Add DList to base

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Dr.Koster

5 Jun 2017 5 Jun '17

3:58 a.m.

Currently GHC already defined DList for internal use in serveral places. This data type is a nature choice when you need CPS your append, which is a common need. I think base should provide it. Cheers~ Winter

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Ivan Lazar Miljenovic

5 Jun 5 Jun

6:21 a.m.

On 5 June 2017 at 17:58, Dr.Koster wrote:

...

Currently GHC already defined DList for internal use in serveral places. This data type is a nature choice when you need CPS your append, which is a common need. I think base should provide it.

It depends if GHC defines DList for use with base or not; if it's something that's required to use base that _may_ be a semi-valid use case. In general though, as annoying as it is to have to add yet another dependency, build, reload ghci, etc. I prefer to have base to be smaller and packages split up and the dlist package serves admirably. If nothing else, it can iterate faster if a new function needs to be added. Call this a weak +0.1(only because of GHC defining it). -- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com http://IvanMiljenovic.wordpress.com

Dr.Koster

10:21 p.m.

New subject: 回复： Add DList to base

Here's a example of Dlist in base: https://github.com/ghc/ghc/blob/master/libraries/base/GHC/Event/PSQ.hs#L468 And here is in GHC module: https://github.com/ghc/ghc/blob/ba597c1dd1daf9643b72dc7aeace8d6b3fce84eb/uti... It's so simple that people often define it with the code it's used, adding it to base can save these key stokes. ------------------ 原始邮件 ------------------ 发件人: "Ivan Lazar Miljenovic";; 发送时间: 2017年6月5日(星期一) 晚上6:21 收件人: "Dr.Koster"; 抄送: "libraries"; 主题: Re: Add DList to base On 5 June 2017 at 17:58, Dr.Koster wrote:

...

Currently GHC already defined DList for internal use in serveral places. This data type is a nature choice when you need CPS your append, which is a common need. I think base should provide it.

David Feuer

11:09 p.m.

I'm -1 on this. For an abstract DList-style list builder, there's the dlist package, which you shouldn't fear depending on (its only dependencies are base and deepseq, both of which are GHC boot packages). The dlist package defines a DList that's an instance of MonadPlus, Traversable, IsList, Ord, Read, Show, IsString, Monoid, and NFData, and provides a variety of functions for working with them. Many of the instances and functions are, inherently, absurdly inefficient. This is because there's basically *nothing* you can do to a DList directly except build one up with `.` and tear one down with `apply`. DLists are extremely efficient precisely when GHC is able to optimize them away altogether. As soon as that is not the case, they're mediocre at best. Now suppose you want a non-abstract DList type (with the constructor exposed). newtype DList a = DL { unDL :: [a] -> [a] } What can that offer in the way of instances? Well, it's clearly not a Functor, so it's certainly not Applicative, Monad, MonadPlus, or Traversable. Ouch. There's no way to write matching Read and Show instances, so you're stuck picking just one. Similarly, IsList and IsString aren't guaranteed to round-trip properly. NFData is utterly absurd, of course. So what's left? Foldable, Monoid, and either Read or Show. The Foldable instance doesn't even interact nicely with the Monoid instance: there's no guarantee that foldMap f xs <> foldMap f ys = foldMap f (xs <> ys). So there's pretty much *nothing going on here*. DList x just doesn't have much more to offer than Endo [x]. We already have Endo; ergo, we don't need DList. On Mon, Jun 5, 2017 at 3:58 AM, Dr.Koster wrote:

...

Currently GHC already defined DList for internal use in serveral places. This data type is a nature choice when you need CPS your append, which is a common need. I think base should provide it.

Cheers~ Winter

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

Henning Thielemann

6 Jun 6 Jun

2:24 a.m.

On Mon, 5 Jun 2017, David Feuer wrote:

...

DList x just doesn't have much more to offer than Endo [x]. We already have Endo; ergo, we don't need DList.

I admit that I already used Endo as a quickly available DList replacement.

Tony Morris

8 Jun 8 Jun

7:41 p.m.

Are there any DList specific functions, that are not provided by existing type-classes (such as Foldable), that are typically used? I can think of a few, but they are all in lens. On Tue, Jun 6, 2017 at 4:24 PM, Henning Thielemann < lemming@henning-thielemann.de> wrote:

...

On Mon, 5 Jun 2017, David Feuer wrote:

DList x just doesn't have much more to offer than Endo [x]. We already

...
have Endo; ergo, we don't need DList.

I admit that I already used Endo as a quickly available DList replacement.

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

Ivan Lazar Miljenovic

8:30 p.m.

On 9 June 2017 at 09:41, Tony Morris wrote:

...

Are there any DList specific functions, that are not provided by existing type-classes (such as Foldable), that are typically used? I can think of a few, but they are all in lens.

singleton, cons, snoc Though if you have singleton (which can be obtained from either Endo or GHC.Ext.IsList.fromList and `(:[])`; alternatively you could use pure/return but that is less pleasant IMO than the singleton function for code legibility purposes) and mappend you can do cons and snoc.

...

On Tue, Jun 6, 2017 at 4:24 PM, Henning Thielemann wrote:

...
On Mon, 5 Jun 2017, David Feuer wrote:

...
DList x just doesn't have much more to offer than Endo [x]. We already have Endo; ergo, we don't need DList.

I admit that I already used Endo as a quickly available DList replacement.

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

-- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com http://IvanMiljenovic.wordpress.com

Tony Morris

9 Jun 9 Jun

12:15 a.m.

Yeah, those are in lens, so I am less concerned that they don't appear in base. Specifically, if DList were not to exist in base, and I use the typeclasses that we all know as well as lens, what am I missing out on? On Fri, Jun 9, 2017 at 10:30 AM, Ivan Lazar Miljenovic < ivan.miljenovic@gmail.com> wrote:

...

On 9 June 2017 at 09:41, Tony Morris wrote:

...
Are there any DList specific functions, that are not provided by existing type-classes (such as Foldable), that are typically used? I can think of a few, but they are all in lens.

singleton, cons, snoc

Though if you have singleton (which can be obtained from either Endo or GHC.Ext.IsList.fromList and `(:[])`; alternatively you could use pure/return but that is less pleasant IMO than the singleton function for code legibility purposes) and mappend you can do cons and snoc.

...
On Tue, Jun 6, 2017 at 4:24 PM, Henning Thielemann wrote:

...
On Mon, 5 Jun 2017, David Feuer wrote:

...
DList x just doesn't have much more to offer than Endo [x]. We already have Endo; ergo, we don't need DList.

I admit that I already used Endo as a quickly available DList

replacement.

...
...
_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

-- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com http://IvanMiljenovic.wordpress.com

Ivan Lazar Miljenovic

12:58 a.m.

On 9 June 2017 at 14:15, Tony Morris wrote:

...

Yeah, those are in lens, so I am less concerned that they don't appear in base. Specifically, if DList were not to exist in base, and I use the typeclasses that we all know as well as lens, what am I missing out on?

...

From a brief skim through the DList module documentation, those are the only functions defined that do not have direct analogues in existing typeclasses in base.

...

On Fri, Jun 9, 2017 at 10:30 AM, Ivan Lazar Miljenovic wrote:

...
On 9 June 2017 at 09:41, Tony Morris wrote:

...
Are there any DList specific functions, that are not provided by existing type-classes (such as Foldable), that are typically used? I can think of a few, but they are all in lens.

singleton, cons, snoc

Though if you have singleton (which can be obtained from either Endo or GHC.Ext.IsList.fromList and `(:[])`; alternatively you could use pure/return but that is less pleasant IMO than the singleton function for code legibility purposes) and mappend you can do cons and snoc.

...
On Tue, Jun 6, 2017 at 4:24 PM, Henning Thielemann wrote:

...
On Mon, 5 Jun 2017, David Feuer wrote:

...
DList x just doesn't have much more to offer than Endo [x]. We already have Endo; ergo, we don't need DList.

I admit that I already used Endo as a quickly available DList replacement.

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

-- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com http://IvanMiljenovic.wordpress.com

-- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com http://IvanMiljenovic.wordpress.com

Dr.Koster

13 Jun 13 Jun

11:26 p.m.

New subject: 回复： Add DList to base

...

What can that offer in the way of instances? Well, it's clearly not a Functor, so it's certainly not Applicative, Monad, MonadPlus, or Traversable. Ouch. There's no way to write matching Read and Show instances, so you're stuck picking just one. Similarly, IsList and IsString aren't guaranteed to round-trip properly. NFData is utterly absurd, of course. So what's left? Foldable, Monoid, and either Read or Show. The Foldable instance doesn't even interact nicely with the Monoid instance: there's no guarantee that foldMap f xs <> foldMap f ys = foldMap f (xs <> ys). So there's pretty much *nothing going on here*. DList x just doesn't have much more to offer than Endo [x]. We already have Endo; ergo, we don't need DList.

I think that's OK: 1) If we're concerning those inefficient instances(My understanding is those defined using `toList`, such as `Foldable` and `Functor`), we can just don't add them. 2) Even `type DList a = Endo [a]` is OK, what i'm asking is CPSed operations (such as cons, replicate...) working on this type, not those instances. BTW. Isn't current `NFData`instance in dlist package problematic? We should directly `seq` the function IMHO. ------------------ 原始邮件 ------------------ 发件人: "David Feuer";; 发送时间: 2017年6月6日(星期二) 中午11:09 收件人: "Dr.Koster"; 抄送: "libraries"; 主题: Re: Add DList to base I'm -1 on this. For an abstract DList-style list builder, there's the dlist package, which you shouldn't fear depending on (its only dependencies are base and deepseq, both of which are GHC boot packages). The dlist package defines a DList that's an instance of MonadPlus, Traversable, IsList, Ord, Read, Show, IsString, Monoid, and NFData, and provides a variety of functions for working with them. Many of the instances and functions are, inherently, absurdly inefficient. This is because there's basically *nothing* you can do to a DList directly except build one up with `.` and tear one down with `apply`. DLists are extremely efficient precisely when GHC is able to optimize them away altogether. As soon as that is not the case, they're mediocre at best. Now suppose you want a non-abstract DList type (with the constructor exposed). newtype DList a = DL { unDL :: [a] -> [a] } What can that offer in the way of instances? Well, it's clearly not a Functor, so it's certainly not Applicative, Monad, MonadPlus, or Traversable. Ouch. There's no way to write matching Read and Show instances, so you're stuck picking just one. Similarly, IsList and IsString aren't guaranteed to round-trip properly. NFData is utterly absurd, of course. So what's left? Foldable, Monoid, and either Read or Show. The Foldable instance doesn't even interact nicely with the Monoid instance: there's no guarantee that foldMap f xs <> foldMap f ys = foldMap f (xs <> ys). So there's pretty much *nothing going on here*. DList x just doesn't have much more to offer than Endo [x]. We already have Endo; ergo, we don't need DList. On Mon, Jun 5, 2017 at 3:58 AM, Dr.Koster wrote:

...

Currently GHC already defined DList for internal use in serveral places. This data type is a nature choice when you need CPS your append, which is a common need. I think base should provide it.

Cheers~ Winter

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

David Feuer

14 Jun 14 Jun

1:58 a.m.

New subject: 回复： Add DList to base

The current NFData instance in dlist is the best you can do. seq on a function (very) rarely does what you want. The instance the package defines guarantees that the DList represents a list that can be reduced to normal form. The semantics are right. Efficiency-wise, it's lousy, but there's no way to fix it (without changing the underlying representation). On Jun 13, 2017 11:26 PM, "Dr.Koster" wrote:

...

What can that offer in the way of instances? Well, it's clearly not a Functor, so it's certainly not Applicative, Monad, MonadPlus, or Traversable. Ouch. There's no way to write matching Read and Show instances, so you're stuck picking just one. Similarly, IsList and IsString aren't guaranteed to round-trip properly. NFData is utterly absurd, of course. So what's left? Foldable, Monoid, and either Read or Show. The Foldable instance doesn't even interact nicely with the Monoid instance: there's no guarantee that foldMap f xs <> foldMap f ys = foldMap f (xs <> ys). So there's pretty much *nothing going on here*. DList x just doesn't have much more to offer than Endo [x]. We already have Endo; ergo, we don't need DList.

I think that's OK: 1) If we're concerning those inefficient instances(My understanding is those defined using `toList`, such as `Foldable` and `Functor`), we can just don't add them. 2) Even `type DList a = Endo [a]` is OK, what i'm asking is CPSed operations (such as cons, replicate...) working on this type, not those instances. BTW. Isn't current `NFData`instance in dlist package problematic? We should directly `seq` the function IMHO. ------------------ 原始邮件 ------------------ *发件人:* "David Feuer";; *发送时间:* 2017年6月6日(星期二) 中午11:09 *收件人:* "Dr.Koster"; *抄送:* "libraries"; *主题:* Re: Add DList to base I'm -1 on this. For an abstract DList-style list builder, there's the dlist package, which you shouldn't fear depending on (its only dependencies are base and deepseq, both of which are GHC boot packages). The dlist package defines a DList that's an instance of MonadPlus, Traversable, IsList, Ord, Read, Show, IsString, Monoid, and NFData, and provides a variety of functions for working with them. Many of the instances and functions are, inherently, absurdly inefficient. This is because there's basically *nothing* you can do to a DList directly except build one up with `.` and tear one down with `apply`. DLists are extremely efficient precisely when GHC is able to optimize them away altogether. As soon as that is not the case, they're mediocre at best. Now suppose you want a non-abstract DList type (with the constructor exposed). newtype DList a = DL { unDL :: [a] -> [a] } What can that offer in the way of instances? Well, it's clearly not a Functor, so it's certainly not Applicative, Monad, MonadPlus, or Traversable. Ouch. There's no way to write matching Read and Show instances, so you're stuck picking just one. Similarly, IsList and IsString aren't guaranteed to round-trip properly. NFData is utterly absurd, of course. So what's left? Foldable, Monoid, and either Read or Show. The Foldable instance doesn't even interact nicely with the Monoid instance: there's no guarantee that foldMap f xs <> foldMap f ys = foldMap f (xs <> ys). So there's pretty much *nothing going on here*. DList x just doesn't have much more to offer than Endo [x]. We already have Endo; ergo, we don't need DList. On Mon, Jun 5, 2017 at 3:58 AM, Dr.Koster wrote:

...

Currently GHC already defined DList for internal use in serveral places. This data type is a nature choice when you need CPS your append, which is a common need. I think base should provide it.

Cheers~ Winter

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

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David Feuer
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Add DList to base

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