I have stripped things down to the bare minimum, and test under GHC
6.10, GHC 6.12, Linux, and Mac OS X. Results are consistent.
In the following code,
1. if you load the code into ghci and evaluate e it will hang, but
(defaultValueD dict) :: Expression returns fine
2. if you change the gunfold instance for Proposition to, error
"gunfold" it stops hanging -- even though this code is never called.
3. if you change, ( Data ctx [Expression], Sat (ctx Expression) =>
Data ctx Expression, to (Data ctx Expression, ....) => ... it stops
hanging.
If someone could explain why each of these cases perform as they do,
that would be awesome! Right now it is a big mystery to me.. e calls
dict .. and there is only one instance of dict available, which should
call error right away. I can't see how something could get in the way
there...
- jeremy
{-# LANGUAGE DeriveDataTypeable, FlexibleContexts, FlexibleInstances,
MultiParamTypeClasses, UndecidableInstances, RankNTypes,
ScopedTypeVariables, KindSignatures, EmptyDataDecls,
NoMonomorphismRestriction #-}
module Main where
import qualified Data.Data as Data
import Data.Typeable
--- syb-with-class
data Constr = Constr deriving (Eq, Show)
data Proxy (a :: * -> *)
class Sat a where
dict :: a
class (Typeable a, Sat (ctx a)) => Data ctx a where
gunfold :: Proxy ctx
-> (forall b r. Data ctx b => c (b -> r) -> c r)
-> (forall r. r -> c r)
-> Constr
-> c a
instance (Sat (ctx [a]),Data ctx a) => Data ctx [a]
--- Default
class (Data DefaultD a) => Default a where
defaultValue :: a
data DefaultD a = DefaultD { defaultValueD :: a }
instance Default t => Sat (DefaultD t) where
dict = error "Sat (DefaultD t) not implemented"
instance Default a => Default [a] where
defaultValue = error "Default [a] not implemented"
--- Trouble
data Proposition = Proposition Expression deriving (Show, Data.Data,
Typeable)
data Expression = Conjunction Expression deriving (Show, Data.Data,
Typeable)
-- instance (Sat (ctx [Expression]), Sat (ctx Expression), Sat (ctx
Proposition)) => Data ctx Proposition where
instance Data DefaultD Proposition where
gunfold _ k z c = k (z Proposition)
-- gunfold _ k z c = error "gunfold"
instance Default Proposition
-- Change Data ctx [Expression] to Data ctx Expression and main works.
instance ( Data ctx [Expression]
, Sat (ctx Expression)
) => Data ctx Expression
instance Default Expression
e :: Expression
e = defaultValueD (dict :: DefaultD Expression)
main = print e
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