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

Hi Bryn Keller, The solution for your problem is very simple. You just have to fetch all values as strings. In this way the library will do all required conversions for you. printRow stmt = do id <- getFieldValue stmt "ID" code <- getFieldValue stmt "Code" name <- getFieldValue stmt "Name" putStrLn (unwords [id, code, name]) Cheers, Krasimir On 5/6/05, Bryn Keller wrote:

Max Vasin wrote:

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Bryn Keller writes:

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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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe