "Add" f)) (const (UserControl "Locked" f))

On Sat, Jul 30, 2016 at 3:01 PM, Gurudev Devanla <gurudev.devanla@gmail.com> wrote:

Hello All,

I have a design question where I could use some of your thoughts and suggestions. Here is the setup. I apologize
for the long email but I have resisted asking this question, since it needs a long explanation, until now.

I have a set of data structures defined as follows

-- An Item that can be of two types, one whose value can be changed, one whose value are frozen once created
data Item = FreeToChange {freeToChangeCount:: Int}
| CannotChange {frozenCount:: Int}

-- The item is part of a basket
data Basket = Basket { name:: String, item::Item }

-- The cart can have both kind of Baskets at the start of the program, or during runtime.
data Cart = List Basket

You can imagine this be a shopping cart with fixed set of items. Where the count of
some of the items in the basket can be changed during shopping but not the count of the
items once they are tagged as frozen.

Therefore, valid operation are:

1. I can create an Basket with either FreeToChange item or CannotChange item.
2. I can update the count for FreeToChange item in the Basket
3. But, once I create an instance of the Basket to contain the CannotChange item,
   we cannot update the count anymore or update the Basket.

One approach I have taken is to throw an error if this happens by pattern matching on type. But, this is
an runtime check.

addToBasket :: Basket -> Basket
addToBasket b = let
i = item b
i' = case i of
    FreeToChange f -> FreeToChange {freeToChangeCount = f + 1}
    CannotChange f -> error ("This operation is not allowed")
in
    b {item=i'}

Here are my questions:

1. Is there a way to design the above data structures in such a way I could use the type system.
2. Since, these are runtime changes, is it even a good design pattern to push this responsibility to a type system?

Thanks
Guru

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