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Data.List.sortBy creates unnecessary thunks
by Siddhanathan Shanmugam 26 Mar '17

26 Mar '17

Data.List.sortBy uses a DList-like representation to efficiently chunk a list, but the final conversion from DList to List is not strict. This leaves unnecessary suspensions while sorting. This was first noticed in https://github.com/ghc-proposals/ghc-proposals/pull/50, where sorting a random list with a naive merge sort is faster than the one in base. Proposing the following change, which should improve performance and space usage. ```diff diff --git a/libraries/base/Data/OldList.hs b/libraries/base/Data/OldList.hs index c3618c42a9..a68c4f42af 100644 --- a/libraries/base/Data/OldList.hs +++ b/libraries/base/Data/OldList.hs @@ -851,7 +851,7 @@ sortBy cmp = mergeAll . sequences ascending a as (b:bs) | a `cmp` b /= GT = ascending b (\ys -> as (a:ys)) bs - ascending a as bs = as [a]: sequences bs + ascending a as bs = *as [a] `seq`* as [a] : sequences bs mergeAll [x] = x mergeAll xs = mergeAll (mergePairs xs) ``` And, as a side note, can we please add more type signatures and comments to the sort/sortBy function? Cheers, Sid

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PROPOSAL: Add Map/IntMap newtypes for different Monoid to containers
by Merijn Verstraaten 25 Mar '17

25 Mar '17

I would like to propose adding a newtype wrapper (with all relevant instances) for Map and IntMap (if I missed any other applicable type in containers, let me know). This newtype should differ in Monoid/Semigroup instance from Map/IntMap by switching: Ord k => Semigroup (Map k v) Ord k => Monoid (Map k v) to: (Ord k, Semigroup v) => Semigroup (Map k v) (Ord k, Monoid v) => Monoid (Map k v) or (Ord k, Semigroup v) => Monoid (Map k v) Any opinions on the overall idea? Opinions on which Monoid instance? Bikeshed for the newtype names? Cheers, Merijn

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Re: Deprecate Foldable for Either
by Anthony Clayden 24 Mar '17

24 Mar '17

> On Thu Mar 23 15:03:36 UTC 2017, Edward Kmett wrote: > > On Thu, Mar 23, 2017 at 6:18 AM, Anthony Clayden wrote: > > > > Why is `[a]` hard-coded in the signature for > > concat? I would expect anything specific to Lists > > to be genericised: > > concat :: (Foldable t1, Foldable t2) => t1 (t2 a) -> > > t2 a > > > > Foldable can't be used to 'construct' in this way. Thanks Edward for the comprehensive reply. Yes I know Foldable isn't powerful enough to construct. I wasn't seriously proposing that as the signature. I was pointing out that there were several value judgments in arriving at the FTP changes. The decision has gone different ways in different places. It's not all as predictable by cold logic as you're trying to make out below. And you've left people surprised. FTP left `concat` hard-coded to Lists. It left `map` hard-coded to Lists. Why didn't it leave `length` hard-coded to Lists? The promise with FTP was to not break code, and for code that already compiled to continue to work with the same behaviour. Mostly that's been delivered. The counterpart hasn't been delivered: inappropriate uses of some methods (which therefore didn't previously compile -- good) do now compile, and whatever they calculate is a surprise/non-intuitive for a significant number. To my intuition, "length" -- as an English word -- is about structures that have length. I can see the benefit you point out below for having a generic method for size of a Foldable. Most of those Foldables I don't see as having "length". > Neither Foldable f nor even Traversable f give you the > power to construct a new 'f' with different shape. > Exactly. So why even bother with concat (Right [...]); or concat (Just [...]) ? > > concat is just a type restricted version of fold = foldMap > id provided basically for legacy compatibility. > OK. It's arbitrary but your justification is it was always arbitrary. FTP `concat` does the same as pre-FTP `concat`. "For legacy compatibility". By that same argument: pre-FTP `length` wasn't even in Foldable. It was arbitrarily restricted to Lists. "For legacy compatibility" it should have stayed restricted to Lists. > > The Foldable/Traversable proposal was to bring the > functions that already existed in base in Data.Foldable > into the Prelude and eliminate the monomorphic versions of > those combinators that made importing these more general > versions painful for users. `length` was never in Data.Foldable. There was never a more general version. You're blowing smoke. > We didn't set out to make new > functions, merely standardize on the ones folks had been > using for a decade. concat was left intact, because there > was already a generalization available, and no particular > reason to break all the existing code that expected it. > > > Why have an instance of Foldable which always returns > > length 1? > > (Or for some instances returns length 0 or 1.) > > > > Say you go to write a function > > toVector :: Foldable f => f a -> Vector a > toVector xs = Vector.fromListN (length xs) (toList xs) > > With length in Foldable, ... No. Length was not in Foldable. You should have stopped to ask why! You give good reasons for having a method in Foldable that consistently gives a size. Those are not good reasons for calling that method `length` or for breaking expectations that `length` applies for Lists, Not for Foldables. > > A number of container types can use 'length' rather than > having to supply their own monomorphic size function. > Would 'length' have been better called 'size'? Yes! Or `cardinality`. > That is another color the bikeshed could have been colored, That's an arrogant comment. You've allowed mopeds, motorbikes and skateboards into the bikeshed. And now there's not enough room for the bikes. Now you blow smoke that it's only about the colour of the shed. > but it would have involved taking a fresh name, possibly in the > Prelude, and the name collisions of a common name with > existing code was considered to be the greater evil. Data.Set has `size`. It used to have `cardinality`, that was removed. OK not the Prelude, but close enough. `size` in Data.Set does exactly what `length` now does. > > > `concat` and `length` should not be in Foldable. > > They should be in a separate class `Consable` (say), > > which would be a sub-class of Foldable. > > > > length being in such a class would mean that every author .. Would use a sensibly-named method (`size`) when they wanted the, er, size of a Foldable. And using `length` would document the code that they were applying to something that might be lengthy. > ... This would have also > meant dumping a brand new abstraction in Prelude without > the preceding decade worth of experimentation on what > works well. As it is, we've got what? A brand new abstraction (called `length`) in Foldable without a preceding decade worth of experimentation on whether length makes sense for Either or Maybe. Furthermore without a preceding decade worth of telling people that length [1, 2] is 2; but length (1, 2) is 1. > In general the FTP didn't seek to go out of > its way to make up new classes, merely to standardize > existing code ... You're blowing smoke again. There was no existing code using `length` of a Foldable, because `length` wasn't a method. > > While working on the Foldable/Traversable Proposal what > happened was we found we needed to add a number of methods > to the Foldable class to avoid silent changes in the > semantics of existing Haskell programs. Some Foldable > combinators folded in different directions than their > monomorphic counterparts. > OK. How well communicated was that? It certainly couldn't have applied for `length`. The semantics of existing Haskell programs was that `length` worked for Lists. Only for Lists. Not Foldables in general. > Along the way as we worked to fix these infelicities and > noticed that as a side-effect that this allowed Foldable > authors some power to execute these operations with better > asymptotics where possible for their containers. This nice > knock-on effect lets folks shed some names that they were > taking from the environment by using what Prelude offers. > During this process some folks noticed that length/null > fit into the scheme, Well those folks were wrong. What they should have noticed was that `size`/null fit. > ... > > With the choices made all of the "mainstream" base modules > now have no name conflicts and can be imported > unqualified. Good. I am not criticising the FTP change in general, nor its objectives. But moving `length` from Prelude to Foldable isn't justified on those grounds. There wasn't previously a `size` (or equivalent) in Foldable. Why introduce it when there was no experience of "using for a decade" -- if that was the criterion? > > Could we have named them flength (or size) and fnull so > that people had to ask for them explicitly and left > length/null alone? Yes. But it would have meant taking new > names from the Prelude or not re-exporting them from > Data.Foldable, and the AMP/FTP tried to very careful about > introducing any new names to the Prelude, and wanted users > to be able to supply instances without having to > explicitly import Data.Foldable. > > We were very dubious about the response we'd get > introducing new, untested, names to Prelude ... So you preferred to introduce new, unattested behaviour. `length` of an Either is unattested. `length` of a Maybe is unattested. That `length` of (1, 2) is 1 is unattested. > as everyone's code is impacted by that choice Again, no: nobody's code could have been using Foldable length. > and folks happen to be using all the "good" names for such things, Yes that's a problem. > and nobody had done so in the preceding 17 years, Because they couldn't. Because `length` applied only for lists. > so any changes along > those lines are surprising. > So do you think it unsurprising asking for the length of a Maybe? Do you think it unsurprising that the length of (1, 2) is 1? Note: "length", not "size". So did it come as a surprise that people are surprised? I'm not surprised newbies are surprised. They're using tutorials saying length applies for Lists. And concat applies for Lists of Lists. It seems to be not just newbies surprised, judging by how often the question comes up. AntC

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Re: Deprecate Foldable for Either
by Anthony Clayden 23 Mar '17

23 Mar '17

> On Thu Mar 2 16:30:21 UTC 2017 Henning Thielemann wrote: >> On Thu, 2 Mar 2017, Andreas Abel wrote: >> Today a student came to me wondering why a certain function >> produced a regular result, where he had expected an error. >> Turned out he had used `concat`, but not on a lists of lists >> as he had thought, but on a lists of `Either a [b]`. > That is, he did (concat (xs :: [Either a [b]]))? > Does GHC actually accept that? > If at all, it would not have to do with Foldable. Good questions Henning. They've got swamped. I found Andreas' description a bit unclear. We have: concat :: Foldable t => t [a] -> [a] So the `concat` you put above would give an error. To get a result, the student must have put (concat (xs :: Either a [b])) And got result either `[]` from (Left _) or a genuine List from (Right ys). Either way, I'd be grumpy: this hasn't and couldn't concatenate anything. Two thoughts: 1) The FTP hasn't done a very thorough job. Why is `[a]` hard-coded in the signature for concat? I would expect anything specific to Lists to be genericised: concat :: (Foldable t1, Foldable t2) => t1 (t2 a) -> t2 a 2) It's obvious why FTP didn't go that far: (concat (xs :: [Either a [b]])) -- as you speculated -- would be daft. concat only applies for structures that can be concatenated. But that surely includes more than Lists(?) So this is essentially the same complaint as for `length`: Why have an instance of Foldable which always returns length 1? (Or for some instances returns length 0 or 1.) `concat` and `length` should not be in Foldable. They should be in a separate class `Consable` (say), which would be a sub-class of Foldable. Note that `length` used not to be in Foldable. `concat` was, but restricted to Lists, as now. If there's a need for a method in Foldable to return 1 for a tuple, or 0..1 for Maybe/Either, don't call it `length`, call it `cardinality` (say). (This is the same approach as for separate `map`, `fmap`, `foldMap`.) That isn't breaking genericity/polymorphism/library rationalisation, `length` was never previously available for non-List Foldables. So no code could have been using it. `concat` was always in the wrong place, but Foldable (when it was a bit of a rag-bag) used to be good enough. AntC

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Proposal: Make unsafeInterleaveST safer
by David Feuer 22 Mar '17

22 Mar '17

Currently, unsafeInterleaveST is considerably less safe than unsafeInterleaveIO. Specifically, very careful discipline is required to make code using it thread-safe. This means that unsafeInterleaveST actually behaves like unsafeDupableInterleaveIO. See Edward Kmett's blog post from several years ago [*] for details. I propose that we should 1. Make unsafeInterleaveST behave only as unsafely as unsafeInterleaveIO, and 2. Add a new unsafeDupableInterleaveST function implementing the current unsafeInterleaveST behavior. The alternative would be to leave unsafeInterleaveST alone and add a new unsafeNonDupableInterleaveST, but that seems like an absolutely terrible mismatch of names. Since any currently-correct code using unsafeInterleaveST will continue to work (albeit more slowly), I don't think that's justified. Thanks, David [*] https://www.schoolofhaskell.com/school/to-infinity-and-beyond/older-but-sti… f

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Laws for GHC.Generics
by Gershom B 20 Mar '17

20 Mar '17

I had a bit of discussion today on how to handle generic instances for abstract types. It resolved to the following question, in a sense -- what are the laws for the Generic typeclass? Should require that `from` and `to` be an isomorphism of data types (i.e. that `from . to` and `to . from` both be the identity) or should we require only the weaker one-sided retract condition (i.e. that `from . to` be the identity.). If the latter, is it better that `from` be partial (which i prefer) or that it quotient together "equivalent" representations (i.e. representing a map as a `fromList` as syb-generics do). The documentation doesn't seem to provide clear guidance here, and it might be worth discussing if some should be added. Cheers, Gershom

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Re: Deprecate Foldable for Either
by Artyom Kazak 19 Mar '17

19 Mar '17

On 20 Mar 2017 1:31 a.m., "Tony Morris" <tonymorris(a)gmail.com> wrote: "But the inconsistency with my intuition for the function named l-e-n-g-t-h that I learned in C programming school in the 1980s" is the best criteria that I have seen, and which commands outright dismissal. This is exactly my criterion (except that I learned English, not C), and I do not agree with you that it "commands outright dismissal". There is value in functions being as general as possible, etc, but there is also value in conforming to human intuitions – because it prevents people from shooting themselves in their feet. Whether those intuitions come from math, or from C, or from English, or elsewhere, is entirely irrelevant. If behavior of `length` fully conforms to your intuitions, that's splendid and you're lucky; however, for me and my usecases it's *only* a source of pain, and I want a way to free myself of that pain (without having to learn another language or spend time trying to change my intuitions). Wanting to make one's life easier is not an unreasonable desire.

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Deprecate Foldable for Either
by Andreas Abel 19 Mar '17

19 Mar '17

Today a student came to me wondering why a certain function produced a regular result, where he had expected an error. Turned out he had used `concat`, but not on a lists of lists as he had thought, but on a lists of `Either a [b]`. With the Foldable instance for Either, which considers Either a b to be a container of 0-1 elements of b, errors are happily swallowed. I think this instance is harmful and should be deprecated (and later removed) from base. There are similarly pointless Foldable instances as well. See a discussion one year ago, which was heated, but had no consequences. https://mail.haskell.org/pipermail/libraries/2016-February/026678.html -- Andreas Abel <>< Du bist der geliebte Mensch. Department of Computer Science and Engineering Chalmers and Gothenburg University, Sweden andreas.abel(a)gu.se http://www.cse.chalmers.se/~abela/

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Re: Deprecate Foldable for Either
by Artyom Kazak 19 Mar '17

19 Mar '17

On 20 Mar 2017 3:31 a.m., "Tony Morris" <tonymorris(a)gmail.com> wrote: What intuition exactly? Demonstrate it to me so that I may empathise with it. What pain? Show it to me. Are you sure it's intuition and not something else? How are you so sure? Okay, I have to admit that in this particular instance it's not quite my intuition which causes the pain. Or rather, it's a different kind of intuition, which I have acquired from Haskell in particular – "I don't have to be particularly careful about minor refactorings, because if I change the type of one thing, the compiler will tell me what other things I have to change if I want to preserve the logic of my code". The behavior of 'length' breaks this intuition because "I'll just pass a tuple here instead of whatever I used before" stops being a (relatively) safe refactoring. (I'm saying "relatively" because of the existence of other extremely polymorphic things – e.g. 'Aeson.encode'. Luckily, they're much less common than 'length'.) (An example of a scenario which more closely fits what I originally had in mind: Set.size is O(1), but HashSet.size is O(n) – this one was pretty fun to hunt down when we found that our code was unexpectedly slow. Unfortunately, I can not adequately expand the intuition behind my initial judgment of "HashSet.size is surely fast", because it's hard to do at 4am.) The pain is pretty easy to demonstrate: I had to waste a day recreating the Foldable/Traversable hierarchy in a custom prelude to ensure that nobody in my team would be bitten by a refactoring. It wasn't *that* painful, sure – but it's somewhat scary to think that there might be dozens of other refactor-unfriendly functions lurking in libraries we're using (and one day they will come to bite us).

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Q Monad State
by Ethan Pailes 11 Mar '17

11 Mar '17

Hi, I've been working on PADS with Kathleen Fisher for the past couple of months, and we have some questions about the Q Monad. In working on extending the PADS language, we have encountered a need to maintain a type environment across invocations of the [pads||] quasi quoter. This allows PADS types to be defined in one quote block and used in a later quote block. Unfortunately, the right way to do this is not obvious from the Q Monad’s interface[1]. The monad provides `qGetQ` and `qPutQ` methods to provide a place to store state across quotes, but unfortunately this state is local to each module. It is desirable from a usability perspective to be able to reuse PADS declarations across modules, but in the particular case of PADS the problem is actually worse than that. PADS ships with a standard library module which uses PADS to define several of the core types required to be productive, so without the ability to do type checking across modules the use of this standard library becomes impossible. There are two solutions to the problem that we have considered. 1. We can generate metadata about each type as it is defined and emit a top level declaration which contains this metadata. 1. This clutters the global namespace. 2. It lets you examine the metadata for each type easily in `ghci` which leads to a nice debugging workflow. 3. If you want to perform any typechecks or other calculations based on the metadata, they must happen after generated code is spliced in. 1. This means you lose a lot of type safety because everything must be represented as an `Exp` (even with typed template haskell there is no way to get a type for an `Exp` of the form `VarE <<name of metadata>>`) 2. It becomes impossible to report type errors at compile time. This is a dealbreaker because otherwise, what is the point? 2. Using `unsafePerformIO` to create an IORef and then using the Q Monand’s `qRunIO` function to access the state cell. 1. This works smoothly across modules 2. You get type safety back because everything happens in the Q Monad. You don’t have to generate code in order to get at the values. 3. It uses `unsafePerformIO` and `qRunIO` so you have to worry about how often everything gets run. 1. It seems like it should be safe with a {-# NONINLINE #-} pragma for the code which creates the IORef. 2. The IORef is hidden, and no one else should be able to modify it. 3. Adding stuff to a `Data.Map.Strict` (the environment implementation we are using) is idempotent, so the quote code getting run multiple times should not be an issue. We have two main questions based on this. 1. Is the IO (ab)use outlined above really as safe as we think it is? 2. Given that environments are such a common requirement in compiling programming languages, and the goal of quasi quotation is allowing easy creation of EDSLs for Haskell, does it make sense to provide some state mechanism for the Q monad which is not restricted by modules. [1]: https://hackage.haskell.org/package/template-haskell-2.11. 1.0/docs/Language-Haskell-TH-Syntax.html Thanks, Ethan Pailes

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