[Haskell-cafe] New type of ($) operator in GHC 8.0 is problematic
Richard Eisenberg
eir at cis.upenn.edu
Fri Feb 5 18:13:23 UTC 2016
Perhaps it will aid the discussion to see that the type of ($) will, for better or worse, be changing again before 8.0.
The problem is described in GHC ticket #11471. The details of "why" aren't all that important for this discussion, but the resolution might be. The new (hopefully final!) type of ($) will be:
> ($) :: forall (r :: RuntimeRep) (a :: *) (b :: TYPE r). (a -> b) -> a -> b
Once again, it's easy enough to tweak the pretty-printer to hide the complexity. But perhaps it's not necessary. The difference as far as this conversation is concerned is that Levity has been renamed to RuntimeRep. I think this is an improvement, because now it's not terribly hard to explain:
---
1. Types of kind * have values represented by pointers. This is the vast majority of data in Haskell, because almost everything in Haskell is boxed.
2. But sometimes, we don't care how a value is represented. In this case, we can be polymorphic in the choice of representation, just like `length` is polymorphic in the choice of list element type.
3. ($) works with functions whose result can have any representation, as succinctly stated in the type. Note that the argument to the function must be boxed, however, because the implementation of ($) must store and pass the argument. It doesn't care at all about the result, though, allowing for representation-polymorphism.
In aid of this explanation, we can relate this all to Java. The reference types in Java (e.g., Object, int[], Boolean) are all like types of kind *. The primitive types in Java (int, boolean, char) do not have kind *. Java allows type abstraction (that is, generics) only over the types of kind *. Haskell is more general, allowing abstraction over primitive types via representation polymorphism.
---
Could this all be explained to a novice programmer? That would be a struggle. But it could indeed be explained to an intermediate programmer in another language just learning Haskell.
For point of comparison, Java is widely used as a teaching language. And yet one of the simplest programs is
public class HelloWorld
{
public static void main(String[] args)
{
System.out.println("Hello, world!");
}
}
When I taught Java (I taught high-school full time for 8 years), I would start with something similar to this and have to tell everyone to ignore 90% of what was written. My course never even got to arrays and `static`! That was painful, but everyone survived. This is just to point out that Haskell isn't the only language with this problem. Not to say we shouldn't try to improve!
We're in a bit of a bind in all this. We really need the fancy type for ($) so that it can be used in all situations where it is used currently. The old type for ($) was just a plain old lie. Now, at least, we're not lying. So, do we 1) lie, 2) allow the language to grow, or 3) avoid certain growth because it affects how easy the language is to learn? I don't really think anyone is advocating for (3) exactly, but it's hard to have (2) and not make things more complicated -- unless we have a beginners' mode or other features in, say, GHCi that aid learning. As I've said, I'm in full favor of adding these features.
Richard
On Feb 5, 2016, at 12:55 PM, Kyle Hanson <me at khanson.io> wrote:
> I am also happy the discussion was posted here. Although I don't teach Haskell professionally, one of the things I loved to do was show people how simple Haskell really was by inspecting types and slowly putting the puzzle pieces together.
>
> Summary of the problem for others:
> From Takenobu Tani
> Before ghc7.8:
>
> Prelude> :t foldr
> foldr :: (a -> b -> b) -> b -> [a] -> b
>
> Prelude> :t ($)
> ($) :: (a -> b) -> a -> b
>
> Beginners should only understand about following:
>
> * type variable (polymorphism)
>
>
> After ghc8.0:
>
> Prelude> :t foldr
> foldr :: Foldable t => (a -> b -> b) -> b -> t a -> b
>
> Prelude> :t ($)
> ($)
> :: forall (w :: GHC.Types.Levity) a (b :: TYPE w).
> (a -> b) -> a -> b
>
>
> With this change it looks like I will no longer be able to keep `$` in my toolbox since telling a beginner its "magic" goes against what I believe Haskell is good at, being well defined and easy to understand (Not well defined in terms of Types but well defined in terms of ability to precisely and concisely explain and define whats going on).
>
> It looks like where the discussion is going is to have these types show by default but eventually have an Alternative prelude for beginners.
>
> From Richard Eisenberg:
> - It's interesting that the solution to the two problems Takenobu pulls out below (but others have hinted at in this thread) is by having an alternate Prelude for beginners. I believe that having an alternate beginners' Prelude is becoming essential. I know I'm not the first one to suggest this, but a great many issues that teachers of Haskell have raised with me and posts on this and other lists would be solved by an alternate Prelude for beginners.
> I don't like the idea of fragmenting Haskell into "beginners" and "advanced" versions. Its hard enough to get people to believe Haskell is easy. If they see that they aren't using the "real" prelude, Haskell will still be this magic black box that is too abstract and difficult to understand. If they have to use a "dumbed down" version of Haskell to learn, its not as compelling.
>
> There is something powerful about using the same idiomatic tools as the "big boys" and have the tools still be able to be easy to understand.... by default. Adding complexity to the default Haskell runs the risk of further alienating newcomers to the language who have a misconception that its too hard.
>
> Admittedly, I am not well informed of the state of GHC 8.0 development and haven't had time to fully look into the situation. I am very interested to see where this conversation and the default complexity of Haskell goes.
>
> --
> Kyle
>
>
> On Fri, Feb 5, 2016 at 8:26 AM, Tom Ellis <tom-lists-haskell-cafe-2013 at jaguarpaw.co.uk> wrote:
> On Fri, Feb 05, 2016 at 05:25:15PM +0100, Johannes Waldmann wrote:
> > > What's changed?
> >
> > I was referring to a discussion on ghc-devs, see
> > https://mail.haskell.org/pipermail/ghc-devs/2016-February/011268.html
> > and mixed up addresses when replying.
>
> I'm glad you did, because this is the first I've heard of it!
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