2011/7/18 Yitzchak Gale
Roger L. Costello wrote:
data Contract = Contract { currency :: Currency , payments :: Double , contracts :: [Contract] } deriving (Show)
David Virebayre wrote:
I'm not sure I would model your datatype this way, I don't like the idea to put unnecessary undefined values in the case of subcontracts.
data Contract = Contract { currency :: Currency, payments :: Double } | SubContract { contracts :: [Contract] }
I think Roger's original design is just fine.
There is nothing "undefined" about the empty list. Roger is saying that every contract has a list of subcontracts. Lists can be empty, and that is legitimate.
I'm not sure you read his post entirely. Here's his and function :
(and) c1 c2 = Contract { currency = undefined, payments = undefined, contracts = [c1, c2] }
Notice how he's setting currency and payments to undefined, since he needs only the contracts field for subcontracts. David.