Thanks for the quick reply Edward. What I'm trying to do is be lazy. I have a longish data structure, say: data Loan = Loan { loan_address :: String, loan_initialAmount :: Double, loan_interestRate :: Double, loan_term:: Int, loan_originated :: Day, loan_minimumPayment :: Double, loan_usualPayment :: Double, loan_familiarName :: String, loan_password :: String, loan_lastVisit :: Day, loan_nextToLastVisit :: Day, loan_emailAddress :: String, } deriving (Read, Show, Eq, Ord, Typeable, Data) where each component is an instance of (Convertible a SqlValue), and I would like to be able to say something like: let vals = magicalGmapIncantation toSql loan liftIO $ withtransaction db $ \d -> run query vals of course I could do: let vals = [toSql (loan_address loan), toSql (loan_initialAmount loan) ...] but that feels so "dirty," when all those fields are just waiting for me inside that loan data type. Best wishes, H

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"EZY" == Edward Z Yang writes:

EZY> Hmm, this is a bit peculiar. The problem is you don't get control EZY> over how gmapQ invokes the function toSql: it will only ever be done EZY> with the type signature Data d => d -> u. There is good reason for EZY> this too: imagined you tried to run gmapQ toSql on a data-type that EZY> contained a member that was not convertible to a SqlValue: then it EZY> ought to fail with a type error! EZY> You may be able to work around this with more generics madness: use EZY> Typeable to check if the types of all the fields are kosher, and then EZY> do an appropriate casts before invoking toSql. But you won't get EZY> particularly good static guarantees doing it this way. EZY> So... what are you really trying to do? :-) ------------------------------------------------------------------------ Greetings. I have a feeling that what I am trying to do is easy, I just don't know how to say it in Haskell. Let's start with:

{-# LANGUAGE DeriveDataTypeable, GeneralizedNewtypeDeriving #-}

import Database.HDBC import Data.Typeable (Typeable) import Data.Data data C = C { str :: String, dbl:: Double } deriving (Eq, Ord, Typeable, Data)

a :: C a = C "twelve" 12.0

Now I load this up in ghci and I can do the following: toSql . str $ a -- result: SqlString "twelve" toSql . dbl $ a -- result: SqlDouble 12.0 but what I would really like to do is something like: gmapQ toSql $ a which results in: <interactive>:1:7: Could not deduce (Convertible d SqlValue) arising from a use of `toSql' from the context (Data d) bound by a type expected by the context: Data d => d -> SqlValue at <interactive>:1:1-11 Possible fix: add (Convertible d SqlValue) to the context of a type expected by the context: Data d => d -> SqlValue or add an instance declaration for (Convertible d SqlValue) In the first argument of `gmapQ', namely `toSql' In the expression: gmapQ toSql In other words, I'm looking for a function with a signature: (Whatever Instances I neeed here) => a -> [SqlValue] I have tried various incantations of type signatures, but thus far I can't get it right. Can someone point me in the right direction? Thanks. -- Nadine & Henry Laxen Belle, Venus, Aphrodite 10580 N. McCarran Blvd. Adonis, Miss Parker & Jarod Suite 115-396 Via Alta # 6 Reno, Nevada Chapala, Jalisco, Mexico 89503-1896 CP 45900 The rest is silence. (Hamlet)

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"EZY" == Edward Z Yang writes:

EZY> Where did 'query' come from? Edward I left that out because it was just a prebuilt db query. It came from something like: dataNames :: Data a => a -> [String] dataNames = constrFields . toConstr loanNames = datanames (loan :: Loan) {- yielding: ["loan_address","loan_initialAmount","loan_interestRate","loan_term","loan_originated","loan_minimumPayment","loan_usualPayment","loan_familiarName","loan_password","loan_lastVisit","loan_nextToLastVisit","loan_emailAddress"] -} let query = "INSERT INTO loans (" ++ loanNames ++ ") values (" ++ (intercalate ", " (replicate (length loanNames) "?")) ++ ")" Best wishes, H