It may come as a surprise to many of you that I, too, am very worried about
Haskell becoming inaccessible to newcomers. If we can't induct new people into
our ranks, we will die. It is for this reason that I have always been unhappy
with the FTP. But that ship has sailed.
I fully agree with George's suggestion below that the default Prelude should be
the beginner's Prelude. I believe I have argued this stance in the past, but
louder voices prevailed. Perhaps I was wrong in branding: we should have a
proper Prelude as the default, and make available a super whiz-bang advanced
Prelude as well. I'm never very good about branding. I'd lend strong support to
someone who articulates a concrete move in this direction, but I don't have the
bandwidth to spearhead it myself.
Despite the various arguments saying that the bits in Java are easier to
understand than the bits in ($), I'm quite unconvinced. (Particularly about
`static`. Even `class` is hard for true beginners.) And the boxed/unboxed
distinction does come up early in Java: just try to write an ArrayList<int> and
now you need to know about boxed types and unboxed ones.
Chris's point that "it's not about the name" is valid. The Levity -->
RuntimeRep change is not about the name, but about the functionality. Levity
distinguished only between lifted and unlifted; RuntimeRep distinguishes
between boxed/lifted, boxed/unlifted, and all the unboxed types with their
different widths. I'm just clarifying that it's not simply a cosmetic
name-change.
The old type of ($) was always a lie. -XMagicHash just changes the parser,
allowing the # suffix. It is only by convention that most (all?) unlifted
things end in #. The old type of ($) was perhaps a harmless lie, but a lie
nonetheless.
Are we comfortable with lying? (Believe me, I'm not trying to impose some moral
dimension to simplifying output!) In my mind, lying about types like this is in
the same space as having a beginner's Prelude. And people will constantly
discover that we're lying and get very confused. Having a whole host of flags
that tell GHC to lie less is somewhat like having two versions of the
language... only the differences manifest only in output instead of input.
If we are comfortable with lying in this way: as I've offered, I can hide the
type of ($) (and other representation-polymorphic things) behind a flag. Easy
to do.
Another great question that has come up is about Haddock output (Hackage). I
think Haddock needs to add a facility where library authors can include
specializations of an overly general type. This can be done in commentary, but
it's not as prominent. Such a new feature would address the ($) problem, as ($)
:: forall (a :: *) (b :: *). (a -> b) -> a -> b is a specialization of its real
type. It would also help a great deal with FTP-related generalizations.
I also want to respond directly to Kyle's comments:
> I think its important to identify who you want your "customers" to be. If you
> only want the most advanced type theorists to use the language, that is
> perfectly fine, but what you lose are thousands of developers that can
> benefit the Haskell community without having to know advanced Typing.
Rest assured, I want my "customers" to be everyone who wants to program. I've
volunteered to teach a bit of Haskell to high schoolers, and I'd love a shot at
a course where I teach it to people who have never programmed.
>
> Needing a "Beginners" mode in a language is *not* a feature, its a
> fundamental design flaw. It shows that the language was not sufficiently
> thought out and designed for everyone.
On an intuitive level, this rings true for me. But when I think about the
details, I'm less convinced. For example, take Scratch (scratch.mit.edu), which
is wonderfully easy to learn and gives kids (and adults!) a great deal of fun.
Yet it's painful to use when you know more. And the Racket folks have invested
a lot of time in coming up with a curriculum to go with their language, and
they explicitly have expertise levels. Needing these levels may just be part of
the game.
So, rest assured, I remain very receptive to these concerns. And I'd love
concrete help in putting them to rest.
Richard
On Feb 5, 2016, at 6:30 PM, George Colpitts <george.colpi...@gmail.com> wrote:
> +1 for Christopher's email
> Richard, I disagree with "But it could indeed be explained to an
> intermediate programmer in another language just learning Haskell." Your
> explanation is good but it assumes you have already explained "types of kind
> *" and the boxed vs unboxed distinction. Admittedly the latter should be
> understood by most Java programmers but I doubt that intermediate programmers
> in other languages do. If I did have to explain "$" I would say, for now
> think of it in terms of it's pre 8.0 type. Alternatively avoid mentioning "$"
> to beginners. I don't believe it is in Hutton's book or any of Bird's
> although I might be wrong.
>
> Most intermediate programmers in another language struggle a lot with
> learning monads, witness all the monad tutorials. Absorbing monads is
> central, there is a lot that has to be explained before that. Minimizing that
> material would be a good thing.
>
> I have mixed feelings about a beginner's prelude best summarized by saying
> the proposed beginner's prelude should be the standard prelude and the
> current one should be an advanced prelude. If we have a beginner's prelude I
> feel we are saying that this is a hard to understand research language and we
> hope that someday you have enough education, energy and tenacity to get to
> the point where you understand it. If we do it the other way we are saying
> you have what you need but if you want more there is lots!
>
> On Fri, Feb 5, 2016 at 3:05 PM, Christopher Allen <c...@bitemyapp.com> wrote:
> Changing the name doesn't fix the issue. The issue is the noise and the
> referent, not the referrer. There's a habit of over-focusing on names in
> programming communities. I think it'd be a mistake to do that here and risk
> missing the point.
>
> You can make all of the keywords in the Java example salient early on, but
> you cannot make the implementation details you're exposing in the type of ($)
> relevant unless they already have a year or two of Haskell under their belts.
> Listing out the keywords:
>
> 1. public
>
> 2. class
>
> 3. (class name)
>
> 4. static
>
> 5. void
>
> 6. (method name)
>
> 7. (method arguments)
>
> Explaining public, class, static, and void usually happens pretty soon after
> the basics in a Java course. Importantly, they're things you _need_ to know
> to get things done properly in Java. The same is not true of what is
> mentioned in the type of ($).
>
> The implicit prenex form and forall are irrelevant for learners until they
> get to Rank2/RankN which is very much beyond, "I am learning Haskell" and
> into, "I am designing an API in Haskell for other people to use". * vs. # is
> something many working and hobbyist Haskellers I've known will scarcely know
> anything about.
>
> There is a big difference, to my mind, between what is being exposed here in
> Java versus what is being exposed in the type ($). Consider that the
> boxed/unboxed distinction exists in Java but needn't come up in any beginner
> tutorials.
>
> >Types of kind * have values represented by pointers. This is the vast
> >majority of data in Haskell, because almost everything in Haskell is boxed.
>
> We can't assume Haskell learners know what pointers are. This, again, creates
> unnecessary noise for learners by forcing exposure to things that are
> irrelevant for a very long time.
>
>
> On Fri, Feb 5, 2016 at 12:13 PM, Richard Eisenberg <e...@cis.upenn.edu> wrote:
> Perhaps it will aid the discussion to see that the type of ($) 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 <m...@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-2...@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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>
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>
> --
> Chris Allen
> Currently working on http://haskellbook.com
>
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