- Haskell-Cafe - Haskell.org

Re: [Haskell] Strictness question
by Ben Lippmeier 08 Jun '05

08 Jun '05

>> To gloss over details: it'll reduce x far enough so it knows that >> it's an Integer, but it won't nessesarally compute that integers >> value. > No, Integers don't contain any lazy components. > It statically knows that it's an integer. I meant that it would reduce to the outermost constructor but nessesarally evaluate the rest of the object. Ok, I actually looked up the implementation of Integer in GHC. > -- | Arbitrary-precision integers. > data Integer > = S# Int# -- small integers > #ifndef ILX > | J# Int# ByteArray# -- large integers > #else > | J# Void BigInteger -- .NET big ints You were right and I was wrong, Integers contain no lazy components. Perhaps that just highlights the folly of guessing how much actually gets evaluated in a lazy language.. :) Ben.

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RE: [Haskell-cafe] [Newbie] Quest for inheritance
by Ralf Lammel 08 Jun '05

08 Jun '05

Hi Cedric, > > http://homepages.cwi.nl/~ralf/OOHaskell/src/PoorMens/ > > Good compromise between complexity, typing and usefulness. [unfortunately also limited, which is the reason that we came up with the monadic version: PoorMens2.] > I noticed that both Rectangle and Circle need to redefine the > operators because of the different names of their respective delegate > to Shape, namely rectangle2shape and circle2shape. > > I we were to give these fields the same name ('parent', or 'super') in > both Rectangle and Circle, could it be that we can avoid to redefine > the operators by moving their definition upwards (and thus requiring > only one definition for both classes) ? Won't work. ;-) Haskell standard records are not polymorphic like this. You can't write an operation that uses a record selector without committing to a specific record type. (With HList that underlies OOHaskell, of course, you can!!!) This is *precisely* the reason that: - Gracjan's code had a get_super. - my code has the generic .?. operator. We could try to give up on using "normal" records. That is, we could use a tuple convention where the first projection returns the data part of the base class, and the second projection returns the contribution of the derivation. However, this won't get us anywhere. We need the nominal types introduced by the new record types in order to represent the inheritance hierarchy in the type system (through the Inherits or Subtype classes). > Would it then be a problem if we further subclass Rectangle in, say, > two subclasses Square and NonSquareRectangle ? Would that still work > or would there be a collision between the multiple 'parent' fields ? Due to the abovementioned reasons, any sort of parent fields will not interfere. Repeated inheritance simply leads to nested data composition; that's Ok. Ralf

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Visual Hashell Studio.NET 2005
by Brian Smith 07 Jun '05

07 Jun '05

Hi, When will VHS support the Visual Studio.NET 2005 Beta? I'd like to volunteer to test VHS.NET 2005 when it is available. (Also, MS is giving away the VS.NET 2005 beta for free, and VS.NET 2003 costs a whopping $15.00 from my school's bookstore). Thanks, Brian

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Infix operators naming conflicts (was: class Ref)
by Dimitry Golubovsky 07 Jun '05

07 Jun '05

Gracjan Polak wrote: > val = readIORef > a=:b = writeIORef a b Pretty shame := is already reserver :(. There is something alike Graphics.Rendering.OpenGL.GL.StateVar. The use $= for assignment. Generalizing "variables" (in respect to some monad) seems to be often reinvented idea :) Indeed, has anyone tried to summarize possible conflicts between infix operators as they are defined in many places to serve different purpose? e. g. (!) :: Ix i => Array i e -> i -> e The value at the given index in an array. (Data.Array) (!) :: a -> [HtmlAttr] -> a (Text.Html) so if a module imports both modules mentioned above simultaneously, will the compiler complain about (!) and ask to use a qualified name? I had the following experience: when trying to create a convenient syntax to access fields of foreign C structures using appropriate Ptr and field label, I tried to define a combinator for that, and to name it (.) (dot, same as dot-composition defined in Prelude, to look similarly to Java notation) Type signature for my (.) was totally different than Prelude's (.), and semantically it is not composition of functions. However the compiler (GHC) asked to use qualified name. Finally I ended up with the name (-->) which looks like C notation. But who knows, whether someone wishes to use (-->) for other purposes? -- Dimitry Golubovsky Anywhere on the Web

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Usage of | in classes
by Frank-Andre Riess 07 Jun '05

07 Jun '05

Hi, Gracjan's question led me to another question: > class Monad m => Ref m r | m -> r where > newRef :: a -> m (r a) > readRef :: r a -> m a > writeRef :: r a -> a -> m () What's the meaning of the bar and the function type in this declaration. I've seen something like that before (with state monads, I think), but couldn't find an explanation in the Haskell grammar (or anywhere else on haskell.org) Thanks in advance, Frank-Andre

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RE: [Haskell-cafe] [Newbie] Quest for inheritance
by Ralf Lammel 07 Jun '05

07 Jun '05

Hi Gracjan, > > http://homepages.cwi.nl/~ralf/OOHaskell/src/PoorMens2/ > > (again *not* using OOHaskell) > > From the quick skim of code: > .?. -- apply function to upcast object > .!. -- apply modification function to upcast object and substitute > returned value (new object), basically update Absolutely. > Is there any description avaliable what is PoorMens2 all about? Let me try. It's just a variation on Chris' encoding of the shape example, while trying to improve code reuse, while trying to highlight the commonalities in the data parts of the objects in the inheritance hierarchy. The approach ends up being similar to yours in so far that getters (and setters) can be made work for derived types once they are defined on the base type. I just notice that there is a short (because non-monadic) version: http://homepages.cwi.nl/~ralf/OOHaskell/src/PoorMens/ Ralf

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RE: [Haskell-cafe] [Newbie] Quest for inheritance
by Ralf Lammel 07 Jun '05

07 Jun '05

> Cédric Paternotte wrote: > ... > > > 5. With this : > http://www.cs.utexas.edu/ftp/pub/techreports/tr01-60/tr01-60.pdf > > > > Gracjan Polak wrote: > > I've been thinking about slight generalization of this lately. Here are > my semi-backed thoughts as of now. I should have mentioned http://homepages.cwi.nl/~ralf/OOHaskell/src/PoorMens2/ (again *not* using OOHaskell) The actual example encodes Chris Rathman's shape benchmark. The underlying story is somewhat similar to Gracjan's email. Compared to the code that Gracjan posted, - this code pinpoints another issue of this poormens' approach: get_super is quite obviously not enough if you end up wanting mutable objects. You need a setter as well. (My feeling here is that the encapsulation could be better. It would need coding effort to avoid that all data is public.) - this code also shows how the "fake inheritance through data composition" approach blends with virtual methods: one uses an extra class per abstract/virtual method. (Clearly, one can potentially try to bundle some methods in a class.) An issue here is that client code might get bloated with constraints: for datatype subtyping *and* all used virtual methods. Alternatively, one can place the OO methods (their functional counterparts) in the data capsules themselves! Then you get into the trouble of self references. A more general and preliminary observation: the entire approach is potentially more about object *composition* (and perhaps delegation) rather than inheritance. Many OO evangelists consider inheritance as a concept that was used too much in early OO times, while object composition is often more appropriate and flexible. So one *might* say that this approach does not encode a Java-inheritance solution but it does an inheritance-to-object-composition migration on the fly. So what Gracjan calls "Inherits" (and I call subtyping or substitution) is perhaps more a "delegates". Ralf

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RE: [Haskell-cafe] [Newbie] Quest for inheritance
by Ralf Lammel 07 Jun '05

07 Jun '05

There are tons of good research questions readily waiting. Such as: - What would we want to explore in OOish Haskell so that we, once more, can provide interesting input for mainstream language development? More specifically: what sorts of type inference would make a difference in Java 1.5+, C# 2.0+? - How would Haskell's type-class type system need to evolve in order to provide completely satisfying programmer experience for the uses of type classes in the various existing styles of OOish Haskell programmning? - We have Template Haskell but we are *still* rather lame when doing (OOish) language extensions. Macro systems with syntax extensions perhaps really open up a wormhole but nevertheless: *how* can we get a more experimental Haskell environment where language experiments make a good impression at the level of syntactic sugar, debugging, error messages, and all that? - How is it possible that Haskell lacks true extensible functions (modulo the kind of encoding I sent around earlier)? This is one of the big immediate benefits of OO mainstream programming. Seeing Haskell not providing them, makes me feel sad. It is also amazing that Haskell scores with complicated modes of extensibility (cf. monad transformers), but has no watertight answer when it comes to silly extensible datatypes (or classes) and functions (or methods) defined upon those. - ... Let's go to Sydney :-) (Or at the very least, let's meet at the Haskell Workshop!) Ralf > -----Original Message----- > From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe- > bounces(a)haskell.org] On Behalf Of Matthew Roberts > Sent: Tuesday, June 07, 2005 12:15 AM > To: haskell-cafe(a)haskell.org > Subject: Re: [Haskell-cafe] [Newbie] Quest for inheritance > > I don't think this is publishable research, there is too much other > stuff already out there. > > matt > > On 06/06/2005, at 5:58 PM, Gracjan Polak wrote: > > > Matthew Roberts wrote: > >> I have a project (for an early research student) waiting for an > >> interested student to take on. It is called "Programming patterns > >> for OO-like programming in Haskell". The basic idea is to identify > >> how to achieve OO-like organisation of your code in vanilla haskell. > >> Hopefully the student can come to a deep understanding of OO and > >> haskell programming in the process.

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Re: [Haskell-cafe] [Newbie] Quest for inheritance
by Matthew Roberts 07 Jun '05

07 Jun '05

I don't think this is publishable research, there is too much other stuff already out there. matt On 06/06/2005, at 5:58 PM, Gracjan Polak wrote: > Matthew Roberts wrote: >> I have a project (for an early research student) waiting for an >> interested student to take on. It is called "Programming patterns >> for OO-like programming in Haskell". The basic idea is to identify >> how to achieve OO-like organisation of your code in vanilla haskell. >> Hopefully the student can come to a deep understanding of OO and >> haskell programming in the process. > > If this could count as graduate student project and could spawn > publication then I would be interested. :) > >> Sounds like exactly what you need. Shame no-one has taken it up yet. >> Perhaps next semester :) Anyone out there want to come to Sydney >> and take it on? > > As I see you did not cc the list, so currently I'm the only one that > received this message. :) > >> Matt >> On 06/06/2005, at 3:55 AM, Gracjan Polak wrote: >>> Cédric Paternotte wrote: >>> > Hi. This is my first message here so Hello to everyone. >>> > >>> > I'm just starting to learn Haskell and I really think it's a cool >>> language. >>> >>> Me too :) >>> >>> > I know OO and inheritance is not really the point of Haskell and >>> that >>> > other mechanisms are provided to somewhat achieve reuse. But it's a >>> > way of programming I've been so used to that I feel lost without >>> it. >>> > You might think I'm heading in the wrong direction. My mistake I >>> have >>> > to agree. Let's take it as a learning exercise then. >>> >>> Me too :) >>> >>> > 5. With this : >>> http://www.cs.utexas.edu/ftp/pub/techreports/tr01-60/tr01-60.pdf >>> > >>> >>> I've been thinking about slight generalization of this lately. Here >>> are my semi-backed thoughts as of now. >>> >>> First of all, in Haskell there will be strict separation between >>> interfaces and data, so almost every method will be declared twice. >>> This is not so strange to anybody programing in Java, but for C++ >>> programmers can be. Inheritance relation is specified after data. >>> There is also separation between two concepts: what interfaces each >>> piece of data implements and which intefaces given interface >>> inherits. So: >>> >>> {-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-} >>> >>> module Main where >>> >>> -- general inheritance relation >>> class Inherits b x where >>> get_super :: x -> b >>> >>> -- declare interface with one method >>> class IA a where >>> get_a :: a -> Int >>> >>> -- define data with one field >>> data DA = DA { da_field :: Int } >>> >>> -- say how data DA conforms to interface IA >>> instance IA DA where >>> get_a x = da_field x >>> >>> -- declare some other interface IB >>> -- note: IB is unrelated to IA >>> class IB a where >>> get_b :: a -> String >>> >>> -- data that inherits fields of DA and adds one another field >>> data DB = DB { db_super :: DA, db_field :: String } >>> >>> -- DB inherits fields and methods of DA >>> instance Inherits DA DB where >>> get_super x = db_super x >>> >>> -- data DB implements interface IB >>> instance IB DB where >>> get_b x = db_field x >>> >>> -- some other random data >>> data DC = DC { dc_super :: DA } >>> >>> -- DC implements interface IB >>> instance IB DC where >>> get_b x = show (get_a x) >>> >>> -- and inherits DA >>> instance Inherits DA DC where >>> get_super x = dc_super x >>> >>> -- now the tricky part: state that every data x inheriting DA >>> -- implements all interfaces of DA (repeat for each interface) >>> instance (Inherits DA x) => IA x where >>> get_a w = da_field (get_super w) >>> >>> main = do >>> let db = DB (DA 123) "zzz" >>> let dc = DC (DA 123) >>> putStrLn $ show (get_a db) >>> putStrLn $ show (get_a dc) >>> putStrLn $ show (get_b db) >>> putStrLn $ show (get_b dc) >>> >>> As you see there is much more writting as in Java. But this gives >>> better control over inheritance and subsumption because everything >>> must be stated explicitly. Multiple inheritance is allowed :) Also >>> it is "private inheritance" (as in C++) by default. >>> >>> There are some problems left: how to update a field? or how to make >>> inheritance transitive. I don't know it yet :) >>> >>> > >>> > I guess my question now is this : Are there other ways to achieve >>> > inheritance in Haskell ? >>> >>> Me too:) >>> >>> My proposal (above) is about the level of 'OO' things done in >>> procedural languages (example: C with GTK+ library). There must be a >>> better way. Any comments? >>> >>> -- >>> Gracjan >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> Haskell-Cafe(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>> > >

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RE: [Haskell-cafe] [Newbie] Quest for inheritance
by Ralf Lammel 07 Jun '05

07 Jun '05

Hi Cedric, Thanks for your investigation. Regarding your guess, the code does *not* use "state monads (for mutable data)". The whole example would make sense without IO. (So this approach is *all* different from OOHaskell :-)) The IO monad is used in Poormens2 because some of the methods might want to do side effects. For instance, in the Shape example, we want to see the progress of drawing at the Console. Also, we added an extra feature to "observe" the invocation of setters. The operators (.?.) and (.!.) can be thought of as generic getters and setters. In fact, close inspection reveals that they rather model reading and writing (or call it object observation and object mutation). Mutability is achieved in Poormens2 merely by having setters. (Optionally, if you like, you *could* use the IO monad to carry around objects. Remember, in OOHaskell, the IO monad provides IORefs for mutable objects; something not used in Poormens2.) (( Some asides: If you remove the IO monad, then the Poormens2 style is really a variation on the style hinted at in Gracjan's email. One interesting difference worth noting is that Gracjan places the actual getters in classes while Poormens2 just uses overloaded functions defined in terms of the generic operators (.!.) and (.?.). But I assert that this is a detail or a minor variation point. There could be monadic and non-monadic versions in the Subtype class; I was just too lazy at that time -- assuming that one needs monads in almost all cases anyhow. (After all, we are trying to understand the migration of imperative OO code to Haskell.) You might say that the fact whether the methods (and getters and setters, say properties) are monadic or not should not disturb the general framework, whereas it does seem to affect the design of the Subtype class used in Poormens2. In particular, you might say that the result type of a getter could be just *any* type: why then expose the monadic status in the framework's types? This is needed for the generic operator (.!.). This operator captures an *in-place* update. That is, it applies the mutator to the supertype component of the datum at hand. Since the aspect of selecting that component *and* putting back the updated component is captured generically, the operator takes a type-preserving function on the supertype and lifts it to a type-preserving function on the subtype. So type preservation is in the type of the generic mutator, and we need the monadic variation on type preservation (i.e., a -> IO a) -- if we anticipate that any mutator will have side effects (in addition to those related to the mutation of the object at hand, which is just explicit in the type!!) It is then just a consistent style to provide a similarly monadic type for generic observers. )) We call this a poor mens' approach because the coding style does not allow for a direct translation of C++/C#/Java OO code -- something we aim to provide by OOHaskell (within limits of course). Ralf > -----Original Message----- > From: Cédric Paternotte [mailto:cpaternotte@gmail.com] > Sent: Monday, June 06, 2005 3:36 PM > To: Ralf Lammel > Cc: haskell-cafe(a)haskell.org; Gracjan Polak > Subject: Re: [Haskell-cafe] [Newbie] Quest for inheritance > > Hi Ralf, > > > I should have mentioned > > http://homepages.cwi.nl/~ralf/OOHaskell/src/PoorMens2/ > > (again *not* using OOHaskell) > > > > It's been an interesting evening. I've been having a go at your > poormen's source code and, although it's different from OOHaskell, I > still find it very similar in the sense that it's using the same > concepts, namely infix operators (for syntactic sugar I assume) and > state monads (for mutable data). But I had a look at it anyway ;) > > From what I gathered today infix operators are just like ordinary > functions, that differ only in the way you pass them parameters. I > understand the .?. and .!. operators in your code are shortcuts that > apply a function to the parent type, respectively for get and set > operations. > > The only thing I couldn't figure is the reason of using monads. I > noticed they (returnIO) were extensively used in the setters and in > the .!. operator. Do monads provide features without which this whole > thing wouldn't be possible ? What is it exactly they provide in this > context ? > > > A more general and preliminary observation: > > the entire approach is potentially more about > > object *composition* (and perhaps delegation) > > rather than inheritance. > > That's also the way I see it. > > Cédric

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