Re: [Haskell-cafe] Re: Type classes and definite types

Max Vasin wrote:

Bryn Keller writes:

...

Hi Max,

Hello Bryn,

...

Thanks for pointing this out. It's odd that I don't see that anywhere in the docs at the HToolkit site: http://htoolkit.sourceforge.net/doc/hsql/Database.HSQL.html but GHC certainly believes it exists. However, this doesn't actually solve the problem. Substituting toSqlValue for show in printRow' gives the same compile error:

Main.hs:22:18: Ambiguous type variable `a' in the constraint: `SqlBind a' arising from use of `getFieldValue' at Main.hs:22:18-30 Probable fix: add a type signature that fixes these type variable(s)

So, like with (show (read s)), we still can't use the function until we've established a definite type for the value, not just a type class.

Yeah... Some more RTFSing shows that we have the

getFieldValueType :: Statement -> String -> (SqlType, Bool)

which allows us to write

printRow stmt = do (id :: Int) <- getFieldValue stmt "ID" let (codeType, _) = getFieldValueType stmt "Code" codestr <- case codeType of SqlChar _ -> do (c :: String) <- getFieldValue stmt "Code" return (toSqlValue c) SqlInteger -> do (i :: Int) <- getFieldValue stmt "Code" return (toSqlValue i) -- etc for all SqlType data constructors putStrLn (unwords [show id, codestr])

At least it compiles. But it's ugly :-(

Ah, good point! Ugly it may be, but at least it works. Thanks for the idea! Bryn