Tom Murphy wrote:

The title is self-explanatory. I'd like to store information from an algebraic data type in an SQL database, but the type signature of toSql (toSql :: Data.Convertible.Base.Convertible a SqlValue => a -> SqlValue) doesn't make sense to me. How is this done (how do I make an instance of a typeclass like that?)

My answer is: This is tiring, redundant work. Honestly, don't do it, if you can avoid it. Rather find a type with a ready-made Convertible instance, which can represent the values of your type and convert to and from that one instead. By the way, if your type is just an enumeration of nullary constructors, note that most database systems support efficient 'ENUM' types. For example in PostgreSQL you can do this: CREATE TYPE server_status AS ENUM( 'online', 'offline', 'not available'); According to the documentation columns of type "server_status" will use a compact, efficient 32 bit integer representation, but on the interface you can work with regular strings, so you can just convert your algebraic type to and from strings. Even better, if you don't mind being tied to the particular database system, you can abstract away such types by using stored procedures. Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://coder.mx/ http://ertes.de/