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]=================================================== -- View this message in context: http://www.nabble.com/typeclasses-comprehension-problems%3A-situation-classe... 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