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, type inference, 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 is 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 languages.
adamc@404: adamc@404: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 build you a local version of this demo: 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@404: 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 a single argument, an integer specifying how many request handler pthreads to spawn. The server accepts requests on port 8080. 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@404:Alias /demo/ "DIR/demo/out/" adamc@404: adamc@404: ProxyPass /Demo/ http://localhost:8080/ adamc@404: ProxyPassReverse /Demo/ http://localhost:8080/
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@405: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 interface 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@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@406:In the implementation of handler, we see the notation {[...]}, which uses type classes to inject values of different types (string and bool 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@403: adamc@403: listShop.urp adamc@403: adamc@403:This is my other favorite.