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 <[EMAIL PROTECTED]> wrote: > Max Vasin wrote: > > >Bryn Keller <[EMAIL PROTECTED]> 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 > ___ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Type classes and definite types
Max Vasin wrote: Bryn Keller <[EMAIL PROTECTED]> 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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Type classes and definite types
Bryn Keller <[EMAIL PROTECTED]> 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 :-( -- WBR, Max Vasin. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Type classes and definite types
Max Vasin wrote: Bryn Keller <[EMAIL PROTECTED]> writes: Hi folks, Hello, [skip] The problem is that getFieldValue returns a value of type (SqlBind a) => a. That is, there's no type information associated with this return value other than it's a valid SQL value. There are no operations in the SqlBind class, it's just a marker as near as I can tell. As of HSQL 1.4 this class has method toSqlValue :: a -> String which probably can help you (I really don't know). Some RTFSing shows that in most cases toSqlValue is implemented with show. Hi Max, 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. Bryn ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Type classes and definite types
Bryn Keller <[EMAIL PROTECTED]> writes: > Hi folks, Hello, [skip] > The problem is that getFieldValue returns a value of type (SqlBind a) > => a. That is, there's no type information associated with this return > value other than it's a valid SQL value. There are no operations in > the SqlBind class, it's just a marker as near as I can tell. As of HSQL 1.4 this class has method toSqlValue :: a -> String which probably can help you (I really don't know). Some RTFSing shows that in most cases toSqlValue is implemented with show. -- WBR, Max Vasin. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
