[Haskell-cafe] generalized newtype deriving allows the definition of otherwise undefinable functions

[Max Cantor]

Isn't this just an extension of the notion that multi-parameter typeclasses without functional dependencies or type families are dangerous and allow for type-naughtiness?  


On Mar 9, 2010, at 5:45 AM, Wolfgang Jeltsch wrote:

> Hello,
> 
> some time ago, it was pointed out that generalized newtype deriving could be 
> used to circumvent module borders. Now, I found out that generalized newtype 
> deriving can even be used to define functions that would be impossible to define 
> otherwise. To me, this is surprising since I thought that generalized newtype 
> deriving was only intended to save the programmer from writing boilerplate 
> code, not to extend expressiveness.
> 
> Have a look at the following code:
> 
>> {-# LANGUAGE
>>    GeneralizedNewtypeDeriving,
>>    MultiParamTypeClasses,
>>    FlexibleInstances
>> #-}
>> 
>> class Iso a b where
>> 
>>    conv :: item a -> item b
>> 
>> instance Iso a a where
>> 
>>    conv = id
>> 
>> newtype Wrapped a = Wrap a deriving (Iso a, Show)
> 
> Now any value whose type contains some type t can be converted into a value of 
> the type that you get if you replace t by Wrap t. Here is some code to 
> demonstrate this for binary operations:
> 
>> newtype BinOp a = BinOp (a -> a -> a)
>> 
>> convBinOp :: (a -> a -> a) -> (Wrapped a -> Wrapped a -> Wrapped a)
>> convBinOp op = let BinOp op' = conv (BinOp op) in op'
> 
> Now, you can enter
> 
>    convBinOp (*) (Wrap 5) (Wrap 3)
> 
> into GHCi, and you will get
> 
>    Wrap 15
> 
> as the result.
> 
> The point is, of course, that such conversions are not only possible for 
> binary operations but for arbitrary values and that these conversions are done 
> by a single generic function conv. I don’t think it would be possible to 
> implement conv without generalized newtype deriving.
> 
> Any thoughts?
> 
> Best wishes,
> Wolfgang
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