<div><div dir="auto">On Mon, 8 Oct 2018 at 8:41 PM, Simon Peyton Jones wrote:</div><br></div><div><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div lang="EN-GB" link="blue" vlink="purple"><div class="m_-1100708644444294181WordSection1">
<p class="MsoNormal"><span>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.</span></p></div></div></blockquote><div dir="auto"><br></div><div dir="auto">No I think you're mis-apprehending. From the abstract to the group's SBLP2016 paper: "<span style="font-family:CMR9;font-size:9pt">This depends on a modularization of instance visibility, as well as on a
redefinition of Haskell’s ambiguity rule.</span>"</div><div dir="auto"><br></div><div dir="auto">You might remember early last year Carlos submitted a proposal (in two rounds). Your comments were very relevant</div><div dir="auto"><div><a href="https://github.com/ghc-proposals/ghc-proposals/pull/48#issuecomment-287124007">https://github.com/ghc-proposals/ghc-proposals/pull/48#issuecomment-287124007</a></div><br></div><div dir="auto">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.</div><div dir="auto"><br></div><div dir="auto">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.</div><div dir="auto"><br></div><div dir="auto">Wadler & Blott's 1988 paper last paragraph had already explained: "But there is no principal type! "</div><div dir="auto"><br></div><div dir="auto"><br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div lang="EN-GB" link="blue" vlink="purple"><div class="m_-1100708644444294181WordSection1"><p class="MsoNormal"><span>
Perhaps that is in line with your thinking.</span></p></div></div></blockquote><div dir="auto"><br></div><div dir="auto">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.</div><div dir="auto"><br></div><div dir="auto">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".</div><div dir="auto"><br></div><div dir="auto"><br></div><div dir="auto">AntC</div><div dir="auto"><br></div><div dir="auto"><br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div lang="EN-GB" link="blue" vlink="purple"><div class="m_-1100708644444294181WordSection1">
<div style="border:none;border-left:solid blue 1.5pt;padding:0cm 0cm 0cm 4.0pt">
<div>
<div style="border:none;border-top:solid #e1e1e1 1.0pt;padding:3.0pt 0cm 0cm 0cm">
<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> Haskell-prime <<a href="mailto:haskell-prime-bounces@haskell.org" target="_blank">haskell-prime-bounces@haskell.org</a>>
<b>On Behalf Of </b>Anthony Clayden<br>
<b>Sent:</b> 06 October 2018 04:19<br>
<b>To:</b> Petr Pudlák <<a href="mailto:petr.mvd@gmail.com" target="_blank">petr.mvd@gmail.com</a>><br>
<b>Cc:</b> <a href="mailto:haskell-prime@haskell.org" target="_blank">haskell-prime@haskell.org</a><br>
<b>Subject:</b> Re: A question about run-time errors when class members are undefined<u></u><u></u></span></p>
</div>
</div></div></div></div><div lang="EN-GB" link="blue" vlink="purple"><div class="m_-1100708644444294181WordSection1"><div style="border:none;border-left:solid blue 1.5pt;padding:0cm 0cm 0cm 4.0pt">
<p class="MsoNormal"><u></u> <u></u></p>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<div>
<div>
<p class="MsoNormal">On Sat, 6 Oct 2018 at 9:47 AM, Petr Pudlák <<a href="mailto:redirect@vodafone.co.nz" target="_blank">redirect@vodafone.co.nz</a>> wrote:<u></u><u></u></p>
</div>
<blockquote style="border:none;border-left:solid #cccccc 1.0pt;padding:0cm 0cm 0cm 6.0pt;margin-left:4.8pt;margin-right:0cm">
<div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">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,<u></u><u></u></p>
</div>
</div>
</blockquote>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">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.<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">The relations between classes (including between single-method classes) can be captured through superclass constraints. For example, in the Haskell 2010 report<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">class (Eq a, Show a) => Num a where ...<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<blockquote style="border:none;border-left:solid #cccccc 1.0pt;padding:0cm 0cm 0cm 6.0pt;margin-left:4.8pt;margin-right:0cm">
<div>
<div>
<p class="MsoNormal">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.<u></u><u></u></p>
</div>
</div>
</blockquote>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">Then make Bind a superclass constraint on `return` (or vice versa, or both ways).<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">Just as the laws for Num's methods are defined in terms of equality<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">x + negate x == fromInteger 0 -- for example<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">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.<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<blockquote style="border:none;border-left:solid #cccccc 1.0pt;padding:0cm 0cm 0cm 6.0pt;margin-left:4.8pt;margin-right:0cm">
<div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">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.<u></u><u></u></p>
</div>
</div>
</blockquote>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">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.<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">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?<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<blockquote style="border:none;border-left:solid #cccccc 1.0pt;padding:0cm 0cm 0cm 6.0pt;margin-left:4.8pt;margin-right:0cm">
<div>
<div>
<p class="MsoNormal"> For example, in Haskell we could have<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">class (Return m, Bind m) => Monad m where<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">without any methods specified. But instances of `Monad` should be only such types for which `return` and `>>=` satisfy the monad laws.<u></u><u></u></p>
</div>
</div>
</blockquote>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">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.<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">Then: what do you think a language could do to detect if some instance satisfies the laws? (Even supposing you could declare them.)<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<blockquote style="border:none;border-left:solid #cccccc 1.0pt;padding:0cm 0cm 0cm 6.0pt;margin-left:4.8pt;margin-right:0cm">
<div>
<div>
<p class="MsoNormal">And this would distinguish them from types that have both `Return` and `Bind` instances, but don't satisfy the laws.<u></u><u></u></p>
</div>
</div>
</blockquote>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">You could have distinct classes/distinct operators. Oh, but then `do` dotation would break.<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<blockquote style="border:none;border-left:solid #cccccc 1.0pt;padding:0cm 0cm 0cm 6.0pt;margin-left:4.8pt;margin-right:0cm">
<div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">Unfortunately I'm not sure if there is a good solution for achieving both these directions.<u></u><u></u></p>
</div>
</div>
</blockquote>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">I don't think there's any solution for achieving "satisfy the xxx laws".<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">AntC<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<blockquote style="border:none;border-left:solid #cccccc 1.0pt;padding:0cm 0cm 0cm 6.0pt;margin-left:4.8pt;margin-right:0cm">
<div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<div>
<div>
<p class="MsoNormal">čt 4. 10. 2018 v 3:56 odesílatel Anthony Clayden <<a href="mailto:anthony_clayden@clear.net.nz" target="_blank">anthony_clayden@clear.net.nz</a>> napsal:<u></u><u></u></p>
</div>
</div>
</div>
</div>
<div>
<div>
<div>
<blockquote style="border:none;border-left:solid #cccccc 1.0pt;padding:0cm 0cm 0cm 6.0pt;margin-left:4.8pt;margin-right:0cm">
<div>
<p class="MsoNormal"><span style="background:white">> We are adding classes and instances to Helium.</span><u></u><u></u></p>
</div>
<div>
<pre style="background:white;white-space:pre-wrap">> We wondered about the aspect that it is allowed to have a class instance<u></u><u></u></pre>
</div>
<div>
<pre style="background:white;white-space:pre-wrap">> of which not all fields have a piece of code/value associated with them, ...<u></u><u></u></pre>
<pre style="background:white;white-space:pre-wrap"><u></u> <u></u></pre>
<pre style="background:white;white-space:pre-wrap">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?<u></u><u></u></pre>
<pre style="background:white;white-space:pre-wrap"><u></u> <u></u></pre>
<pre style="background:white;white-space:pre-wrap">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.<u></u><u></u></pre>
<pre style="background:white;white-space:pre-wrap"><u></u> <u></u></pre>
<pre style="background:white;white-space:pre-wrap">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:<u></u><u></u></pre>
<pre style="background:white;white-space:pre-wrap"><u></u> <u></u></pre>
<pre style="background:white;white-space:pre-wrap">class (Additive a, Subtractive a, Negative a, Multiplicative a, Divisive a) => Num a<u></u><u></u></pre>
<pre style="background:white;white-space:pre-wrap"><u></u> <u></u></pre>
<pre style="background:white;white-space:pre-wrap">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:<u></u><u></u></pre>
<pre style="background:white;white-space:pre-wrap">* the need to provide an overloading for every method, even though it may not make sense<u></u><u></u></pre>
<pre style="background:white;white-space:pre-wrap"> (or suffer a run-time error, as you say)<u></u><u></u></pre>
<pre style="background:white;white-space:pre-wrap">* the inability to 'fine tune' methods for a specific datatype [**]<u></u><u></u></pre>
<pre style="background:white;white-space:pre-wrap">* an internal compiler/object code cost of passing a group of methods in a dictionary as tuple<u></u><u></u></pre>
<pre style="background:white;white-space:pre-wrap"> (as apposed to directly selecting a single method)<u></u><u></u></pre>
<pre style="background:white;white-space:pre-wrap"><u></u> <u></u></pre>
<pre style="background:white;white-space:pre-wrap">[**] 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.<u></u><u></u></pre>
<pre style="background:white;white-space:pre-wrap"><u></u> <u></u></pre>
<pre style="background:white;white-space:pre-wrap">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.<u></u><u></u></pre>
<pre style="background:white;white-space:pre-wrap"><u></u> <u></u></pre>
<pre style="background:white;white-space:pre-wrap">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 (==)<u></u><u></u></pre>
<pre style="background:white;white-space:pre-wrap"><u></u> <u></u></pre>
<pre style="background:white;white-space:pre-wrap">show @Int = primShowInt -- in effect pattern matching on the type<u></u><u></u></pre>
<pre style="background:white;white-space:pre-wrap">(==) @Int = primEqInt -- so see showList below<u></u><u></u></pre>
<pre style="background:white;white-space:pre-wrap">That is: I'm giving an overloading for those methods on type `Int`. How do I declare those methods are overloadable? In their signature:<u></u><u></u></pre>
<pre style="background:white;white-space:pre-wrap"><u></u> <u></u></pre>
<pre style="background:white;white-space:pre-wrap">show @a :: a -> String -- compare show :: Show a => a -> String<u></u><u></u></pre>
<pre style="background:white;white-space:pre-wrap">(==) @a :: a -> a -> Bool<u></u><u></u></pre>
<pre style="background:white;white-space:pre-wrap">Non-overladable functions don't have `@a` to the left of `::`.<u></u><u></u></pre>
<pre style="background:white;white-space:pre-wrap">How do I show that a class has a superclass constraint? That is: a method has a supermethod constraint, we'll still use `=>`:<u></u><u></u></pre>
<pre style="background:white;white-space:pre-wrap"><u></u> <u></u></pre>
<pre style="background:white;white-space:pre-wrap">show @a :: showsPrec @a => a -> String -- supermethod constraint<u></u><u></u></pre>
<pre style="background:white;white-space:pre-wrap">show @[a] :: show a => [a] -> String -- instance decl, because not bare a, with constraint =><u></u><u></u></pre>
<pre style="background:white;white-space:pre-wrap">show @[a] xss = showList xss<u></u><u></u></pre>
<pre style="background:white;white-space:pre-wrap">(*) @a :: (+) @a => a -> a -> a<u></u><u></u></pre>
<pre style="background:white;white-space:pre-wrap"><u></u> <u></u></pre>
<pre style="background:white;white-space:pre-wrap">Is this idea completely off the wall? Take a look at Wadler's original 1988 memo introducing what became type classes. <u></u><u></u></pre>
<div>
<pre style="background:white"><a href="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" target="_blank">http://homepages.inf.ed.ac.uk/wadler/papers/class-letter/class-letter.txt</a><u></u><u></u></pre>
</div>
<pre style="background:white;white-space:pre-wrap"><u></u> <u></u></pre>
<pre style="background:white;white-space:pre-wrap">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:<u></u><u></u></pre>
<pre style="background:white;white-space:pre-wrap"><u></u> <u></u></pre>
<pre style="background:white;white-space:pre-wrap">square x = x * x -- no explicit signature given<u></u><u></u></pre>
<pre style="background:white;white-space:pre-wrap">square :: (*) @a => a -> a -- signature inferred, because (*) is overloaded<u></u><u></u></pre>
<pre style="background:white;white-space:pre-wrap">rms = sqrt . square -- no explicit signature<u></u><u></u></pre>
<pre style="background:white;white-space:pre-wrap">rms :: sqrt @a => a -> a -- signature inferred<u></u><u></u></pre>
<pre style="background:white;white-space:pre-wrap"><u></u> <u></u></pre>
<pre style="background:white;white-space:pre-wrap">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`.<u></u><u></u></pre>
<pre style="background:white;white-space:pre-wrap"><u></u> <u></u></pre>
<pre style="background:white;white-space:pre-wrap">> ... a run-time error results.<u></u><u></u></pre>
<pre style="background:white">><u></u> <u></u></pre>
<pre style="background:white">> Does anyone know of a rationale for this choice, since it seems rather unhaskell-like.<u></u><u></u></pre>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">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.<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">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?<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">AntC<u></u><u></u></p>
</div>
</div>
<p class="MsoNormal">_______________________________________________<br>
Haskell-prime mailing list<br>
<a href="mailto:Haskell-prime@haskell.org" target="_blank">Haskell-prime@haskell.org</a><br>
<a href="https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fmail.haskell.org%2Fcgi-bin%2Fmailman%2Flistinfo%2Fhaskell-prime&data=02%7C01%7Csimonpj%40microsoft.com%7C5ec7c3a23a9746bb154b08d62b3a7ba2%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636743927613528630&sdata=%2FVDtNLEzevef69Qs9rzrd5V2lBJzMLjRAxBBYvbsPAs%3D&reserved=0" target="_blank">http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-prime</a><u></u><u></u></p>
</blockquote>
</div>
</div>
</div>
</blockquote>
</div>
</div>
</div>
</div>
</div>
</blockquote></div></div>