Ur/Web is a domain-specific language for programming web applications backed by SQL databases. It is (strongly) statically-typed (like ML and Haskell) and purely functional (like Haskell). Ur is the base language, and the web-specific features of Ur/Web (mostly) come only in the form of special rules for parsing and optimization. The Ur core looks a lot like Standard ML, with a few Haskell-isms added, and kinder, gentler versions added of many features from dependently-typed languages like the logic behind Coq. The type system is much more expressive than in ML and Haskell, such that well-typed web applications cannot "go wrong," not just in handling single HTTP requests, but across their entire lifetimes of interacting with HTTP clients. Beyond that, Ur is unusual in using ideas from dependent typing to enable very effective metaprogramming, or programming with explicit analysis of type structure. Many common web application components can be built by Ur/Web functions that operate on types, where it seems impossible to achieve similar code re-use in more established statically-typed languages.
adamc@404: adamc@896:This demo is built automatically from Ur/Web sources and supporting files. If you unpack the Ur/Web source distribution, then the following steps will (if you're lucky) build you a local version of this demo. If you're not lucky, you can consult the beginning of the manual for more detailed instructions. adamc@404: adamc@404:
adamc@404: adamc@404:./configure adamc@404: make adamc@404: sudo make install adamc@404: urweb -demo /Demo demo
The -demo /Demo flag says that we want to build a demo application that expects its URIs to begin with /Demo. The final argument demo gives the path to a directory housing demo files. One of the files in that directory is prose, a file describing the different demo pieces with HTML. Some lines of prose have the form foo.urp, naming particular project files (with the extension .urp) in that directory.
adamc@404: adamc@404:These project files can also be built separately. For example, you could run adamc@404: adamc@404:
adamc@404: adamc@498: to build the "Hello World" demo application. Whether building the pieces separately or all at once with the -demo flag, a standalone web server executable is generated. The -demo command line will generate demo/demo.exe, and the other command line will generate demo/hello.exe. Either can be run with no arguments to start a single-threaded server accepting requests on port 8080. Pass the flag -h to see which options are available. adamc@404: adamc@404:urweb demo/hello
The -demo version also generates some HTML in a subdirectory out of the demo directory. It is easy to set Apache up to serve these HTML files, and to proxy out to the Ur/Web web server for dynamic page requests. This configuration works for me, where DIR is the location of an Ur/Web source distribution. adamc@404: adamc@404:
adamc@404: adamc@409:Alias /demo/ "DIR/demo/out/" adamc@404: adamc@863: ProxyPass /Demo/ http://localhost:8080/Demo/ adamc@863: ProxyPassReverse /Demo/ http://localhost:8080/Demo/
Building the demo also generates a demo.sql file, giving the SQL commands to run to define all of the tables and sequences that the applications expect to see. The file demo.urp contains a database line with the PostgreSQL database that the demo web server will try to connect to.
adamc@409: adamc@896:The easiest way to get a demo running locally is probably with this alternate command sequence: adamc@896: adamc@896:
adamc@896: adamc@896:urweb -dbms sqlite -db /path/to/database/file -demo /Demo demo adamc@896: sqlite3 /path/to/database/file <demo/demo.sql adamc@896: demo/demo.exe
Then you can skip the static content and connect directly to the demo server at http://localhost:8080/Demo/Demo/main, which contains links to the individual demos.
adamc@896: adamc@404:The rest of the demo focuses on the individual applications. Follow the links in the lefthand frame to visit the applications, commentary, and syntax-highlighted source code. (An Emacs mode is behind the syntax highlighting.) I recommend visiting the applications in the order listed, since that is the order in which new concepts are introduced.
adamc@380: adamc@380: hello.urp adamc@380: adamc@405:We must, of course, begin with "Hello World."
adamc@405: adamc@405:The project file justs list one filename prefix, hello. This causes both hello.urs and hello.ur to be pulled into the project. .urs files are like OCaml .mli files, and .ur files are like OCaml .ml files. That is, .urs files provide interfaces, and .ur files provide implementations. .urs files may be omitted for .ur files, in which case most permissive interfaces are inferred.
adamc@405: adamc@503:Ur/Web features a module system very similar to those found in SML and OCaml. Like in OCaml, interface files are treated as module system signatures, and they are ascribed to structures built from implementation files. hello.urs tells us that we only export a function named main, taking no arguments and running a transaction that results in an HTML page. transaction is a monad in the spirit of the Haskell IO monad, with the intent that every operation performable in transaction can be undone. By design, Ur/Web does not provide a less constrained way of running side-effecting actions. This particular example application will employ no side effects, but the compiler requires that all pages be generated by transactions.
adamc@405: adamc@405:Looking at hello.ur, we see an SML-looking function definition that returns a fragment of XML, written with special syntax. This fragment is returned to browsers that request the URI /Demo/Hello/main. That is, we take the demo-wide prefix /Demo and add a suffix that indicates we want to call the main function in the Hello module. This path convention generalizes to arbitrary levels of nested module definitions and functor applications (which we will see later).
adamc@380: adamc@380: link.urp adamc@380: adamc@406:In link.ur, we see how easy it is to link to another page. The Ur/Web compiler guarantees that all links are valid. We just write some Ur/Web code inside an "antiquote" in our XML, denoting a transaction that will produce the new page if the link is clicked.
adamc@406: adamc@407: rec.urp adamc@407: adamc@407:Crafting webs of interlinked pages is easy, using recursion.
adamc@407: adamc@416: counter.urp adamc@416: adamc@416:It is also easy to pass state around via functions, in the style commonly associated with "continuation-based" web servers. As is usual for such systems, all state is stored on the client side. In this case, it is encoded in URLs.
adamc@416: adamc@416:In the implementation of Counter.counter, we see the notation {[...]}, which uses type classes to inject values of different types (int in this case) into XML. It's probably worth stating explicitly that XML fragments are not strings, so that the type-checker will enforce that our final piece of XML is valid.
adamc@416: adamc@406: form.urp adamc@406: adamc@406:Here we see a basic form. The type system tracks which form inputs we include, and it enforces that the form handler function expects a record containing exactly those fields, with exactly the proper types.
adamc@406: adamc@455: nested.urp adamc@455: adamc@455:Here is an implementation of the tiny challenge problem from this web framework comparison. Using nested function definitions, it is easy to persist state across clicks.
adamc@455: adamc@465: cookie.urp adamc@465: adamc@465:Often, it is useful to associate persistent data with particular web clients. Ur/Web includes an easy facility for using type-safe cookies. This example shows how to use a form to set a named cookie.
adamc@465: adamc@465:After setting the cookie, try browsing back to this demo from the main index. The data you entered should still be there.
adamc@465: adamc@774: url.urp adamc@774: adamc@774:Up to this point, we haven't included a single URL in our source code. This may be very surprising to programmers used to working with traditional web frameworks! In Ur/Web, we avoid writing URLs explicitly wherever possible. To link to an external web page, we rely on an abstract type url. Strings can't be treated implicitly as URLs; rather, they must be "blessed" explicitly. This helps avoid some classes of code injection attacks.
adamc@774: adamc@774:Further, each Ur/Web application enforces a global condition on which strings are allowed as URLs. The .urp file for this demo shows an example that specifies particular rules about which URLs are allowed. You can try entering a variety of URLs on the form on the front page. Only those satisfying the allow url/deny url conditions should be permitted.
adamc@774: adamc@775: css.urp adamc@775: adamc@775:Ur/Web supports a structured approach to Cascading Style Sheets, where each style is a first-class value within a module. This demo shows the importing of an external style sheet with one style. By default, like other Ur/Web entities, the name of the style would be Css_quote. We use the rewrite directive in the .urp file to specify an alternate name for a particular canonical module path. The external style sheet contains a definition of a style with the alternate name that we give.
adamc@775: adamc@776: upload.urp adamc@776: adamc@776:HTTP file upload is made convenient, via the abstract types blob and file in the standard library. A blob is a binary sequence, and a file combines a blob with MIME type information. An upload form input can be used to accept files from the user.
adamc@776: adamc@776:In the .urp file for this example, we give a whitelist of MIME types to be accepted. The application will echo back to the user any file he uploads as one of those types. You can try submitting other kinds of files to verify that they are rejected.
adamc@776: adamc@780: subforms.urp adamc@780: adamc@780:In the examples so far, the number of inputs per form has been constant. Often it is useful to have a varying set of form inputs. Ur/Web provides the <subforms> and <entry> tags for grouping a list of forms with the same field names and types into a single, list-valued composite form. This demo shows those tags in action, in a simple form echoing application that lets the user add and remove inputs.
adamc@780: adamc@403: listShop.urp adamc@403: adamc@408:This example shows off algebraic datatypes, parametric polymorphism, and functors.
adamc@408: adamc@408:The List module defines a list datatype, much in the style of SML, but with type parameters written more in Haskell style. The types of List.length and List.rev indicate that they are polymorphic. Types like t ::: Type -> ... indicate polymorphism, with the triple colon denoting that the value of this type parameter should be inferred at uses. A double colon would mean that the type argument must be provided explicitly at uses. In contrast to ML and Haskell, all polymorphism must be declared explicitly in Ur, while instantiations may be inferred at uses.
adamc@408: adamc@408:The ListFun module defines a functor for building list editing sub-applications. An argument to the functor Make must give the type to be stored in the lists, along with marshaling and unmarshaling functions. In return, the functor returns an entry point function.
adamc@408: adamc@408:The ListShop modules ties everything together by instantiating ListFun.Make with structures for integers and strings. show and read can be used for marshaling and unmarshaling in both cases because they are type-class-generic.
adamc@410: adamc@410: sql.urp adamc@410: adamc@410:We see a simple example of accessing a SQL database. The project file specifies the database to connect to.
adamc@410: adamc@410:A table declaration declares a SQL table with rows of a particular record type. We can use embedded SQL syntax in a way that leads to all of our queries and updates being type-checked. Indeed, Ur/Web makes strong guarantees that it is impossible to execute invalid SQL queries or make bad assumptions about the types of tables for marshaling and unmarshaling (which happen implicitly).
adamc@410: adamc@410:The list function implements an HTML table view of all rows in the SQL table. The queryX function takes two arguments: a SQL query and a function for generating XML fragments from query result rows. The query is run, and the fragments for the rows are concatenated together.
adamc@410: adamc@410:Other functions demonstrate use of the dml function, for building a transaction from a SQL DML command. It is easy to insert antiquoted Ur code into queries and DML commands, and the type-checker catches mistakes in the types of the expressions that we insert.
adamc@410: adamc@410:adamc@413: adamc@420: ref.urp adamc@420: adamc@420:
This example shows how to mix the module system with SQL to implement a kind of "abstract data type." The functor RefFun.Make takes in a type belonging to the type class of those types that may be included in SQL. The functor output includes an abstract type ref, along with operations for working with refs via transactions. In the functor implementation, we see that ref is implemented as int, treated as primary keys of a SQL table.
adamc@420: adamc@420:The functor creates a new encapsulated SQL sequence and table on each call. These local relations show up in the automatically-generated SQL file that should be run to prepare the database for use, but they are invisible from client code. We could change the functor to create different SQL relations, without needing to change client code.
adamc@420: adamc@643:Note that, in ref.ur, the inj components of functor arguments are omitted. Since these arguments are type class witnesses, the compiler infers them automatically based on the choices of data.
adamc@643: adamc@501: tree.urp adamc@501: adamc@501:Here we see how we can abstract over common patterns of SQL queries. In particular, since standard SQL does not help much with queries over trees, we write a function for traversing an SQL tree, building an HTML representation, based on a user-provided function for rendering individual rows.
adamc@501: adamc@643:The signature of TreeFun.Make tells us that, to instantiate the functor, we must provide
adamc@643:Ur/Web supports attaching SQL table constraints to table definitions. We've sprinkled a few such constraints throughout our examples so far, without mentioning them. This example shows a table with all four of the supported kinds of constraints. An application would generally try to avoid inserting data that violates constraints, but, in this example, we let you insert arbitrary data, so that you can see each of the constraints failing.
adamc@773: adamc@773:SQL outer joins are no problem, as this demo shows. Unlike with SQL, here we have static type inference determining for us which columns may become nullable as a result of an outer join, and the compiler will reject programs that make the wrong assumptions about that process. The details of that nullification don't appear in this example, where the magic of type classes determines both the post-join type of each field and the right pretty-printing and parsing function for each of those types.
adamc@777: adamc@778: view.urp adamc@778: adamc@778:SQL views are also supported with a special declaration form, analogous to table. A multi-parameter type class fieldsOf is used to characterize places where both tables and views are allowed. For instance, the polymorphic function list shown here lists the contents of any table or view containing just a single int column named A.
adamc@778: adamc@779: cookieSec.urp adamc@779: adamc@779:Ur/Web guarantees that compiled applications are immune to certain kinds of cross site request forgery. For instance, a "phisher" might send many e-mails linking to a form that he has set up to look like your web site. The form is connected to your web site, where it might, say, transfer money from your bank account to the phisher's account. The phisher doesn't know your username, but, if that username is stored in a cookie, it will be sent automatically by your browser. Ur/Web automatically signs cookie values cryptographically, with the signature included as a POST parameter and not part of a cookie, to prevent such attacks.
adamc@779: adamc@779:This demo shows a simple mock-up of a situation where such an attack is often possible with traditional web frameworks. You can set an arbitrary username for yourself in a cookie, and you can modify the database in a way that depends on the current cookie value. Try getting the latter action to succeed without first setting your desired username in the cookie. This should be roughly as impossible as cracking the particular cryptographic hash function that is used.
adamc@779: adamc@413: sum.urp adamc@413: adamc@413:Metaprogramming is one of the most important facilities of Ur. This example shows how to write a function that is able to sum up the fields of records of integers, no matter which set of fields the particular record has.
adamc@413: adamc@413:Ur's support for analysis of types is based around extensible records, or row types. In the definition of the sum function, we see the type parameter fs assigned the kind {Unit}, which stands for records of types of kind Unit. The Unit kind has only one inhabitant, (). The kind Type is for "normal" types.
adamc@413: adamc@643:The sum function also takes an argument fl of type folder fs. Folders represent permutations of the elements of type-level records. We can use a folder to iterate over a type-level record in the order indicated by the permutation.
adamc@413: adamc@643:The unary $ operator is used to build a record Type from a {Type} (that is, the kind of records of types). The library function mapU takes in a type t of kind
Another library function foldUR is defined at the level of expressions, while mapU is a type-level function. foldUR takes 7 arguments, some of them types and some values. Type arguments are distinguished by being written within brackets. The arguments to foldUR respectively tell us: adamc@413: adamc@413:
The general syntax for constant row types is [Name1 = t1, ..., NameN = tN], and there is a shorthand version [Name1, ..., NameN] for records of Units.
adamc@413: adamc@643:With sum defined, it is easy to make some sample calls. The form of the code for main does not make it apparent, but the compiler must "reverse engineer" the appropriate {Unit} from the {Type} available from the context at each call to sum. The compiler also infers a folder for each call, guessing at the desired permutations by examining the orders in which field names are written in the code.
adamc@417: adamc@417: tcSum.urp adamc@417: adamc@417:It's easy to adapt the last example to use type classes, such that we can sum the fields of records based on any numeric type.
adamc@418: adamc@418: metaform1.urp adamc@418: adamc@420:We can use metaprogramming with row types to build HTML forms (and their handlers) generically. The functor Metaform.Make takes in a unit row fs and a value-level record names assigning string names to the fields of fs. The functor implementation builds a form handler with a library function foldURX2, which runs over two value-level records in parallel, building an XML fragment.
adamc@420: adamc@643:The form itself is generated using the more primitive foldUR. We see the type xml form [] (mapU string cols) as the result of the fold. This is the type of XML fragments that are suitable for inclusion in forms, require no form fields to be defined on entry, and themselves define form fields whose names and types are given by mapU string cols. The useMore function "weakens" the type of an XML fragment, so that it "pretends" to require additional fields as input. This weakening is necessary to accommodate the general typing rule for concatenating bits of XML. adamc@643:
The functor use in Metaform1 is trivial. The compiler infers the values of the structure members fs and fl from the type of the value provided for names.
adamc@420: adamc@418: metaform2.urp adamc@419: adamc@420:This example showcases code reuse by applying the same functor as in the last example. The Metaform2 module mixes pages from the functor with some new pages of its own.
adamc@421: adamc@421: crud1.urp adamc@421: adamc@421:This example pulls together much of what we have seen so far. It involves a generic "admin interface" builder. That is, we have the Crud.Make functor, which takes in a description of a table and outputs a sub-application for viewing and editing that table.
adamc@421: adamc@421:The signature of Crud.Make is based around a type function colMeta, which describes which supporting values we need for each column. This function is declared with the keyword con, which stands for "constructor," the general class of "compile-time things" that includes types. An argument to colMeta has kind (Type * Type), which means that it must be a type-level tuple. The first type is how the column is represented in SQL, and the second is how we represent it in HTML forms. In order, the components of the resulting record give: adamc@421: adamc@421:
The function colsMeta lifts colMeta over type-level records of type pairs. The Crud module also defines reasonable default colMeta values for some primitive types.
adamc@421: adamc@421:The functor signature tells us (in order) that an input must contain: adamc@421: adamc@421:
Looking at crud1.ur, we see that a use of the functor is almost trivial. Only the value components of the argument structure must be provided. The column row type is inferred, and the disjointness constraint is proved automatically.
adamc@421: adamc@643:We won't go into detail on the implementation of Crud.Make. The types of the functions used there can be found in the signatures of the built-in Basis module and the Top module from the standard library. The signature of the first and the signature and implementation of the second can be found in the lib/ur directory of the Ur/Web distribution.
adamc@422: adamc@422: crud2.urp adamc@422: adamc@422:This example shows another application of Crud.Make. We mix one standard column with one customized column. We write an underscore for the Inject field of meta-data, since the type class facility can infer that witness.
adamc@644: adamc@781: crud3.urp adamc@781: adamc@781:One thing that is unclear from the previous examples is how to provide more complex, multi-input widgets for taking input meant for particular fields. The signature of Crud.Make forces every widget to define exactly one input. The <subform> tag, the simpler cousin of the <subforms> tag that we saw earlier, provides a fix for this problem. Via <subform>, an arbitrary form can be turned into a single record-valued input.
adamc@781: adamc@781:We use that possibility here to define a silly widget for a string column, which concatenates the values entered into two different textboxes.
adamc@781: adamc@644: alert.urp adamc@644: adamc@644:Ur/Web makes it easy to write code whose execution should be distributed between the web server and client web browsers. Server-side code is compiled to efficient native code, and client-side code is compiled to JavaScript. Ur/Web programmers don't need to worry about these details, because the language and standard library provide a uniform ML-like interface for the whole process.
adamc@644: adamc@644:Here's an example of a button that, when clicked, opens an alert dialog on the client.
adamc@645: adamc@645: react.urp adamc@645: adamc@645:Most client-side JavaScript programs modify page contents imperatively, but Ur/Web is based on functional-reactive programming instead. Programs allocate data sources and then describe the page as a pure function of those data sources. When the sources change, the page changes automatically.
adamc@645: adamc@645:Here's an example where a button modifies a data source that affects some text on the page. The affected portion of the page is indicated with the pseudo-HTML tag dyn, whose signal attribute specifies one of these pure functions over mutable sources. A source containing data of type t has type source t and is created with the source operation within the transaction monad. Functions over sources are represented in the monad signal. Like in Haskell, we overload monad notations, so that the same return and bind operators can be used to write signals and transactions. The signal function coerces a source to a signal.
adamc@647: adamc@647: listEdit.urp adamc@647: adamc@647:This is a more involved functional-reactive example, involving recursive data structures that contain sources. We build a list editor similar to the one from the ListShop example, but with all editing happening on the client side.
adamc@647: adamc@647:The central data structure is the rlist, a list whose individual elements are sources, enabling fine-grained mutation. Every rlist is either nil or is a cons cell made up of a source for a string data element, another source to serve as a scratchpad for GUI-based edits to the data element, and a final source that stores the remainder of the list.
adamc@647: adamc@647:The main program provides operations to append to a list and to edit the data stored at any cell of the list. Append is implemented by maintaining a source head, which points to the first list element; and a source tailP, which points to a source rlist where we should place the next appended node.
adamc@648: adamc@648: increment.urp adamc@648: adamc@648:Here's an example where client-side code needs to run more code on the server. We maintain a (server-side) SQL sequence. When the user clicks a button, an AJAX request increments the remote sequence and gets the new value.
adamc@649: adamc@651: noisy.urp adamc@651: adamc@651:This example shows how easy it is to make the flow of control "ping pong" back and forth between the client and the server. Clicking a button triggers three queries to the server, with an alert generated after each query.
adamc@651: adamc@649: batch.urp adamc@649: adamc@649:This example shows more of what is possible with mixed client/server code. The application is an editor for a simple database table, where additions of new rows can be batched in the client, before a button is clicked to trigger a mass addition.
adamc@650: adamc@650: batchG.urp adamc@650: adamc@650:We can redo the last example with a generic component, like we did in the Crud examples. The module BatchFun is analogous to the Crud module. It contains a functor that builds a batching editor, when given a suitable description of a table.
adamc@650: adamc@650:The signature of the functor is the same as for Crud. We change the definition of colMeta to reflect the different kinds of column metadata that we need. Each column is still described by a pair of types, and the first element of each pair still gives the SQL type for a column. Now, however, the second type in a pair gives a type of local state to be used in a reactive widget for inputing that column.
adamc@650: adamc@650:The first three fields of a colMeta record are the same as for Crud. The rest of the fields are:
adamc@650:BatchFun.Make handles the plumbing of allocating the local state, using it to create widgets, and reading the state values when the user clicks "Batch it."
adamc@650: adamc@650:batchG.ur contains an example instantiation, which is just as easy to write as in the Crud1 example.
adamc@697: adamc@697: threads.urp adamc@697: adamc@697:Ur/Web makes it easy to write multi-threaded client-side code. This example demonstrates two threads writing to a page at once.
adamc@697: adamc@697:First, we define a useful component for sections of pages that can have lines of text added to them dynamically. This is the Buffer module. It contains an abstract type of writable regions, along with functions to create a region, retrieve a signal representing its HTML rendering, and add a new line to it.
adamc@697: adamc@697:The entry point to the main module Threads begins by creating a buffer. The function loop implements writing to that buffer periodically, incrementing a counter each time. The arguments to loop specify a prefix for the messages and the number of milliseconds to wait between writes.
adamc@697: adamc@697:We specify some client-side code to run on page load using the onload attribute of <body>. The onload code in this example spawns two separate threads running the loop code with different prefixes, update intervals, and starting counters.
adamc@697: adamc@697:Old hands at concurrent programming may be worried at the lack of synchronization in this program. Ur/Web uses cooperative multi-threading, not the more common preemptive multi-threading. Only one thread runs at a time, and only particular function calls can trigger context switches. In this example, sleep is the only such function that appears.
adamc@698: adamc@698: roundTrip.urp adamc@698: adamc@698:So far, we've seen examples of client-side code triggering the execution of server-side code. Such remote calls only happen in response to client-side events. It is often useful to allow a client to trigger events on other clients, and Ur/Web facilitates this with a simple asynchronous message-passing facility. The current example introduces the basics of message-passing with a trivial use case, and the next example shows a more realistic case where several clients can communicate.
adamc@698: adamc@698:We are going to provide a silly service where a client can send messages to the server, which the server then echoes back to the client. The SQL table channels stores a mapping from client IDs to message channels. The abstract type client holds unique client IDs, which Ur/Web generates automatically as needed. A channel T is a channel to which messages of type T can be sent. Every channel belongs to a single client; anyone can send to a channel, but only the channel's owner can read the messages. Every client is associated with a particular open page on a particular web browser somewhere. Since web browsing sessions are ephemeral, clients and their channels are garbage-collected automatically as the web server loses contact with browsers. When a client is garbage-collected, any database row mentioning it or one of its channels is deleted. It's also possible to include option clients (and likewise for channels) in databases, in which case such columns are merely nulled out when they refer to dead clients.
adamc@698: adamc@698:The main function begins by retrieving the current client ID, allocating a new channel, and associating that channel with the current client in the database. Next, we allocate a buffer and return the page, which in its onload attribute starts two loops running in parallel. In contrast to in the last example, here we only use spawn with the call to the first loop, since every client-side event handler is implicitly started in a new thread. adamc@698: adamc@698:
The first loop, receiver, repeatedly reads messages from the channel and writes them to the buffer. The second loop, sender, periodically sends messages to the channel. Client code can't send messages directly. Instead, we must use server-side functions to do the sending. Clients aren't trusted to pass channels to the server, so our server-side function writeBack instead keys off of the client ID, looking up the corresponding channel in the database.
adamc@699: adamc@699: chat.urp adamc@699: adamc@699:This example provides a simple anonymous online chatting system, with multiple named channels.
adamc@699: adamc@699:First, we build another useful component. Recall that each channel has an owning client, who has the exclusive ability to read messages sent to it. On top of that functionality, we can build a kind of broadcast channel that accepts multiple subscribers. The Broadcast module contains a functor with such an implementation. We instantiate the functor with the type of data we want to send over the channel. The functor output gives us an abstract type of "topics," which are subscribable IDs. When a client subscribes to a topic, it is handed a channel that it can use to read new messages on that topic. We also have an operation to count the number of subscribers to a topic. This number shouldn't be treated as too precise, since some clients that have surfed away from the application may still be considered subscribed until a timeout period elapses.
adamc@699: adamc@699:The main Chat application includes some standard management of a table of named channels. All of the interesting client-server work is done with the recv function and with the functions provided by Broadcast.