March 2001 - Libraries

RE: Proposal: module namespaces.
by Simon Marlow 09 Mar '01

09 Mar '01

[ leaving out haskell(a)haskell.org from the recipient list ] > Malcolm Wallace wrote: > > > > Christian writes: > > > What about the module declaration? Should it be: > > > module Text.Xml.Parser where ... > > > or just > > > module Parser where ... -- located in Text/Xml/Parser.hs? > > > > The former. The reason is that a compiler needs to > generate a unique > > linker symbol for each defined function. If the full module name is > > not encoded in the source file, you will need to add a commandline > > option to the compiler, which is the wrong way to go in my opinion. > > What?? The compiler knows the full name of the module without > the module > clause. If it didn't do that, it can't find the modules to > compile! Does > the compiler opens every file on the Internet to check > whether it is the > file to compile? How does the compiler find the file to compile in the > first place? What should the command line option you mentioned do? The compiler finds the file because you tell it what the filename is. That's the way it works now, and if I understand correctly Malcolm isn't suggesting we change that (and I agree). There are really two issues here: * how do you find a module to compile in the first place (not an issue for interpreters, only for batch compilers). Just run the compiler giving it the filename. * when you import a module, how does the compiler find the interface (or source, in the case of an interpreter) for the target. My feeling is that this happens with a small extension to the current scheme: at the moment, the compilers all have a list of search paths in which to find interfaces/sources. The change is that to find a module A.B.C you search in D/A/B/ (for each D in the search path) rather than just D. GHC's package mechanism will actually work pretty much unchanged with this scheme, I believe. Cheers, Simon

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module namespaces
by Johannes Waldmann 08 Mar '01

08 Mar '01

I think the namespace proposal wants to be strictly upward compatible, therefore it's perhaps not the right time to discuss this ... but still I think the question "why should the source text of a module contain its name?" wants an answer. Even if we stick to module headers, I don't really like the idea of `module Fully.Qualified.Name where ..' The proposal mentions the analogy to Java. My knowledge of that is minimal, but in Java, they have `package Foo.Bar.Frobs' (full name required) and `class Frobble' (only last name there). So, a Haskell module corresponds to what? I understand the argument "the compiler needs to generate unique names in the object files" but this is a matter of implementation, about which the programmer (at least theoretically) should not care. I think the underlying design decision here is: how far should the user of a set of modules expect that he can move their source/interface/object files around, and still compile them/link against them correctly. The present proposal seems to imply: not at all. This is a conservative answer. It is easy to implement, and guarantees some consistency. Is it too restrictive? But assume this policy is adopted. Then I am not allowed to move (precompiled) modules around physically. That's why I want some means of renaming them (during importing), and as I wrote earlier, not only for single modules (`import Foo as Bar') but also for complete hierarchies (`import Leipzig.Standard as Standard') A potential problem is that the root of some hierarchy perhaps is just a directory (that contains subdirs and modules) but not itself a module. One would need to write a dummy module that just (qualified-)imports and re-exports all submodules. But this had to be extended whenever a submodule is added. This doesn't feel right. (On the other hand, the libFoo.a file had to be rebuilt anyway on such an occasion.) This leads to another question - during the linking stage: when should the compilation manager look for a single object file, and when should it expect an archive? Anyway I think even the most restricitve form of hierarchical module names would be very helpful. I certainly would start using it. -- -- Johannes Waldmann ---- http://www.informatik.uni-leipzig.de/~joe/ -- -- joe(a)informatik.uni-leipzig.de -- phone/fax (+49) 341 9732 204/252 --

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Re: Proposal: module namespaces.
by Christian Brolin 08 Mar '01

08 Mar '01

Marcin 'Qrczak' Kowalczyk wrote: > > Wed, 28 Feb 2001 17:31:13 +0100, Christian Brolin <Christian.Brolin(a)carmen.se> pisze: > > > This dilemma comes from the fact that you don't tell the compiler what > > module to compile, but just give it some Haskell code. You should always > > compile the module with: > > compile A.B.C.D, i.e. tell the compiler which module you want to > > compile, and the compiler will find the source code. > > It must work well with Makefiles. I can imagine that it's doable in > this case, but generally it should work to specify the filename too. If it isn't double, it is not possible to automatically find the files of imported modules! > > > Instead I would let the current directory play as an implicit module > > > root. I.e. you can refer to D2 as D2, and in this case you cannot refer > > > to the global module D2 if it exists (but you can refer to D2.Other > > > if D2 is a global directory and there is no local directory named D2). > > > > I think this is very confusing! And restrictive. > > It may be confusing (but not more than having your own function > called sortBy). sortBy? > It's not restrictive. When you want to use a module, don't provide > a module of the same name - *that* would be confusing when used > (if allowed). You can't import a child module without mention the name of the importing module. > It has an advantage that it's simpler. The compiler has a set of > roots and there is just one form of module paths. I think it is better to explicitly distinguish between absolute and relative addresses of imported modules. It would be easier for a reader (e.g. a compiler) to find the modules. Absolutely imported modules are found by looking in the set of roots, while the others are found relative to the position of the current module. -- Christian Brolin

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Proposal: module namespaces.
by Malcolm Wallace 08 Mar '01

08 Mar '01

This is an annoucement of a new mailing list, and a proposal for three things: * An extended mechanism for module namespaces in Haskell. * A "standard" namespace for new libraries, common across all systems. * A social process for adding new libraries to the "standard" set. A formatted version of this proposal appears on the web at http://www.cs.york.ac.uk/fp/libraries/ The new mailing list is for the discussion of these proposals. Please subscribe if you are interested. Follow-ups set accordingly. Mailing list details -------------------- libraries(a)haskell.org The purpose for this new list is to: (a) discuss an extension to Haskell to provide a richer module namespace, (b) discuss how to partition this namespace and populate it with libraries, (c) discuss how to provide a consistent set of libraries for all compilers, and the setting up of a common library repository. To subscribe: http://haskell.org/mailman/listinfo/libraries/ Introduction ------------ Everyone agrees that Haskell needs good, useful, libraries: lots of them, well-specified, well-implemented, well-documented. A problem is that the current "Standard Libraries" defined by the Haskell'98 Report number only about a dozen. But there are actually many more libraries out there: some are in GHC's hslibs collection, others are linked from haskell.org, even more are used only by their original author and have no public distribution. What is more, there is no Haskell Committee. There is no-one to decide which candidate libraries are worthy to be added to the "Standard" set. This stifles the possible distribution of great libraries, because no-one knows how to get /my/ library "accepted". Furthermore, the existing libraries that people distribute from their own websites often run into problems when used alongside other people's libraries. A library usually consists of several modules, but often the constituent modules have simple names that can easily clash with modules from another library package. This leads people to ad hoc solutions such as prefixing all their modules with a cryptic identifier e.g. HsParse XmlParse HOGLParse THIHParse Just counting the libraries currently available from GHC's hslibs, and haskell.org's links, there are currently over 200 separate modules in semi-"standard" use. As more libraries are written, the possibility of clashes can only increase. Related to this problem, although not identical, is the difficulty of finding a library that provides exactly the functionality you need to help you write a specific application program. How do you go about searching through 200+ modules for interesting-looking datatypes and signatures, starting only from the module names? My View ------- My view is that many of these problems are rooted in Haskell's restriction to a flat module namespace. If we can address that issue adequately, then I believe that many of the difficulties surrounding the provision of good libraries for Haskell will simply fall away. Proposal 1 ---------- Introduce nested namespaces for modules. The key concept here is to map the module namespace into a hierarchical directory-like structure. I propose using the dot as a separator, analogous to Java's usage for namespaces. So for instance, the four example module names above using cryptic prefixes could perhaps be more clearly named Haskell.Language.Parse Text.Xml.Parse Graphics.Drawing.HOpenGL.ConfigFile.Parse TypeSystem.Parse Naming proceeds from the most general category on the left, through more specific subdivisions towards the right. For most compilers and interpreters, this extended module namespace maps directly to a directory/file structure in which the modules are stored. Storing unrelated modules in separate directories (and related modules in the same directory) is a useful and common practice when engineering large systems. (But note that, just as Haskell'98 does not *insist* that modules live in files of the same name, this proposal does not insist on it either. However, we expect most tools to use the close correspondance to their advantage.) There are several issues arising from the particular proposal here. * This is a surface change to the module naming convention. It does not introduce nested /definition/ of modules. * The syntax I propose (a dot separator) is familiar from other languages such as Java, but could in principle be something else, for instance a prime ' or underscore _ or centred dot · or something different again. * Of the choices of separator, dot requires a change to the Haskell'98 lexical syntax, allowing modid -> qconid where currently the syntax is modid -> conid * The use of qualified imports becomes more verbose: for instance import qualified XmlParse ... XmlParse.element f ... becomes import qualified Text.Xml.Parse ... Text.Xml.Parse.element f ... However, I propose that every import have an implicit "as" clause to use as an abbreviation, so in import qualified Text.Xml.Parse [ as Parse ] the clause "as Parse" would be implicit, unless overridden by the programmer with her own "as" clause. The implicit "as" clause always uses the final subdivision of the module name. So for instance, either the fully-qualified or abbreviated-qualified names Text.Xml.Parse.element Parse.element would be accepted and have the same referent, but a partial qualification like Xml.Parse.element would not be accepted. * Another consequence of using the dot as the module namespace separator is that it steals one extremely rare construction from Haskell'98: A.B.C.D in Haskell'98 means the composition of constructor D from module C, with constructor B from module A: (.) A.B C.D No-one so far thinks this is any great loss, and if you really want to say the latter, you still can by simply inserting spaces: A.B . C.D Further down this document, I give more motivation and a rationale for this proposal of nested namespaces. But first, two other proposals which rest on the first one. Proposal 2 ---------- Adopt a standardised namespace layout to help those looking for or writing libraries, and a "Std" namespace prefix for genuinely standard libraries. (These are two different things.) The hslibs collection of modules is a great starting place for finding common libraries that could become standards. I propose that we adopt a "standardised" namespace hierarchy, based on the current hslibs layout, into which Haskell programmers can plug their own libraries relatively easily (whether they intend to release them or not). The aim is to make it clear where to place a new module, and where to search for a possible existing module. For instance, in ASCII art, here is a small part of a suggested tree. + Data + Structures + Trees + AVL | | | + RedBlack | | | | | + Queue + Bankers | | + FIFO | + Encoding + Binary | + MD5 | + Graphics + UI + Gtk + Widget | | | + Pane | | | + Text | | | | | + FranTk | | | + Drawing + HOpenGL + .... | | + Vector | | | + Format + Jpeg | + PPM + Haskell + .... | A fuller proposed layout appears on the web at http://www.cs.york.ac.uk/fp/libraries/layout.html In addition to a standardised hierarchy layout, I propose a truly Standard-with-a-capital-S namespace. A separate discussion is needed on what exactly would consitute "Standard" quality, but by analogy with Java where everything beginning "java." is sanctioned by Sun, I propose that every module name beginning "Std." is in some sense sanctioned by the whole Haskell community. So for instance, an experimental, or not-quite-complete, library could be called Text.Xml but only a guaranteed-to-be-stable, complete, library could be called Std.Text.Xml The implication of the Std. namespace is that all such "standard" libraries will be distributed with all Haskell systems. In other words, you can rely on a standard library always being there, and always having the same interface on all systems. Proposal 3 ---------- Develop a process by which candidate libraries can be proposed to enter the Std namespace. Since Haskell'98 is fixed, and there is no longer a Haskell Committee, there is no official body capable of deciding new standards for libraries. However, we do have a Haskell community which will use or not use libraries, depending on their quality. So libraries will become standards by a de-facto process, rather than de-jure. Apart from the Haskell compiler implementers, we wanted a means to encourage the whole community to be involved in recognising de facto "standard" libraries. The mailing list 'libraries(a)haskell.org' is one contribution. We hope this will work on the same model as the FFI mailing list, which has been pretty successful at allowing a community of designers and implementers to explore their FFI needs and solidify a design that is common across at least three Haskell systems. On top of this discussion however, some final decisions will have to be made on which libraries achieve entry to the "Std." namespace. The Haskell implementers have collectively proposed a ruling troika, one representing each of the three main Haskell systems (Hugs,ghc,nhc98). These are Simon Marlow, representing ghc, and current keeper of the hslibs collection; Malcolm Wallace, representing nhc98; and Andy Gill, representing Hugs users. Some obvious criteria for entry to the "Std." namespace would be: * The interface is stable and unlikely to change significantly; * The library is written in pure Haskell'98. This criterion is likely to be the most contentious, so perhaps a better idea would be that ... * ... an implementation exists for at least the three Haskell systems Hugs, ghc, and nhc98; * The library is already in current use, so bugs in its coding and design have been ironed out; * The Haskell community recognises it as solving a common task, or encapsulating a common programming idiom. These suggested criteria need some discussion and improvement. After the initial period of deciding what belongs in the "Std." namespace, I would expect any further candidate libraries that are proposed for standardisation to spend some time in another part of the namespace hierarchy whilst they gain stability and common acceptance, before being moved to "Std.". Rationale and Motivation for Proposal 1 (nested namespaces) ----------------------------------------------------------- Scenario 1 ---------- Imagine you have just written a new library of, say, Pretty-printing combinators. You want to release it to the Haskell public. So what do you call it? module Pretty -- already taken (several times) module UU_Pretty -- also taken module PrettyLib -- already exists as well Ok, so lacking any further inspiration, you end up deciding to call it module MyPretty -- ! Surely there must be a better solution. Of course there is - namespaces. Let's classify libraries that do similar jobs together: module Text.PrettyPrinter.Hughes -- the original Hughes design module Text.PrettyPrinter.HughesPJ -- later modified by Simon PJ module Text.PrettyPrinter.UU -- the Utrecht design module Text.PrettyPrinter.Chitil -- Olaf's new design These are exactly the same Pretty libs as before, but named more sensibly. It is still clear that each is a pretty-printing library, but it is also clear that they are different. Incidentally, have you ever tried to write your own module called Pretty? You may have discovered with GHC (which has a Pretty already in the hslibs collection), that you get strange errors. This is because sometimes the compiler can be confused into reading one Pretty.hi interface file (i.e. yours), yet linking the other Pretty.o object file (i.e. from hslibs), ending in a core dump. With proper module namespaces, this confusion should never happen again. Scenario 2 ---------- You are writing a complex library that has a couple of layers of abstraction. For some users, you want to expose just a small high-level set of types and functions. Other users will need more detailed access to lower-level stuff. With namespaces, you can use the directory-like structure to make these kinds of access explicit. For instance, imagine a socket library: module Network.Socket It exports an /abstract/ type Socket for ordinary users - they only need to know its name. More advanced hackers however can play with the details of the type, because you also have: module Network.Socket.Types which exports the Socket type non-abstractly i.e. Socket(..). And of course this abstraction is easy for the library-writer to manage, because the implementation of the more abstract layer simply imports and re-exports a careful selection of the more detailed layers. Don't forget that, in terms of the actual filesystem layout, it is perfectly OK to have e.g. file Network/Socket.hs dir NetWork/Socket file Network/Socket/Types.hs Scenario 3 ---------- You are managing a software engineering project. Several people are working more-or-less independently on different sections of the program. To avoid mistakes with files, you give each one a separate directory to place their code in. But in Haskell'98 this is not enough to ensure that they invent module names that do not clash with other developers' modules. So you insist that everyone also uses a prefix-naming scheme for each appropriate sub-task. For instance, here is a sketch of the layout of the Galois Connection team's entry in the ICFP 2000 programming contest: dir CSG -- constructive solid geometry file CSG/CSG.hs file CSG/CSGConstruct.hs file CSG/CSGGeometry.hs file CSG/CSGInterval.hs dir Fran -- Fran-style animation file Fran/FranLite.hs file Fran/FranCSG.hs dir GML -- interpreter for little language file GML/GMLData.hs file GML/GMLParse.hs file GML/GMLPrimitives.hs So now the problem is that to actually build the software, you need to write a Makefile that descends into these directories. Or maybe you use 'hmake' like so: hmake examples/chess.hs -ICSG -IFran -IGML -IRayTrace -package text Note how many sub-directories you must remember to add to the command line (this applies equally for compiler options in Makefiles). Note also the inconsistency between compiling and linking /my/ modules, against using and linking a "standard" hslibs module from package text. Isn't there a simpler way? Yes. Namespaces. Prefix naming is no longer needed inside directories, because the directory name is /part/ of the module name: file CSG.hs -- re-exports everything from the CSG dir dir CSG file CSG/Construct.hs file CSG/Geometry.hs file CSG/Interval.hs dir Fran file Fran/Lite.hs file Fran/CSG.hs -- does not conflict with top-level CSG.hs dir GML file GML/Data.hs file GML/Parse.hs file GML/Primitives.hs And now, the commandline to 'hmake' (or compiler options in a Makefile) becomes simply: hmake examples/chess.hs -I. You only need to specify the root of the module tree (-I.), and all modules in all subdirectories can be found via their full namespace path as used in the source files. Note also that, whereas previously we needed to specify a package for whatever hslibs modules were used, now the compiler/hmake already knows the root of the installed hslibs tree and can use the same mechanism to find and link "standard" modules as for user modules. >From this example it should be clear that the use of module namespaces is of benefit to ordinary programs that may never become public, quite aside from any benefits we expect to derive in managing publically-distributed library code. What now? --------- Ok, so that's my proposal. The implementers of some of the main Haskell systems have seen a presentation of these ideas, and seemed to like them. Namespaces are already implemented in nhc98 (v1.02) and hmake (v2.02) if you want to play with them. I expect some discussion to refine this proposal on the 'libraries(a)haskell.org' list, to which everyone interested is invited. Once we have nailed down the precise design, we need to get matching implementations in all systems. I have rashly volunteered to implement the lexical/parsing/module-search changes in any Haskell system that no-one else volunteers for (probably ghc, Hugs, possibly hbc). But after that we will still have many more decisions to take about individual libraries, precise naming, build systems, and so on, not to mention actually writing the libraries. Get involved. Contribute. Regards, Malcolm

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qualified exports?
by Johannes Waldmann 05 Mar '01

05 Mar '01

While thinking of top-level modules for libraries (packages), I thought it would be uesful if the module could export indentifiers from submodules, but with their qualified names. The effect would be that after module Top ( module Foo , module Bar ) where import qualified Foo import qualified Bar one could just write `import Top' and then use the names `Foo.frobs' etc. What does the languages standard say about the above example? Section 5.2.5 only mentions "The set of all entities brought into scope from a module m by one or more unqualified import declarations ..." Best regards, -- -- Johannes Waldmann ---- http://www.informatik.uni-leipzig.de/~joe/ -- -- joe(a)informatik.uni-leipzig.de -- phone/fax (+49) 341 9732 204/252 --

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Re: Proposal: module namespaces.
by Ashley Yakeley 01 Mar '01

01 Mar '01

At 2001-02-26 09:59, Malcolm Wallace wrote: >Proposal 2 >---------- >Adopt a standardised namespace layout to help those looking for or >writing libraries, I'm a big fan of the Java reversed DNS style. Whatever, I think it's important that anyone with a domain name should be able to obtain a unique namespace without any further bureaucracy. In fact, whatever you decide it's likely to happen anyway, since people will decide that for instance "Com.Microsoft.Research.MyModule" is unlikely to clash with anyone outside the appropriate domains and subdivisions. I'm assuming that module name components have enforced capitalisation, like all other Haskell identifiers. >and a "Std" namespace prefix for genuinely >standard libraries. (These are two different things.) Eeesh, let's hope ICANN doesn't register a 'std' TLD. I would prefer "Standard" for this reason and also because the abbreviation seems pretty pointless. >In addition to a standardised hierarchy layout, I propose a truly >Standard-with-a-capital-S namespace. A separate discussion is needed >on what exactly would consitute "Standard" quality, but by analogy >with Java where everything beginning "java." is sanctioned by Sun, >I propose that every module name beginning "Std." is in some sense >sanctioned by the whole Haskell community. Do you have any kind of guarantees of copyright openness in mind? In Java, everything under java.* is supposed to be owned by Sun. Will it be standard practice for versions of Standard be included with Haskell compilers? Could the Prelude make use of Standard? Could Standard become an alternative to the Prelude? If answers to these last three are all "no", an alternative would be to put it under "Org.Haskell.Standard". -- Ashley Yakeley, Seattle WA

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