[Haskell-cafe] typeclasses comprehension problems: situation classes?

[Belka]

Hello, cafe visitors!

I'm trying to learn Haskell typeclasses, - about how to use them, - but
can't handle some conceptiual problems, which confuses me a lot. I took one
real problem (ErrorInfo gragual gathering), to tackle it in my studies: I
have a class of situations: there is an object, and it gets cumulatively
filled (or updated) with content. The code is in the end - it consists of 3
versions:
1. My first try. Fast written, based on intuitive understanding. Failed.
2. The second try - exploring an open world assumption. I hoped this would
set me on the right path. Failure.
3. Surrendered to compiler - statisfied all it's requirements. This code
looks absurd to me: parameter-never-to-be-used, unwanted-defaults. Compiler
accepted this one though.

Problems: 
1. How to define *fillerRole* correctly, so that it depends on the
type-value of "src_t"?
2. How to define *initFillable* correctly, so that it depends only on the
type-value "filled_t", which is specified by the context of evaluation?
3. What are my misconcepts in the use of Haskell typeclasses here? 
4. Maybe I should distinguish *situation class* (as this one), as something
unavailable in Haskell? This assumption is the last one to make... I'd
rather belive, that there is something I'm not aware of (for a considerably
long time already) in Haskell. A lack of some programming technique

Please, Help!

Regards, 
Belka

==TRY=1===DOESN'T=COMPILE==================================================

{-# LANGUAGE MultiParamTypeClasses  #-}

class FillsConsideringRoles src_t filled_t role_t where
     initFillable :: filled_t
     fillerRole   :: role_t
     fill         :: src_t -> filled_t -> filled_t

------------------------------------------------------------

data Role = Role1 | Role2 deriving (Show)
data FillableObject = FillableObject 
                            { foData1 :: Maybe (Int, Role)
                            , foData2 :: Maybe (Int, Role)
                            } deriving (Show)

newEmptyFillableObject :: FillableObject
newEmptyFillableObject = FillableObject Nothing Nothing

data Constructor1 = Constructor1 Int
data Constructor2 = Constructor2 Int

instance FillsConsideringRoles Constructor1 FillableObject Role where
     initFillable = newEmptyFillableObject
     fillerRole = Role1
     fill c fo = let (Constructor1 i) = c in fo { foData1 = Just (i,
fillerRole) }

instance FillsConsideringRoles Constructor2 FillableObject Role where
     initFillable = newEmptyFillableObject
     fillerRole = Role2
     fill c fo = let (Constructor2 i) = c in fo { foData2 = Just (i,
fillerRole) }

main = putStrLn $ show $ fill c2 $ fill c1 initFillable
   where
      c1 = Constructor1 76
      c2 = Constructor2 43

==TRY=1==[END]===================================================

==TRY=2===DOESN'T=COMPILE========================================
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE OverlappingInstances #-}

class FillsConsideringRoles src_t filled_t role_t where
     initFillable :: filled_t
     fillerRole   :: role_t
     fill         :: src_t -> filled_t -> filled_t

----------------------------------------------------------------------

data Role = DefaultRole | Role1 | Role2 deriving (Show)
data FillableObject = FillableObject 
                            { foData1 :: Maybe (Int, Role)
                            , foData2 :: Maybe (Int, Role)
                            } deriving (Show)

newEmptyFillableObject :: FillableObject
newEmptyFillableObject = FillableObject Nothing Nothing

data Constructor1 = Constructor1 Int
data Constructor2 = Constructor2 Int

instance FillsConsideringRoles filler_t filled_t Role where
     fillerRole = DefaultRole
instance FillsConsideringRoles Constructor2 filled_t Role where
     fillerRole = Role2
instance FillsConsideringRoles Constructor1 filled_t Role where
     fillerRole = Role1

instance FillsConsideringRoles filler_t FillableObject role_t where
     initFillable = newEmptyFillableObject

instance FillsConsideringRoles Constructor1 FillableObject Role where
     fill c fo = let (Constructor1 i) = c in fo { foData1 = Just (i,
fillerRole) }

instance FillsConsideringRoles Constructor2 FillableObject Role where
     fill c fo = let (Constructor2 i) = c in fo { foData2 = Just (i,
fillerRole) }

main = putStrLn $ show $ fill c2 $ fill c1 initFillable
   where
      c1 = Constructor1 76
      c2 = Constructor2 43
==TRY=2==[END]===================================================

==TRY=3===WORKS=================================================
{-# LANGUAGE MultiParamTypeClasses  #-}

class FillsConsideringRoles src_t filled_t role_t where
     initFillable :: ((),src_t, role_t) -> filled_t
     fillerRole   :: ((),src_t, filled_t) -> role_t
     fill         :: ((),role_t) -> src_t -> filled_t -> filled_t

------------------------------------------------------------

data Role = DefaultRole | Role1 | Role2 deriving (Show)
data FillableObject = FillableObject 
                            { foData1 :: Maybe (Int, Role)
                            , foData2 :: Maybe (Int, Role)
                            } deriving (Show)

newEmptyFillableObject :: FillableObject
newEmptyFillableObject = FillableObject Nothing Nothing

data Constructor1 = Constructor1 Int
data Constructor2 = Constructor2 Int

instance FillsConsideringRoles Constructor1 FillableObject Role where
     initFillable _ = newEmptyFillableObject
     fillerRole _ = Role1
     fill _ c fo = let (Constructor1 i) = c in fo { foData1 = Just (i,
fillerRole ((), c, fo)) }

instance FillsConsideringRoles Constructor2 FillableObject Role where
     initFillable _ = newEmptyFillableObject
     fillerRole _ = Role2
     fill _ c fo = let (Constructor2 i) = c in fo { foData2 = Just (i,
fillerRole ((), c, fo)) }

main = putStrLn $ show $ fill ((), DefaultRole) c2 $ fill ((), DefaultRole)
c1 ((initFillable ((), Constructor1 (-1), DefaultRole)) :: FillableObject)
   where
      c1 = Constructor1 76
      c2 = Constructor2 43
==TRY=3==[END]===================================================
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