Dear Group,
Greetings. I have a feeling that what I am trying to do is easy, I
just don't know how to say it in Haskell. Let's start with:
> {-# LANGUAGE
> DeriveDataTypeable, GeneralizedNewtypeDeriving #-}
>
> import Database.HDBC
> import Data.Typeable (Typeable)
> import Data.Data
> data C = C { str :: String, dbl:: Double }
> deriving (Eq, Ord, Typeable, Data)
>
> a :: C
> a = C "twelve" 12.0
>
Now I load this up in ghci and I can do the following:
toSql . str $ a -- result: SqlString "twelve"
toSql . dbl $ a -- result: SqlDouble 12.0
but what I would really like to do is something like:
gmapQ toSql $ a
which results in:
<interactive>:1:7:
Could not deduce (Convertible d SqlValue)
arising from a use of `toSql'
from the context (Data d)
bound by a type expected by the context: Data d => d -> SqlValue
at <interactive>:1:1-11
Possible fix:
add (Convertible d SqlValue) to the context of
a type expected by the context: Data d => d -> SqlValue
or add an instance declaration for (Convertible d SqlValue)
In the first argument of `gmapQ', namely `toSql'
In the expression: gmapQ toSql
In other words, I'm looking for a function with a signature:
(Whatever Instances I neeed here) => a -> [SqlValue]
I have tried various incantations of type signatures, but thus far I
can't get it right. Can someone point me in the right direction? Thanks.
Henry Laxen
--
Nadine & Henry Laxen Belle, Venus, Aphrodite
10580 N. McCarran Blvd. Adonis, Miss Parker & Jarod
Suite 115-396 Via Alta # 6
Reno, Nevada Chapala, Jalisco, Mexico
89503-1896 CP 45900
The rest is silence. (Hamlet)
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