On 2017-10-01 07:55 PM, Guru Devanla wrote:
Having to not have something which I can quickly start off on troubles me and makes me wonder if the reason is my lack of understanding or just the pain of using static typing.
Something, somewhere has to keep track of the type of each column, and since the data doesn’t have that itself you have to store it somewhere else. That could be in another data file of some kind which would be loaded at runtime, but then you would lose the benefit of static type checking by the compiler. So it’s better to have it in source code, even if that’s generated by TH or some other process. I recommend taking a look at the Cassava library. You can do some pretty neat things with it, including defining your own mapping from rows to records. In particular, if you need only a small subset of the 100 columns, you can provide a (de)serializer that looks at only the columns it needs. The library reads the row into a vector of Text, and your serialization code works with just the elements it needs. You could even have different record types (and associated serializers) for different tasks, all working off the same input record, since the serialization methods are from a typeclass and each record type can be a different instance of the class. Cassava supports Applicative, which makes for some very succinct code, and it can make use of a header record at the start of the data. Here’s an example: |data Account = Business | Visa | Personal | Cash | None deriving (Eq, Ord, Show, Read, Enum, Bounded) instance FromField Account where parseField f | f == "B" = pure Business | f == "V" = pure Visa | f == "P" = pure Personal | f == "C" = pure Cash | f == "CC" = pure Visa | f == "" = pure None | otherwise = fail $ "Invalid account type: \"" ++ B.unpack f ++ "\"" instance ToField Account where toField Business = "B" toField Visa = "V" toField Personal = "P" toField Cash = "C" toField None = "" type Money = Centi data Transaction = Transaction { date :: Day , description :: Text , category :: Text , account :: Account , debit :: Maybe Money , credit :: Maybe Money , business :: Money , visa :: Money , personal :: Money , cash :: Money } deriving (Eq, Ord, Show, Read) instance FromNamedRecord Transaction where parseNamedRecord r = Transaction <$> r .: "Date" <*> r .: "Description" <*> r .: "Category" <*> r .: "Account" <*> r .: "Debit" <*> r .: "Credit" <*> r .: "Business" <*> r .: "Visa" <*> r .: "Personal" <*> r .: "Cash" instance ToNamedRecord Transaction where toNamedRecord r = namedRecord [ "Date" .= date r, "Description" .= description r, "Category" .= category r, "Account" .= account r, "Debit" .= debit r, "Credit" .= credit r, "Business" .= business r, "Visa" .= visa r, "Personal" .= personal r, "Cash" .= cash r] | Note that the code doesn’t assume fixed positions for the different columns, nor a total number of columns in a row, because it indirects through the column headers. There could be 1000 columns and the code wouldn’t care.