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

[Krasimir Angelov]

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 <xoltar at xoltar.org> wrote:
> Max Vasin wrote:
> 
> >Bryn Keller <xoltar at xoltar.org> 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
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