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
