Re: [Haskell-cafe] Most Important GHC extensions to learn/use?
On Fri, Jun 1, 2012 at 4:37 PM, James Cook mo...@deepbondi.net wrote: On Jun 1, 2012, at 6:11 AM, Gábor Lehel wrote: On Fri, Jun 1, 2012 at 6:29 AM, wren ng thornton w...@freegeek.org wrote: TypeFamilies (aka TFs) These are really nifty and they're all the rage these days. In a formal sense they're equivalent to fundeps, but in practice they're weaker than fundeps. Is that still true? The reason used to be that we didn't have superclass equalities, but we do have them now since 7.2. The only drawbacks I know of relative to FDs are that it's sometimes more typing, not supported by GeneralizedNewtypeDeriving, and doesn't allow OverlappingInstances (ick). In addition to other things mentioned today in the Fundeps and overlapping instances thread, type families have no way of defining injective type functions where the range includes already-existing types. For example, if you define: type family Succ a there is no way (that I've found) to define it in such a way that the compiler can see that Succ a ~ Succ b = a ~ b. The equivalent in MPTCs+FDs would be: class Succ a b | a - b, b - a class (S a ~ b, P b ~ a) = Succ a b where type S a type P b (Succ a c, Succ b c) = (S a ~ c, P c ~ a, S b ~ c, P c ~ b) = (P c ~ a, P c ~ b) = (a ~ P c, P c ~ b) = (a ~ b) There is more discussion of this particular weakness at http://hackage.haskell.org/trac/ghc/ticket/6018 . Also, there are less-common usages of fundeps that may be translatable to type families but not easily, when there are complex interrelationships between type variables. For example, type-level binary operations will sometimes have fundeps such as a b - c, a c - b, b c - a - that is to say, any two determines the third. Like above: class (FD1 a b ~ c, FD2 b c ~ a, FD3 c a ~ b) = BinOp a b c where type FD1 a b type FD2 b c type FD3 c a You can mechanically translate MPTCs with FDs into MPTCs with ATs and superclass equalities in this way, and your fingers will get a lot of exercise. But that's the basis for the claim that TFs with superclass equalities are no less powerful than FDs. It's true that this doesn't always allow you to express everything as just plain top-level type families, but then, neither do FDs :). @wren, did you have some other examples in mind? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Most Important GHC extensions to learn/use?
On Fri, Jun 1, 2012 at 6:29 AM, wren ng thornton w...@freegeek.org wrote: TypeFamilies (aka TFs) These are really nifty and they're all the rage these days. In a formal sense they're equivalent to fundeps, but in practice they're weaker than fundeps. Is that still true? The reason used to be that we didn't have superclass equalities, but we do have them now since 7.2. The only drawbacks I know of relative to FDs are that it's sometimes more typing, not supported by GeneralizedNewtypeDeriving, and doesn't allow OverlappingInstances (ick). ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Most Important GHC extensions to learn/use?
On Jun 1, 2012, at 6:11 AM, Gábor Lehel wrote: On Fri, Jun 1, 2012 at 6:29 AM, wren ng thornton w...@freegeek.org wrote: TypeFamilies (aka TFs) These are really nifty and they're all the rage these days. In a formal sense they're equivalent to fundeps, but in practice they're weaker than fundeps. Is that still true? The reason used to be that we didn't have superclass equalities, but we do have them now since 7.2. The only drawbacks I know of relative to FDs are that it's sometimes more typing, not supported by GeneralizedNewtypeDeriving, and doesn't allow OverlappingInstances (ick). In addition to other things mentioned today in the Fundeps and overlapping instances thread, type families have no way of defining injective type functions where the range includes already-existing types. For example, if you define: type family Succ a there is no way (that I've found) to define it in such a way that the compiler can see that Succ a ~ Succ b = a ~ b. The equivalent in MPTCs+FDs would be: class Succ a b | a - b, b - a There is more discussion of this particular weakness at http://hackage.haskell.org/trac/ghc/ticket/6018 . Also, there are less-common usages of fundeps that may be translatable to type families but not easily, when there are complex interrelationships between type variables. For example, type-level binary operations will sometimes have fundeps such as a b - c, a c - b, b c - a - that is to say, any two determines the third. -- James ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Most Important GHC extensions to learn/use?
wren ng thornton w...@freegeek.org writes: There are a bunch which are mostly just syntax changes. The important ones are: Also, if you have new GHC, it will often tell you if/when you need to enable extensions. E.g.: Line 8: 1 error(s), 0 warning(s) `Pos' has no constructors (-XEmptyDataDecls permits this) In the data type declaration for `Pos' -k -- If I haven't seen further, it is by standing in the footprints of giants ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Most Important GHC extensions to learn/use?
On 6/1/12 12:45 AM, Jonathan Geddes wrote: Thanks, Wren, I really appreciate the detailed response! Though I am surprised that Template Haskell isn't on your list. From the little I know of TH it seems like all of the interesting generic/generative stuff is done with TH. Do the other extensions subsume the need for TH, or is it just not terribly interesting? TH is plenty interesting, but it's a very different sort of direction to head. The extensions I mentioned are the ones I think everyone assumes a seasoned Haskeller will know. With the exception of TFs and GADTs, those extensions have all been around since the days of GHC 6.6 and Hugs. Thus, most people consider them normal parts of Haskell even if they're not in in the Report. Consequently, understanding them is necessary to understand most of the non-H98 code on Hackage. TH, on the other hand, is very much GHC-only and that's unlikely to change. Also, there's a lot of unnecessary(?) grunge there. The basic theory of staged computation which led to TH was laid out in the MetaML/MetaOCaml papers. The version in MetaML is a lot nicer in terms of the theory, so I'd suggest people start there before diving into TH. One of the nice things in MetaML is that there's no limit on the levels of meta-ness. This was removed from TH because it would require having the compiler present in every executable; a sensible limitation, though it complicates the theory. Another major difference is that MetaML only did staging at the term level, whereas TH also allows splices in type signatures, splices which generate type class instances, etc. These extensions make TH *much* more powerful than MetaML, but also make it *much* harder to understand and reason about. IMO the formal theory of these non-term splices hasn't been worked out very well, which is part of the reason why TH is so grungy to work with. I'm a big fan of staged computation, and I'd definitely suggest any sophisticated functional programmer should read the MetaML papers (and the TH papers if so inclined, and the Flask papers to get a different perspective on the theme). But very few packages require understanding TH in order to understand them, and even fewer require understanding TH in order to use them. -- Live well, ~wren ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Most Important GHC extensions to learn/use?
On 6/1/12 6:11 AM, Gábor Lehel wrote: On Fri, Jun 1, 2012 at 6:29 AM, wren ng thorntonw...@freegeek.org wrote: TypeFamilies (aka TFs) These are really nifty and they're all the rage these days. In a formal sense they're equivalent to fundeps, but in practice they're weaker than fundeps. Is that still true? The reason used to be that we didn't have superclass equalities, but we do have them now since 7.2. The only drawbacks I know of relative to FDs are that it's sometimes more typing, not supported by GeneralizedNewtypeDeriving, and doesn't allow OverlappingInstances (ick). The superclass equalities was a big thing, but the disparity remains even still. The main problem is that type inference with fundeps remains more powerful than type inference for TFs. There are still patterns of use which infer/check easily with fundeps but which don't have an equivalent TF implementation. I haven't messed with TFs enough to know the details here, but there are other folks on the list who do. In addition there's the issue of overlapping instances, which is currently being discussed elsewhere. -- Live well, ~wren ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Most Important GHC extensions to learn/use?
Haskell Hackers,
I'm pretty comfortable with all of Haskell 98 (and 2010, really). But I've
always sort of avoided extensions. I realize that this is a bit silly and
if I want to continue learning, it probably means delving into the
extensions. Which ones are the most important to know from a practical
point of view? And which ones from a {Language,Category,Math}-theoretical
point of view? (Any other interesting/important points of view I'm missing?
:D )
As always, thanks for the feedback.
Cheers,
--J Arthur
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Re: [Haskell-cafe] Most Important GHC extensions to learn/use?
On 5/31/12 7:15 PM, Jonathan Geddes wrote:
Haskell Hackers,
I'm pretty comfortable with all of Haskell 98 (and 2010, really). But I've
always sort of avoided extensions. I realize that this is a bit silly and
if I want to continue learning, it probably means delving into the
extensions. Which ones are the most important to know from a practical
point of view? And which ones from a {Language,Category,Math}-theoretical
point of view? (Any other interesting/important points of view I'm missing?
:D )
There are a bunch which are mostly just syntax changes. The important
ones are:
ForeignFunctionInterface (aka FFI)
Not technically part of H98, though it was a quick addition. It
is part of H2010, so it's not really an extension anymore.
ScopedTypeVariables
This one's really easy, and in the cases where you want it you
really really want it.
KindSignatures
This one's simple, and it helps expose you to the idea of
kinds, which is helpful for what's to come.
TypeOperators
This one's trivial, but it makes things a bit prettier.
FlexibleContexts, FlexibleInstances
These are essential for actually using MPTCs (described below).
IMO they should be enabled automatically whenever MPTCs are on.
And there are also a bunch of ones about extending the deriving
mechanic to work with new classes or with newtypes.
Then there are the ones that actually change the language in a
significant way. I'd say the critical ones to learn are:
RankNTypes (or Rank2Types if you're squeamish)
This is used in lots of nice tricks like list fusion. Learning
list fusion is a good place for the H98 veteran to explore
next, since it's easy to pick up and has many applications
outside of just doing list fusion. Also, it's been around
forever and isn't going anywhere anytime soon.
MultiParamTypeClasses (aka MPTCs)
This has been around forever, and is considered standard
Haskell by most people, even though it hasn't made it into the
Report yet (due the the fundeps vs TFs issue).
FunctionalDependencies (aka fundeps)
This is helpful for making certain MPTCs usable without too
many type signatures. Also, it's good for understanding the
fundeps vs TFs issue. Also, this one has been around forever,
and although it's fallen into disfavor it is still
indispensable due to limitations in TFs.
TypeFamilies (aka TFs)
These are really nifty and they're all the rage these days. In
a formal sense they're equivalent to fundeps, but in practice
they're weaker than fundeps.
GADTs
These are really nifty and they're all the rage these days.
Though beware, GADTs are a rabbit hole leading off to the world
of dependent types. You should be aware of the basic ideas
here, though don't worry too much about the theory (unless you
want to spend a lot of time worrying about the theory).
--
Live well,
~wren
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Re: [Haskell-cafe] Most Important GHC extensions to learn/use?
Thanks, Wren, I really appreciate the detailed response! Though I am
surprised that Template Haskell isn't on your list. From the little I know
of TH it seems like all of the interesting generic/generative stuff is done
with TH. Do the other extensions subsume the need for TH, or is it just not
terribly interesting?
--J Arthur
On Thu, May 31, 2012 at 10:29 PM, wren ng thornton w...@freegeek.orgwrote:
On 5/31/12 7:15 PM, Jonathan Geddes wrote:
Haskell Hackers,
I'm pretty comfortable with all of Haskell 98 (and 2010, really). But I've
always sort of avoided extensions. I realize that this is a bit silly and
if I want to continue learning, it probably means delving into the
extensions. Which ones are the most important to know from a practical
point of view? And which ones from a {Language,Category,Math}-**
theoretical
point of view? (Any other interesting/important points of view I'm
missing?
:D )
There are a bunch which are mostly just syntax changes. The important ones
are:
ForeignFunctionInterface (aka FFI)
Not technically part of H98, though it was a quick addition. It
is part of H2010, so it's not really an extension anymore.
ScopedTypeVariables
This one's really easy, and in the cases where you want it you
really really want it.
KindSignatures
This one's simple, and it helps expose you to the idea of
kinds, which is helpful for what's to come.
TypeOperators
This one's trivial, but it makes things a bit prettier.
FlexibleContexts, FlexibleInstances
These are essential for actually using MPTCs (described below).
IMO they should be enabled automatically whenever MPTCs are on.
And there are also a bunch of ones about extending the deriving mechanic
to work with new classes or with newtypes.
Then there are the ones that actually change the language in a significant
way. I'd say the critical ones to learn are:
RankNTypes (or Rank2Types if you're squeamish)
This is used in lots of nice tricks like list fusion. Learning
list fusion is a good place for the H98 veteran to explore
next, since it's easy to pick up and has many applications
outside of just doing list fusion. Also, it's been around
forever and isn't going anywhere anytime soon.
MultiParamTypeClasses (aka MPTCs)
This has been around forever, and is considered standard
Haskell by most people, even though it hasn't made it into the
Report yet (due the the fundeps vs TFs issue).
FunctionalDependencies (aka fundeps)
This is helpful for making certain MPTCs usable without too
many type signatures. Also, it's good for understanding the
fundeps vs TFs issue. Also, this one has been around forever,
and although it's fallen into disfavor it is still
indispensable due to limitations in TFs.
TypeFamilies (aka TFs)
These are really nifty and they're all the rage these days. In
a formal sense they're equivalent to fundeps, but in practice
they're weaker than fundeps.
GADTs
These are really nifty and they're all the rage these days.
Though beware, GADTs are a rabbit hole leading off to the world
of dependent types. You should be aware of the basic ideas
here, though don't worry too much about the theory (unless you
want to spend a lot of time worrying about the theory).
--
Live well,
~wren
__**_
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/**mailman/listinfo/haskell-cafehttp://www.haskell.org/mailman/listinfo/haskell-cafe
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