[Haskell-cafe] The Related monad and constant values in type
classes
Jonas Almström Duregård
jonas.duregard at gmail.com
Thu Feb 18 13:01:44 EST 2010
Hi Edward,
Nothing new under the sun it would seem :).
Perhaps these functions could be useful in the Tagged library?
>on1 :: Tagged a v -> Tagged (x a) v
>on1 = retag
>
>on2 :: Tagged a v -> Tagged (x a x0) v
>on2 = retag
>
>on3 :: Tagged a v -> Tagged (x a x0 x1) v
>on3 = retag
>
>on4 :: Tagged a v -> Tagged (x a x0 x1 x2) v
>on4 = retag
They allow the user to perform operations on the type parameters of an
instantiated type without adding a lot of additional type signatures
etc., e.g.
>instance SomeClass a => SomeClass [a] where
> someFunction = on1 someFunction
/Jonas
2010/2/18 Edward Kmett <ekmett at gmail.com>:
> I've needed something similar in the past.
>
> I used it in the reflection library, and its present on its own on hackage
> as 'tagged'.
>
> http://hackage.haskell.org/packages/archive/tagged/0.0/doc/html/Data-Tagged.html
>
> I talked a bit about using it here:
>
> http://comonad.com/reader/2009/clearer-reflection/
>
> -Edward Kmett
>
> 2010/2/17 Jonas Almström Duregård <jonas.duregard at gmail.com>
>>
>> Hi,
>>
>> 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
>>
>>
>> Regards,
>> Jonas Almström Duregård
>> _______________________________________________
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>> Haskell-Cafe at haskell.org
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>
>
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