Clarification: classy prelude

July 16, 2012

GravatarBy Michael Snoyman

Firstly, I wanted to mention that the markdown engine powering this blog is now available on Hackage. I made a small release announcement. Basically: it's experimental, and I'd love feedback on how it works for others, but it should be quite usable as-is.

Anyway, onto the main event.

When I announced classy prelude, there was quite a mixed reaction from the community. There was quite a bit of positive feedback, and lots of people seemed interested. And on the flip side, a number of people very unequivocally declared it a horrible, horrible idea.

I'm not going to mince words: I think a couple of the detractors are making a number of baseless assertions, attacking strawmen, and engaging in hyperbole. Declaring war on a new package and swearing to never use it or any of its dependencies to bring about its eventually demise does not really fit into a normal, healthy discussion. Making claims about "brittle typeclass extensions" without any clarification is not helpful. When discussions devolve to such a level, I don't think there's any point in engaging.

Part of the fault in all of this is that I did not clarify the purpose of the library well enough in the initial post, instead focusing more on how it works. I will try to rectify that in this post, and in doing so hope to answer some of the more well-stated arguments against classy prelude.

I strongly encourage discussion about classy prelude, and certainly want to hear critiques. But please try to make them based on actual facts with well reasoned arguments, not just asserting that typeclasses are horrible or that this library will break all of Haskell's type safety.

Let's start over: what is classy prelude?

  • It is a library which leverages type classes to provide name overloading, thereby reducing the number of import statements made and decreasing the syntactic overhead of qualified imports.

    Said another way: it's nothing more than syntactic sugar. There may certainly be better theoretical approaches to such syntactic sugar (a new namespace language extension, or a better module system, or maybe something like TDNR), but none of those actually exist today. In today's Haskell, the only approach possible to achieve this goal is typeclasses. (If someone knows something I don't, please say so.)

  • The motivation here is a simple hypothesis: programmers are lazy. Writing:

    "foo" ++ "bar"

    is far easier than:

    import qualified Data.Text as T
    T.pack "foo" `T.append` T.pack "bar"

    Therefore, people will tend towards the easier path, all else being equal. The goal is to lower that resistance to the right way to do things, and therefore encourage better code overall.

  • The purpose of classy prelude is not to encourage writing polymorphic code based on the typeclasses provided. Though it's certainly possible to write code such as:

    {-# LANGUAGE NoImplicitPrelude #-}
    import ClassyPrelude
    import ClassyPrelude.Classes
    foo :: (CanLookup t k v, CanInsertVal t k v)
        => k
        -> v
        -> t
        -> Maybe v
    foo x y z = lookup x $ insert x y z

    That's not my intention. Instead, the idea would be to actually nail this down to concrete types, e.g.:

    {-# LANGUAGE NoImplicitPrelude #-}
    import ClassyPrelude
    foo :: (Eq k, Hashable k) => k -> v -> LHashMap k v -> Maybe v
    foo x y z = lookup x $ insert x y z

    Compare that to the equivalent without classy-prelude:

    import Data.HashMap.Lazy (HashMap)
    import qualified Data.HashMap.Lazy as HashMap
    import Data.Hashable
    foo :: (Eq k, Hashable k) => k -> v -> HashMap k v -> Maybe v
    foo x y z = HashMap.lookup x $ HashMap.insert x y z

    When used this way, the only difference from classy-prelude is syntactic.

  • That said, if people want to try to write polymorphic code with classy-prelude, I see no problem with trying it out. It's true that there are no typeclass laws defined for the classes provided, and therefore such generic code may not work correctly, but it's certainly worth trying it out. Which brings me to the most important point...

  • classy-prelude above all else is an experiment. It is in no way intended to be a replacement for the actual prelude (and probably never will be). I've used it in a few smaller projects at work to remove redundant code... and that's it. If someone sent me a pull request on one of the Yesod libraries today which used classy-prelude, I would reject it, because the library is not ready for prime time.

    Are there questionable choices made right now? Absolutely. Some people have pointed out that trying to unify a ByteString map and conduit's map into a single class may be overkill. I completely agree: it may be overkill. However, I vehemently disagree with anyone claiming that they know it's wrong. How can you know it's wrong, bad, evil, and kills kittens until you've actually tried it?

So far, my experience has been that error messages do not get significantly more confusing, that code tends to just work the way you'd expect it to (since it's all just syntactic sugar for existing, well-tested and thoroughly type-safe libraries), and that the code becomes more legible since we've removed a bunch of unnecessary line noise (i.e., qualified usage of functions).

Are there downsides to this library? Certainly. Is it going to become the de-facto library used for all Haskell development? (Almost) certainly not. Has it already proven itself useful in some actual, real world code? Yes. And that last point is the important one: even if you personally can't see a need for this, others (myself included) do. Even if you believe that it violates every principle of Haskell you hold dear, your belief isn't enough to win an argument. And even if you try to declare a holy crusade against this thing, you won't kill it. If people find it useful, they'll use it. If not, then it will die without your help.

What I'm really saying is this: let's bump up the level of interaction here. If you see a flaw, demonstrate the flaw. If you believe something is wrong, explain that you think it's wrong, but don't start claiming to have absolute truth on your side. I'm happy to engage in a healthy conversation about this library, but I have better things to do with my time then engage in pointless flame wars.


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