A question about run-time errors when class members are undefined

[Anthony Clayden]

On Mon, 8 Oct 2018 at 8:41 PM, Simon Peyton Jones wrote:

You may be interested in Carlos Camarao’s interesting work.  For a long
> time now he has advocated (in effect) making each function into its own
> type class, rather that grouping them into classes.
>

No I think you're mis-apprehending. From the abstract to the group's
SBLP2016 paper: "This depends on a modularization of instance visibility,
as well as on a redefinition of Haskell’s ambiguity rule."

You might remember early last year Carlos submitted a proposal (in two
rounds). Your comments were very relevant
https://github.com/ghc-proposals/ghc-proposals/pull/48#issuecomment-287124007

Relevant because not just was it difficult to understand the proposal, the
proposal had no answer to how instance resolution was to behave.
"expression ambiguity" turned out to mean: use module scope to resolve
overloading.

In the second round of the proposal and in an extended email exchange
off-forum with (I think it was) Rodrigo Ribeiro in Carlos' group I tried to
tease out how module-scoped instances were going to work for a method
exported to a module where there was a different instance in scope. Of
course 'orphan instances' are the familiar symptom in GHC.

Wadler & Blott's 1988 paper last paragraph had already explained: "But
there is no principal type! "


   Perhaps that is in line with your thinking.
>

Not at all. My thinking is coming directly from Wadler's early 1988 memo
that I referenced (note *not* the W&B paper) + using some of GHC's more
recent features like explicit type application in terms; and its
counterpart: explicit method application in types.

I wonder how different would have been the history of Haskell if Wadler had
not borrowed the terminology "class" and "method". Since Helium has a focus
on Haskell learners/beginners: I wonder how much confusion we might have
saved those coming from OOP where the terms mean something really quite
different. We might have avoided "class" altogether; and talked of
"overloaded function".


AntC


*From:* Haskell-prime <haskell-prime-bounces at haskell.org> *On Behalf
Of *Anthony
> Clayden
> *Sent:* 06 October 2018 04:19
> *To:* Petr Pudlák <petr.mvd at gmail.com>
> *Cc:* haskell-prime at haskell.org
> *Subject:* Re: A question about run-time errors when class members are
> undefined
>
>
>
>
>
> On Sat, 6 Oct 2018 at 9:47 AM, Petr Pudlák <redirect at vodafone.co.nz>
> wrote:
>
>
>
> IIRC one of the arguments against having many separate classes is that a
> class is not a just set of methods, it's also the relations between them,
>
>
>
> Hi Petr, I was talking about splitting out Haskell's current class
> hierarchy as a step towards doing away with classes altogether. If your
> language insists on methods being held in classes, that's just tedious
> bureacracy to invent class names.
>
>
>
> The relations between classes (including between single-method classes)
> can be captured through superclass constraints. For example, in the Haskell
> 2010 report
>
>
>
> class (Eq a, Show a) => Num a where ...
>
>
>
> such as the important laws between `return` and `>>=`. And then for
> example a class with just `return` doesn't give any information what
> `return x` means or what should be its properties.
>
>
>
> Then make Bind a superclass constraint on `return` (or vice versa, or both
> ways).
>
>
>
> Just as the laws for Num's methods are defined in terms of equality
>
>
>
> x + negate x == fromInteger 0          -- for example
>
>
>
> Talking about laws is a red herring: you can't declare the laws/the
> compiler doesn't enforce them or rely on them in any way. Indeed the
> Lensaholics seem to take pleasure in building lenses that break the (van
> Laarhoven) laws.
>
>
>
>
>
>
>
> That said, one of really painful points of Haskell is that refactoring a
> hierarchy of type-classes means breaking all the code that implements them.
> This was also one of the main reasons why reason making Applicative a
> superclass of Monad took so long. It'd be much nicer to design type-classes
> in such a way that an implementation doesn't have to really care about the
> exact hierarchy.
>
>
>
> Yes that's what I was saying. Unfortunately for Haskell's Num class, I
> think it's just too hard. So a new language has an opportunity to avoid
> that. If OTOH Helium wants to slavishly follow Haskell, I'm wondering what
> is the point of Helium.
>
>
>
> With Applicative, IIRC, refactoring had to wait until we got Constraint
> kinds and type families that could produce them. Would Helium want to put
> all that into a language aimed at beginners?
>
>
>
>
>
>  For example, in Haskell we could have
>
>
>
> class (Return m, Bind m) => Monad m where
>
>
>
> without any methods specified. But instances of `Monad` should be only
> such types for which `return` and `>>=` satisfy the monad laws.
>
>
>
> First: what does "satisfy the xxx laws" mean? The Haskell report and GHC's
> Prelude documentation state a bunch of laws; and it's a good discipline to
> write down laws if you're creating a class; but it's only documentation.
> Arguably IO, the most commonly used Monad, breaks the Monad laws in rather
> serious ways because it imposes sequence of execution; and it would be
> unfit for purpose if it were pure/lazy function application.
>
>
>
> Then: what do you think a language could do to detect if some instance
> satisfies the laws? (Even supposing you could declare them.)
>
>
>
>
>
> And this would distinguish them from types that have both `Return` and
> `Bind` instances, but don't satisfy the laws.
>
>
>
> You could have distinct classes/distinct operators. Oh, but then `do`
> dotation would break.
>
>
>
>
>
> Unfortunately I'm not sure if there is a good solution for achieving both
> these directions.
>
>
>
> I don't think there's any solution for achieving "satisfy the xxx laws".
>
>
>
>
>
> AntC
>
>
>
>
>
> čt 4. 10. 2018 v 3:56 odesílatel Anthony Clayden <
> anthony_clayden at clear.net.nz> napsal:
>
> > We are adding classes and instances to Helium.
>
> > We wondered about the aspect that it is allowed to have a class instance
>
> > of which not all fields have a piece of code/value associated with them, ...
>
>
>
> I have a suggestion for that. But first let me understand where you're going with Helium. Are you aiming to slavishly reproduce Haskell's classes/instances, or is this a chance for a rethink?
>
>
>
> Will you want to include associated types and associated datatypes in the classes? Note those are just syntactic sugar for top-level type families and data families. It does aid readability to put them within the class.
>
>
>
> I would certainly rethink the current grouping of methods into classes. Number purists have long wanted to split class Num into Additive vs Multiplicative. (Additive would be a superclass of Multiplicative.) For the Naturals perhaps we want Presburger arithmetic then Additive just contains (+), with `negate` certainly in a different class, perhaps (-) subtract also in a dedicated class. Also there's people wanting Monads with just `bind` not `return`. But restructuring the Prelude classes/methods is just too hard with all that legacy code. Even though you should be able to do:
>
>
>
> class (Additive a, Subtractive a, Negative a, Multiplicative a, Divisive a) => Num a
>
>
>
> Note there's a lot of classes with a single method, and that seems to be an increasing trend. Historically it wasn't so easy in Haskell to do that superclass constraints business; if it had been perhaps there would be more classes with a single method. Then there's some disadvantages to classes holding multiple methods:
>
> * the need to provide an overloading for every method, even though it may not make sense
>
>   (or suffer a run-time error, as you say)
>
> * the inability to 'fine tune' methods for a specific datatype [**]
>
> * an internal compiler/object code cost of passing a group of methods in a dictionary as tuple
>
>   (as apposed to directly selecting a single method)
>
>
>
> [**] Nats vs Integrals vs Fractionals for `Num`; and (this will be controversial, but ...) Some people want to/some languages do use (+) for concatenating Strings/lists. But the other methods in `Num` don't make any sense.
>
>
>
> If all your classes have a single method, the class name would seem to be superfluous, and the class/instance decl syntax seems too verbose.
>
>
>
> So here's a suggestion. I'll need to illustrate with some definite syntax, but there's nothing necessary about it. (I'll borrow the Explicit Type Application `@`.) To give an instance overloading for method `show` or (==)
>
>
>
> show @Int = primShowInt                     -- in effect pattern matching on the type
>
> (==) @Int = primEqInt                       -- so see showList below
>
> That is: I'm giving an overloading for those methods on type `Int`. How do I declare those methods are overloadable? In their signature:
>
>
>
> show @a :: a -> String                      -- compare show :: Show a => a -> String
>
> (==) @a :: a -> a -> Bool
>
> Non-overladable functions don't have `@a` to the left of `::`.
>
> How do I show that a class has a superclass constraint? That is: a method has a supermethod constraint, we'll still use `=>`:
>
>
>
> show @a :: showsPrec @a => a -> String      -- supermethod constraint
>
> show @[a] :: show a => [a] -> String        -- instance decl, because not bare a, with constraint =>
>
> show @[a] xss = showList xss
>
> (*) @a :: (+) @a => a -> a -> a
>
>
>
> Is this idea completely off the wall? Take a look at Wadler's original 1988 memo introducing what became type classes.
>
> http://homepages.inf.ed.ac.uk/wadler/papers/class-letter/class-letter.txt <https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fhomepages.inf.ed.ac.uk%2Fwadler%2Fpapers%2Fclass-letter%2Fclass-letter.txt&data=02%7C01%7Csimonpj%40microsoft.com%7C5ec7c3a23a9746bb154b08d62b3a7ba2%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636743927613528630&sdata=vXgR1YLqw8ERzEbhVZBGr%2FpB5fMLMmYtnwt6Bpp4wGs%3D&reserved=0>
>
>
>
> It reviews several possible designs, but not all those possibilities made it into his paper (with Stephen Blott) later in 1988/January 1989. In particular look at Section 1's 'Simple overloading'. It's what I'm suggesting above (modulo a bit of syntax). At the end of Section 1, Wadler rejects this design because of "potential blow-ups". But he should have pushed the idea a bit further. Perhaps he was scared to allow function/method names into type signatures? (I've already sneaked that in above with constraints.) These days Haskell is getting more relaxed about namespaces: the type `@`pplication exactly allows type names appearing in terms. So to counter his example, the programmer writes:
>
>
>
> square x = x * x                             -- no explicit signature given
>
> square :: (*) @a => a -> a                   -- signature inferred, because (*) is overloaded
>
> rms = sqrt . square                          -- no explicit signature
>
> rms :: sqrt @a => a -> a                     -- signature inferred
>
>
>
> Note the inferred signature for `rms` doesn't need `(*) @a` even though it's inferred from `square`. Because (*) is a supermethod of `sqrt`. `sqrt` might also have other supermethods, that amount to `Floating`.
>
>
>
> > ... a run-time error results.
>
> >
>
> > Does anyone know of a rationale for this choice, since it seems rather unhaskell-like.
>
>
>
> If you allow default method implementations (in the class, as Cale points
> out), then I guess you have to allow instance decls that don't mention all
> the methods. I think there should at least be a warning if there's no
> default method. Also beware the default method might have a more specific
> signature, which means it can't be applied for some particular instance.
>
>
>
> Altogether, I'd say, the culprit is the strong bias in early Haskell to
> bunch methods together into classes. These days with Haskell's richer/more
> fine-tuned typeclass features: what do typeclasses do that can't be done
> more precisely at method level -- indeed that would _better_ be done at
> method level?
>
>
>
>
>
> AntC
>
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