[Haskell-cafe] I hate Haskell's typeclasses

Jonathan Cast jonathanccast at fastmail.fm
Sat Apr 19 01:01:13 EDT 2008

On 18 Apr 2008, at 9:29 PM, Ryan Ingram wrote:


>   Hopefully this fires off some productive
> discussion on how to fix these problems!

{-# GHC_OPTIONS -foverlapping-instances -fundecidable-instances #-} :)

What you want to work is precisely what this allows.

> Don't get me wrong:  I think the idea of typeclasses is great.  Their
> implementation in Haskell comes so close to being awesome and then
> falls short, and that's almost worse than not being awesome in the
> first place!

We've noticed.  The literature on extending Haskell type classes is,  
um, enormous.

> Some examples of things I think you should be able to do, that just Do
> Not Work.  Examples like these are trivial in many other languages,

I call.  Name a language that is

a) Completely statically typed (no type errors at runtime),
b) Has an ad-hoc overloading mechanism powerful enough to encode Num  
and Monad, and
c) Is substantially better than Haskell + extensions for your examples.

The examples aren't all that long; comparison code snippets shouldn't  
be all that long either.

> and they shouldn't be that hard here, either!
> 1) You can't make sensible default implementations.  For example, it'd
> be nice to make all my Monads be Applicatives and Functors without
> resorting to Template Haskell or infinite boilerplate.  Why can't I
> just write this?
> instance Monad m => Applicative m where
>     pure = return
>     (<*>) = ap
> Sure, I get that there might be ambiguity of which instance to choose.
>  But why not warn me about that ambiguity, or let me choose somehow on
> a case-by-case basis when it happens?

You can already choose on a case-by-case basis.  In this specific  
case, you can only think of one super-instance, but I can think of  

instance Arrow a => Applicative (a alpha) where
   pure = arr . const
   a <*> b = (a &&& b) >>> arr ($)

I think Conal Elliot's recent work of FRP can be extended to show  
that Fudgets-style stream processors can be made instances of  
Applicative by both these methods, with different instances.  So as  
soon as both are present, you have to choose the instance you want  
every time.  Having something like this spring up and bite you  
because of a change in some library you pulled off of Haddock does  
not make for maintainable code.

More generally, specifying what you want is really not hard.  Do you  
really have gazillions of monads in your code you have to repeat this  
implementation for?

> 2) You can't add sensible superclasses.  I was playing with QuickCheck
> and wanted to write "equal with regards to testing".  So I wrote up a
> class for it:
> class TestableEq a where
>     (~=) :: a -> a -> Property
> instance Eq a => TestableEq a where
>     -- should be a superclass of Eq instead!
>     a ~= b = a == b

Again, this is one (*) line per type.  How many types do you declare?

> instance (Arbitrary a, TestableEq b) => TestableEq (a -> b) where
>     f ~= g = forAll arbitrary (\a -> f a ~= g a)
> But this doesn't work without overlapping & undecidable instances!
> Sure, there is an alternative: I could manually declare instances of
> TestableEq for EVERY SINGLE TYPE that is an instance of Eq.  I am sure
> nobody here would actually suggest that I do so.

Bzzzt.  Wrong.  Thanks for playing!

> And sure, these extensions are both safe here, because the intent

What?  By that reasoning, perl is `safe'.  Haskell is not perl.

> is
> that you won't declare instances of TestableEq for things that are
> already instances of Eq, and you won't do something stupid like
> "instance TestableEq a => Eq a".
> But why do I need to jump through these hoops for a perfectly safe &
> commonly desired operation?

It's called a proof obligation.  Haskell is not here to stop you from  
jumping through hoops.  In fact, it is here precisely to force you to  
jump through hoops.  That's why it's called a bondage and discipline  

> 3) There's no reflection or ability to choose an implementation based
> on other constraints.
> In QuickCheck, (a -> b) is an instance of Arbitrary for appropriate a,
> b.  But you can't use this instance in forAll or for testing functions
> without being an instance of Show.  Now, this is probably a design
> mistake, but it's the right choice with the current typeclass system
> (see (2)).  But it'd be a million times better to have something like
> the following:
> class Arbitrary a => MkArbitrary a where
>    mkArbitrary :: Gen (a, String)
> case instance MkArbitrary a where
>    Show a =>
>        mkArbitrary = do
>            x <- arbitrary
>            return (x, show x)
>    otherwise =>
>        mkArbitrary = do
>            st <- getGenState
>            x <- arbitrary
>            return (x, "evalGen arbitrary " ++ show st)

So we compile in a table of every instance and every datatype, add a  
Typeable constraint to forAll (since parametricity just got shot to  
heck), and scan through that table on every test.  Millions of times  
better.  And slower.  And more likely to develop odd changes and hard- 
to-debug errors.

> With this, QuickCheck could print reproducible test cases painlessly
> without adding the requirement that everything is an instance of Show!

QuickCheck makes testing so easy, I think the Arbitrary (a -> b)  
instance is almost unnecessary; (btw., functions /are/ instances of  
Show).  You can easily write a showable ADT encoding the functions  
you want to test.

> Now, you could say that mkArbitrary should be a member function of
> Arbitrary, but then you clutter up your instance definitions with tons
> of "mkArbitrary = defaultMkArbitrary" for types that have a Show
> instance.

Thousands and thousands of pounds!  You have too many types.  Look  
for ways to re-factor, and move your duplication into functors.

> 4) Every concrete type should be an instance of Typeable without
> having to do anything,

Sure.  And seq should go back to being a class method.  (See earlier  
about parametricity being shot to heck).  I have an excellent design  
which will preserve the language's semantics (which are fine the way  
they are, thank you), while being convenient for programmers, which  
this margin is too small to contain.[1]

> and Typeable should give you typecase &
Type case is easy:

   genericShow :: Typeable a => a -> String
   genericShow x = fromJust $ do
                                 s <- cast x :: Maybe String
                                 return s
                      `mplus` do
                                 n <- cast x :: Maybe Int
                                 return (show n)
                      `mplus` do
                                 return "<unknown>"

> reflection:
> genericShow :: Typeable a => a -> String
> genericShow x = typecase x of
>     String -> x
>     (Show t => t) -> show x -- any instance of Show
>     _ -> "<unknown>"

Reflection is harder, because of the need for the lookup table with  
every instance of every class I mentioned earlier.  (And you get to  
figure out how to encode polymorphic instances, too!  Good luck[2]).


[1, 2] These are the non-sarcastic bits.

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