[Haskell-cafe] Rank-2-polymorphism problem

[Martin Huschenbett]

Hi,

I'm writing some database code using HSQL and had to stop on a problem 
with rank-2-polymorphism that I can't solve. The essence of my code is:


module Value where

import Data.Maybe

class SqlBind a where
   fromSqlValue :: String -> a

data Field
data Value

emptyValue :: Field -> Value
emptyValue _ = ...

readValue :: Field -> (forall s. SqlBind s => s) -> Value
readValue _ = ...


That works just fine. But now I want a version of readValue that has a 
Maybe wrapped around the second parameter and that shall call readValue 
in the case of a Just and emptyValue in the case of Nothing. But I can't 
figure out how to write this function as I always get compiler errors.

My trials were:


-- The type I want to get.
readValue' :: Field -> (forall s. SqlBind s => Maybe s) -> Value

-- First trial:
readValue' fld s =
   if isJust s then readValue fld (fromJust s) else emptyValue fld

-- Second trial:
readValue' fld s
   | isJust s  = readValue fld (fromJust s)
   | otherwise = emptyValue fld

-- Third trial:
readValue' fld (Just s) = readValue fld s
readValue' fld Nothing  = emptyValue fld

-- Fourth trial:
readValue fld s = case s of
   Just s' -> readValue fld s'
   Nothing -> emptyValue fld


But none of these trials worked. Is there any solution that works with 
GHC-6.6 for now?

Thanks in advance,

Martin.