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

5 May 2005

      Max Vasin wrote:
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Bryn Keller  writes:
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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