Re: [Haskelldb-users] Newbie question

[Justin B. <jgb...@gm...>]

On Tue, Nov 16, 2010 at 1:31 PM, Paul Graphov <gr...@gm...> wrote:
> Hello Justin,
> I've made a minimal example:
> http://hpaste.org/41544/haskelldb_example

> Error with removed type signature is as follows:
> $ ghc --make Example.hs
> [1 of 1] Compiling Main             ( Example.hs, Example.o )
> Example.hs:31:7:
>     Ambiguous type variable `a' in the constraints:
>       `Database.HaskellDB.Database.GetValue a'
>         arising from a use of `get_as' at Example.hs:31:7-12
>       `Show a' arising from a use of `print' at Example.hs:31:18-22
>     Probable fix: add a type signature that fixes these type variable(s)

If you give the 'a' attribute a type everything works. You have to
tell HaskellDB what type 'a' represents. Without a signature it
assumes:

  a :: Attr A x

where 'x' isn't known. With it, 'a' has a concrete type

  a :: Attr A Int

I updated your paste:

  http://hpaste.org/41546/fixing_the_type_error

Thanks for providing the example!

Justin