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.

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:

./configure
make
sudo make install
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.

These project files can also be built separately. For example, you could run

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.

Alias /demo/ "DIR/demo/out/"

ProxyPass /Demo/ http://localhost:8080/
ProxyPassReverse /Demo/ http://localhost:8080/

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.

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.

We must, of course, begin with "Hello World."

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.

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.

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).

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.

Crafting webs of interlinked pages is easy, using recursion.

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.

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.

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.

This example shows off algebraic datatypes, parametric polymorphism, and functors.

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.

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.

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.

We see a simple example of accessing a SQL database. The project file specifies the database to connect to.

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).

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.

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.

sum.urp

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.

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.

The unary $ operator is used to build a record Type from a {Type} (that is, the kind of records of types). The library function mapUT takes in a Type t and a {Unit} r, and it builds a {Type} as long as r, where every field is assigned value t.

Another library function foldUR is defined at the level of expressions, while mapUT is a type-level function. foldUR takes 6 arguments, some of them types and some values. Type arguments are distinguished by being written within brackets. The arguments to foldUR respectively tell us:

1. The type we will assign to each record field
2. The type of the final and all intermediate results of the fold, expressed as a function over the portion of the {Unit} that has been traversed so far
3. A function that updates the accumulator based on the current record field name, the rest of the input record type, the current record field value, and the current accumulator
4. The initial accumulator value
5. The input record type
6. The input record value

The general syntax for constant row types is [Name1 = t1, ..., NameN = tN], and there is a shorthand version [Name1, ..., NameN] for records of Units.

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.

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.