On Thu, Feb 03, 2011 at 12:20:12PM -0500, Patrick Lynch wrote:
Good morning,
I'm using "Real World Haskell" and a Windows Vista PC and I have Sqlite3 installed...[note: I had to change test1.db to /users/user/test1.db in order to get this to work, otherwise, neither the database nor the table could be created -- also, note c:/users/user/test1.db gives a syntax error], ghci doesn't like c:/]:
I tried to use the following example from the book but it failed...see following:
Prelude> :m Database.HDBC Database.HDBC.Sqlite3 Prelude Database.HDBC Database.HDBC.Sqlite3> conn <- connectSqlite3 "/users/user/test1.db" ... Prelude Database.HDBC Database.HDBC.Sqlite3> run conn "INSERT INTO test VALUES (?, ?)" [toSql 0, toSql "zero"]
<interactive>:1:43: No instance for (Data.Convertible.Base.Convertible t SqlValue) arising from a use of `toSql' at <interactive>:1:43-49 Possible fix: add an instance declaration for (Data.Convertible.Base.Convertible t SqlValue) In the expression: toSql 0 In the third argument of `run', namely `[toSql 0, toSql "zero"]' In the expression: run conn "INSERT INTO test VALUES (?, ?)" [toSql 0, toSql "zero"]
I think the problem is that 0 is polymorphic, so GHC does not know what type you want it to have, but its type determines which Convertible instance is chosen. It should work if you put a type signature on the 0, like toSql (0 :: Int) or toSql (0 :: Integer) or whatever. -Brent