I just ran into a strange behaviour of both Hugs and GHC. I discussed it
with John Hughes who managed to find a workaround, and suggested that I
ask you whether this is really expected behaviour.
The attached file contains a small example that exhibits the problem.
--
/MpB
-----------------------------------------------------------------------------
-- Class Default - used to define default values for types
class Default a where
defaultVal :: a
instance Default () where
defaultVal = ()
instance Default Bool where
defaultVal = False
instance Default Int where
defaultVal = 0
-----------------------------------------------------------------------------
-- Data type with annotations
data MyAnnotatedType a = MAT { theInt :: Int
, theBool :: Bool
, theAnnotation :: a
}
-- If the annotation type a is an instance of default, the we can define
-- default values for MyAnnotatedType a:
instance Default a => Default (MyAnnotatedType a) where
defaultVal = MAT defaultVal defaultVal defaultVal
-----------------------------------------------------------------------------
-- Defining values of MyAnnotatedType (this is where we run into problems)
noProblem :: MyAnnotatedType ()
noProblem = defaultVal { theInt = 42 }
noProblem2 :: MyAnnotatedType Int
noProblem2 = defaultVal { theInt = 42 }
problematic :: MyAnnotatedType Int
problematic = defaultVal { theInt = 42
, theAnnotation = 10
}
-- The line ", theAnnotation = 10" is the problematic one. It seems impossible
-- to fix the problem by rewriting this line (I've tried things such as
-- defaultVal, error "", or adding the type explicitly).
-----------------------------------------------------------------------------
-- A workaround (thanks to John Hughes)
-- define an unoverloaded version of defaultVal
defaultMAT = defaultVal :: MyAnnotatedType Int
workaround :: MyAnnotatedType Int
workaround = defaultMAT { theInt = 42
, theAnnotation = 10
}
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