[Haskell-cafe] The Related monad and constant values in type classes

Jonas Almström Duregård jonas.duregard at gmail.com
Wed Feb 17 10:48:45 EST 2010


This literate haskell file was intended to be a quick question about a
problem i have been pondering, but it developed into a short
presentation instead. What i want to know is if there is already
something like this (and suggestions for improvement of course).

>{-#LANGUAGE GeneralizedNewtypeDeriving#-}

Sometimes i find myself needing to associate a constant with a type
or, more precisely, with a type class instance. Something like this
would be nice:

class Sized a where
  size :: Int

instance Sized Int where
  size = 32

Of course this will not work since there is no way of knowing which
instance i refer to when i use "size". A common work-around is to use
a dummy parameter:

>class SizedDummy a where
>  sizeDummy :: a -> Int
>instance SizedDummy Int where
>  sizeDummy = const 32

The size function is typically passed an undefined value. This is not
very pretty, and somewhat unsafe. Another workaround is to define a
newtype with a type parameter.

>newtype SizeOf a = MkSize {toInt :: Int}
>class SizedNewType a where
>  sizeNewType :: SizeOf a
>instance SizedNewType Int where
>  sizeNewType = MkSize 32

If we want the size of a pair to be the sum of it's components,
something like this is needed:

>instance (SizedNewType a, SizedNewType b) => SizedNewType (a,b) where
>  sizeNewType = sizeNewType' sizeNewType sizeNewType where
>    sizeNewType' :: SizeOf a -> SizeOf b -> SizeOf (a,b)
>    sizeNewType' a b = MkSize $ toInt a + toInt b

This is way to much code say that "size = size a + size b". A more
general solution can be achieved by making "Int" another type variable
of "SizeOf". I call the resulting type "Related":

>newtype Related a b = Related {unrelated :: b} deriving
>          (Eq,Ord,Show,Read,Bounded,Enum,Fractional,Num,
>           Real,Integral,RealFrac,Floating,RealFloat)

This type is highly reusable and the GeneralizedNewtypeDeriving
language extension is very practical (although the instances could be
written manually). It can also be used as an Identity monad:

>instance Functor (Related a) where
>  fmap f (Related a) = Related $ f a
>instance Monad (Related a) where
>  return = Related
>  (Related a) >>= f = f a

This allows the Sized class and instances to be specified in a slim
fashion using a familiar monadic interface:

>class Sized a where
>  size :: Related a Int
>instance Sized Int where
>  size = return 32
>instance (Sized a, Sized b) => Sized (a,b) where
>  size = do
>    a <- return size :: Sized a => Related (a,b) (Related a Int)
>    b <- return size :: Sized b => Related (a,b) (Related b Int)
>    return $ unrelated a + unrelated b

This still requires a lot of type signatures, some additional magic is
required. It is possible to write general versions of the type
signatures above, which allows the following instance definition for

>instance (Sized a, Sized b, Sized c) => Sized (a,b,c) where
>  size = do
>    a <- on3 size
>    b <- on2 size
>    c <- on1 size
>    return $ a + b + c

With the derivation of Num, this can be done even more compact:

>instance (Sized a, Sized b, Sized c, Sized d) => Sized (a,b,c,d) where
>  size = on1 size + on2 size + on3 size + on4 size

The code for the onN functions:

>rerelate :: Related a b -> Related c b
>rerelate = return . unrelated

>on1 :: Related a v -> Related (x a) v
>on1 = rerelate

>on2 :: Related a v -> Related (x a x0) v
>on2 = rerelate

>on3 :: Related a v -> Related (x a x0 x1) v
>on3 = rerelate

>on4 :: Related a v -> Related (x a x0 x1 x2) v
>on4 = rerelate

Jonas Almström Duregård

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