Re: [Haskell-cafe] How to do this with associated types?

[Ryan Ingram]

On Sun, Jul 25, 2010 at 1:53 PM, Alexey Karakulov ankaraku...@gmail.com wrote:
 Suppose I have one piece of code like this:

 class Result r e | r - e where
    failure :: e - r a
    success :: a - r a

 Maybe instance is discarding failure information:

 instance Result Maybe e where
     failure _ = Nothing
     success x = Just x

I would argue that this shouldn't successfully compile.

Your class declaration claims that given r, you can uniquely determine e.

But you've effectively defined

instance Result Maybe ()
instance Result Maybe Int
... many more instances ...

which means that isn't the case.

I wonder if there is a potential type safety failure in the compiler
with instances of this form?

  -- ryan
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[Haskell-cafe] How to do this with associated types?

[Alexey Karakulov]

Suppose I have one piece of code like this:

 class Result r e | r - e where
    failure :: e - r a
    success :: a - r a

 at :: Result r String = [a] - Int - r a
 at xs i = if i = 0  i  length xs
     then success (xs !! i)
     else failure Wrong index

Either instance of Result is quite straightforward:

 instance Result (Either e) e where
     failure e = Left e
     success x = Right x

Maybe instance is discarding failure information:

 instance Result Maybe e where
     failure _ = Nothing
     success x = Just x

Some tests it in ghci:

ghci let xs = [0,1,2]
ghci at xs 2 :: Either String Integer
Right 2
ghci at xs 3 :: Either String Integer
Left Wrong index
ghci at xs 2 :: Maybe Integer
Just 2
ghci at xs 3 :: Maybe Integer
Nothing

I'd like to replace functional dependencies with type families
(associated types in my case):

 class Result r where
   type Failure
   failure :: Failure r - r a
   success :: a - r a

Either instance is ok:

 instance Result (Either e) where
   type Failure (Either e) = e
   failure e = Left e
   success x = Right x

But what to do with Maybe?

 instance Result Maybe where
   type Failure Maybe = forall e. e -- can't do this
   failure _ = Nothing
   success x = Just x
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Re: [Haskell-cafe] How to do this with associated types?

[Christopher Lane Hinson]

But what to do with Maybe?


instance Result Maybe where
  type Failure Maybe = forall e. e -- can't do this
  failure _ = Nothing
  success x = Just x


Normally, I would use: type Failure Maybe = ()

Unless the ability to discard information of any type is somehow a salient 
feature.

Friendly,
--L

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