annotate demo/prose @ 773:74a090ff296e

constraints demo
author Adam Chlipala <adamc@hcoop.net>
date Sun, 03 May 2009 12:01:55 -0400
parents 311ca1ae715d
children 412ccd97ab71
rev   line source
adamc@643 1 <p><b>Ur/Web</b> 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). <b>Ur</b> 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 <a href="http://sml.sourceforge.net/">Standard ML</a>, with a few <a href="http://www.haskell.org/">Haskell</a>-isms added, and kinder, gentler versions added of many features from dependently-typed languages like the logic behind <a href="http://coq.inria.fr/">Coq</a>. 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.</p>
adamc@404 2
adamc@404 3 <p>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 4
adamc@404 5 <blockquote><pre>./configure
adamc@404 6 make
adamc@404 7 sudo make install
adamc@404 8 urweb -demo /Demo demo</pre></blockquote></p>
adamc@404 9
adamc@404 10 <p>The <tt>-demo /Demo</tt> flag says that we want to build a demo application that expects its URIs to begin with <tt>/Demo</tt>. The final argument <tt>demo</tt> gives the path to a directory housing demo files. One of the files in that directory is <tt>prose</tt>, a file describing the different demo pieces with HTML. Some lines of <tt>prose</tt> have the form <tt><i>foo</i>.urp</tt>, naming particular project files (with the extension <tt>.urp</tt>) in that directory.</p>
adamc@404 11
adamc@404 12 <p>These project files can also be built separately. For example, you could run
adamc@404 13
adamc@404 14 <blockquote><pre>urweb demo/hello</pre></blockquote>
adamc@404 15
adamc@498 16 to build the "Hello World" demo application. Whether building the pieces separately or all at once with the <tt>-demo</tt> flag, a standalone web server executable is generated. The <tt>-demo</tt> command line will generate <tt>demo/demo.exe</tt>, and the other command line will generate <tt>demo/hello.exe</tt>. Either can be run with no arguments to start a single-threaded server accepting requests on port 8080. Pass the flag <tt>-h</tt> to see which options are available.</p>
adamc@404 17
adamc@404 18 <p>The <tt>-demo</tt> version also generates some HTML in a subdirectory <tt>out</tt> 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 <tt>DIR</tt> is the location of an Ur/Web source distribution.
adamc@404 19
adamc@404 20 <blockquote><pre>Alias /demo/ "DIR/demo/out/"
adamc@404 21
adamc@404 22 ProxyPass /Demo/ http://localhost:8080/
adamc@404 23 ProxyPassReverse /Demo/ http://localhost:8080/</pre></blockquote></p>
adamc@404 24
adamc@409 25 <p>Building the demo also generates a <tt>demo.sql</tt> file, giving the SQL commands to run to define all of the tables and sequences that the applications expect to see. The file <tt>demo.urp</tt> contains a <tt>database</tt> line with the PostgreSQL database that the demo web server will try to connect to.</p>
adamc@409 26
adamc@404 27 <p>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.</p>
adamc@380 28
adamc@380 29 hello.urp
adamc@380 30
adamc@405 31 <p>We must, of course, begin with "Hello World."</p>
adamc@405 32
adamc@405 33 <p>The project file justs list one filename prefix, <tt>hello</tt>. This causes both <tt>hello.urs</tt> and <tt>hello.ur</tt> to be pulled into the project. <tt>.urs</tt> files are like <a href="http://caml.inria.fr/ocaml/">OCaml</a> <tt>.mli</tt> files, and <tt>.ur</tt> files are like OCaml <tt>.ml</tt> files. That is, <tt>.urs</tt> files provide interfaces, and <tt>.ur</tt> files provide implementations. <tt>.urs</tt> files may be omitted for <tt>.ur</tt> files, in which case most permissive interfaces are inferred.</p>
adamc@405 34
adamc@503 35 <p>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. <tt>hello.urs</tt> tells us that we only export a function named <tt>main</tt>, taking no arguments and running a transaction that results in an HTML page. <tt>transaction</tt> is a monad in the spirit of the Haskell IO monad, with the intent that every operation performable in <tt>transaction</tt> 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.</p>
adamc@405 36
adamc@405 37 <p>Looking at <tt>hello.ur</tt>, 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 <tt>/Demo/Hello/main</tt>. That is, we take the demo-wide prefix <tt>/Demo</tt> and add a suffix that indicates we want to call the <tt>main</tt> function in the <tt>Hello</tt> module. This path convention generalizes to arbitrary levels of nested module definitions and functor applications (which we will see later).</p>
adamc@380 38
adamc@380 39 link.urp
adamc@380 40
adamc@406 41 <p>In <tt>link.ur</tt>, 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.</p>
adamc@406 42
adamc@407 43 rec.urp
adamc@407 44
adamc@407 45 <p>Crafting webs of interlinked pages is easy, using recursion.</p>
adamc@407 46
adamc@416 47 counter.urp
adamc@416 48
adamc@416 49 <p>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.</p>
adamc@416 50
adamc@416 51 <p>In the implementation of <tt>Counter.counter</tt>, we see the notation <tt>{[...]}</tt>, which uses type classes to inject values of different types (<tt>int</tt> in this case) into XML. It's probably worth stating explicitly that XML fragments <i>are not strings</i>, so that the type-checker will enforce that our final piece of XML is valid.</p>
adamc@416 52
adamc@406 53 form.urp
adamc@406 54
adamc@406 55 <p>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.</p>
adamc@406 56
adamc@455 57 nested.urp
adamc@455 58
adamc@455 59 <p>Here is an implementation of the tiny challenge problem from <a href="http://www.accursoft.co.uk/web/">this web framework comparison</a>. Using nested function definitions, it is easy to persist state across clicks.</p>
adamc@455 60
adamc@465 61 cookie.urp
adamc@465 62
adamc@465 63 <p>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.</p>
adamc@465 64
adamc@465 65 <p>After setting the cookie, try browsing back to this demo from the main index. The data you entered should still be there.</p>
adamc@465 66
adamc@403 67 listShop.urp
adamc@403 68
adamc@408 69 <p>This example shows off algebraic datatypes, parametric polymorphism, and functors.</p>
adamc@408 70
adamc@408 71 <p>The <tt>List</tt> module defines a list datatype, much in the style of SML, but with type parameters written more in Haskell style. The types of <tt>List.length</tt> and <tt>List.rev</tt> indicate that they are polymorphic. Types like <tt>t ::: Type -> ...</tt> indicate polymorphism, with the triple colon denoting that the value of this type parameter should be <i>inferred</i> 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 <i>declared</i> explicitly in Ur, while instantiations may be inferred at uses.</p>
adamc@408 72
adamc@408 73 <p>The <tt>ListFun</tt> module defines a functor for building list editing sub-applications. An argument to the functor <tt>Make</tt> 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.</p>
adamc@408 74
adamc@408 75 <p>The <tt>ListShop</tt> modules ties everything together by instantiating <tt>ListFun.Make</tt> with structures for integers and strings. <tt>show</tt> and <tt>read</tt> can be used for marshaling and unmarshaling in both cases because they are type-class-generic.</p>
adamc@410 76
adamc@410 77 sql.urp
adamc@410 78
adamc@410 79 <p>We see a simple example of accessing a SQL database. The project file specifies the database to connect to.</p>
adamc@410 80
adamc@410 81 <p>A <tt>table</tt> 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).</p>
adamc@410 82
adamc@410 83 <p>The <tt>list</tt> function implements an HTML table view of all rows in the SQL table. The <tt>queryX</tt> 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.</p>
adamc@410 84
adamc@410 85 <p>Other functions demonstrate use of the <tt>dml</tt> 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.</p>
adamc@410 86
adamc@410 87 <p>
adamc@413 88
adamc@420 89 ref.urp
adamc@420 90
adamc@420 91 <p>This example shows how to mix the module system with SQL to implement a kind of "abstract data type." The functor <tt>RefFun.Make</tt> 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 <tt>ref</tt>, along with operations for working with <tt>ref</tt>s via transactions. In the functor implementation, we see that <tt>ref</tt> is implemented as <tt>int</tt>, treated as primary keys of a SQL table.</p>
adamc@420 92
adamc@420 93 <p>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.</p>
adamc@420 94
adamc@643 95 <p>Note that, in <tt>ref.ur</tt>, the <tt>inj</tt> components of functor arguments are omitted. Since these arguments are type class witnesses, the compiler infers them automatically based on the choices of <tt>data</tt>.</p>
adamc@643 96
adamc@501 97 tree.urp
adamc@501 98
adamc@501 99 <p>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.</p>
adamc@501 100
adamc@643 101 <p>The signature of <tt>TreeFun.Make</tt> tells us that, to instantiate the functor, we must provide</p>
adamc@643 102 <ol>
adamc@773 103 <li>A primary key type <tt>key</tt></li>
adamc@773 104 <li>SQL field names <tt>id</tt> (for primary keys) and <tt>parent</tt> (for parent links)</li>
adamc@773 105 <li>A type-level record <tt>cols</tt> of field names besides <tt>id</tt> and <tt>parent</tt></li>
adamc@773 106 <li>"Proofs" that <tt>id</tt> is distinct from <tt>parent</tt> and that neither of <tt>id</tt> and <tt>parent</tt> appears in <tt>cols</tt></li>
adamc@773 107 <li>A witness that <tt>key</tt> belongs to the type class <tt>sql_injectable_prim</tt>, which indicates that both <tt>key</tt> and <tt>option key</tt> are fair game to use with SQL</li>
adamc@773 108 <li>An SQL table <tt>tab</tt>, containing a field <tt>id</tt> of type <tt>key</tt>, a field <tt>parent</tt> of type <tt>option key</tt>, and every field of <tt>cols</tt></li>
adamc@773 109 </ol>
adamc@773 110
adamc@773 111 constraints.urp
adamc@773 112
adamc@773 113 <p>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.</p>
adamc@773 114
adamc@773 115 <ol>
adamc@773 116 <li>The <tt>PRIMARY KEY</tt> constraint establishes the field of the table that we expect to use as a key in looking up specific rows. It is an error for two rows to share the same primary key.</li>
adamc@773 117 <li>The <tt>UNIQUE</tt> constraint is like <tt>PRIMARY KEY</tt>, with the difference being that a table may have many <tt>UNIQUE</tt> constraints but no more than one primary key.</li>
adamc@773 118 <li>The <tt>CHECK</tt> constraint declares a boolean assertion that must hold for every row of the table.</li>
adamc@773 119 <li>The <tt>FOREIGN KEY</tt> constraint declares that a row of the table references a particular column of another table, or of the same table, as we see in this example. It's a static type error to reference a foreign key column that has no <tt>PRIMARY KEY</tt> or <tt>UNIQUE</tt> constraint.</li>
adamc@643 120 </ol>
adamc@643 121
adamc@413 122 sum.urp
adamc@413 123
adamc@413 124 <p>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.</p>
adamc@413 125
adamc@413 126 <p>Ur's support for analysis of types is based around extensible records, or <i>row types</i>. In the definition of the <tt>sum</tt> function, we see the type parameter <tt>fs</tt> assigned the <i>kind</i> <tt>{Unit}</tt>, which stands for records of types of kind <tt>Unit</tt>. The <tt>Unit</tt> kind has only one inhabitant, <tt>()</tt>. The kind <tt>Type</tt> is for "normal" types.</p>
adamc@413 127
adamc@643 128 <p>The <tt>sum</tt> function also takes an argument <tt>fl</tt> of type <tt>folder fs</tt>. 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.</p>
adamc@413 129
adamc@643 130 <p>The unary <tt>$</tt> operator is used to build a record <tt>Type</tt> from a <tt>{Type}</tt> (that is, the kind of records of types). The library function <tt>mapU</tt> takes in a type <i>t</i> of kind <t>K</t> and a <tt>{Unit}</tt> <i>r</i>, and it builds a <tt>{K}</tt> as long as <i>r</i>, where every field is assigned value <i>t</i>.</p>
adamc@643 131
adamc@643 132 <p>Another library function <tt>foldUR</tt> is defined at the level of expressions, while <tt>mapU</tt> is a type-level function. <tt>foldUR</tt> takes 7 arguments, some of them types and some values. Type arguments are distinguished by being written within brackets. The arguments to <tt>foldUR</tt> respectively tell us:
adamc@413 133
adamc@413 134 <ol>
adamc@773 135 <li>The type we will assign to each record field</li>
adamc@773 136 <li>The type of the final and all intermediate results of the fold, expressed as a function over the portion of the <tt>{Unit}</tt> that has been traversed so far</li>
adamc@773 137 <li>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</li>
adamc@773 138 <li>The initial accumulator value</li>
adamc@773 139 <li>The input record type</li>
adamc@773 140 <li>A folder for that type</li>
adamc@773 141 <li>The input record value</li>
adamc@413 142 </ol>
adamc@413 143
adamc@413 144 An unusual part of the third argument is the syntax <tt>[t1 ~ t2]</tt> within a multi-argument <tt>fn</tt>. This syntax denotes a proof that row types <tt>t1</tt> and <tt>t2</tt> have no field names in common. The proof is not named, because it is applied automatically as needed. Indeed, the proof appears unused in this case, though it is actually needed to ensure the validity of some inferred types, as well as to unify with the type of <tt>foldUR</tt>.</p>
adamc@413 145
adamc@413 146 <p>The general syntax for constant row types is <tt>[Name1 = t1, ..., NameN = tN]</tt>, and there is a shorthand version <tt>[Name1, ..., NameN]</tt> for records of <tt>Unit</tt>s.</p>
adamc@413 147
adamc@643 148 <p>With <tt>sum</tt> defined, it is easy to make some sample calls. The form of the code for <tt>main</tt> does not make it apparent, but the compiler must "reverse engineer" the appropriate <tt>{Unit}</tt> from the <tt>{Type}</tt> available from the context at each call to <tt>sum</tt>. The compiler also infers a <tt>folder</tt> for each call, guessing at the desired permutations by examining the orders in which field names are written in the code.</p>
adamc@417 149
adamc@417 150 tcSum.urp
adamc@417 151
adamc@417 152 <p>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.</p>
adamc@418 153
adamc@418 154 metaform1.urp
adamc@418 155
adamc@420 156 <p>We can use metaprogramming with row types to build HTML forms (and their handlers) generically. The functor <tt>Metaform.Make</tt> takes in a unit row <tt>fs</tt> and a value-level record <tt>names</tt> assigning string names to the fields of <tt>fs</tt>. The functor implementation builds a form handler with a library function <tt>foldURX2</tt>, which runs over two value-level records in parallel, building an XML fragment.</p>
adamc@420 157
adamc@643 158 <p>The form itself is generated using the more primitive <tt>foldUR</tt>. We see the type <tt>xml form [] (mapU string cols)</tt> 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 <tt>mapU string cols</tt>. The <tt>useMore</tt> 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.</tt>
adamc@643 159 <p>The functor use in <tt>Metaform1</tt> is trivial. The compiler infers the values of the structure members <tt>fs</tt> and <tt>fl</tt> from the type of the value provided for <tt>names</tt>.</p>
adamc@420 160
adamc@418 161 metaform2.urp
adamc@419 162
adamc@420 163 <p>This example showcases code reuse by applying the same functor as in the last example. The <tt>Metaform2</tt> module mixes pages from the functor with some new pages of its own.</p>
adamc@421 164
adamc@421 165 crud1.urp
adamc@421 166
adamc@421 167 <p>This example pulls together much of what we have seen so far. It involves a generic "admin interface" builder. That is, we have the <tt>Crud.Make</tt> functor, which takes in a description of a table and outputs a sub-application for viewing and editing that table.</p>
adamc@421 168
adamc@421 169 <p>The signature of <tt>Crud.Make</tt> is based around a type function <tt>colMeta</tt>, which describes which supporting values we need for each column. This function is declared with the keyword <tt>con</tt>, which stands for "constructor," the general class of "compile-time things" that includes types. An argument to <tt>colMeta</tt> has kind <tt>(Type * Type)</tt>, 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@421 170
adamc@421 171 <ol>
adamc@773 172 <li>A display name</li>
adamc@773 173 <li>A way of pretty-printing values of the column</li>
adamc@773 174 <li>A way of generating an HTML form widget to input this column</li>
adamc@773 175 <li>A way of generating an HTML form widget with an initial value specified</li>
adamc@773 176 <li>A way of parsing values of the column from strings</li>
adamc@773 177 <li>A type class witness, showing that the SQL representation can really be included in SQL</li>
adamc@421 178 </ol></p>
adamc@421 179
adamc@421 180 <p>The function <tt>colsMeta</tt> lifts <tt>colMeta</tt> over type-level records of type pairs. The <tt>Crud</tt> module also defines reasonable default <tt>colMeta</tt> values for some primitive types.</p>
adamc@421 181
adamc@421 182 <p>The functor signature tells us (in order) that an input must contain:
adamc@421 183
adamc@421 184 <ol>
adamc@773 185 <li>A type pair record <tt>cols</tt></li>
adamc@773 186 <li>A proof that <tt>cols</tt> does not contain a field named <tt>Id</tt></li>
adamc@773 187 <li>A SQL table <tt>tab</tt> with an <tt>Id</tt> field of type <tt>int</tt> and other fields whose names and types are read off of <tt>cols</tt></li>
adamc@773 188 <li>A display title for the admin interface</li>
adamc@773 189 <li>A record of meta-data for the columns</li>
adamc@421 190 </ol></p>
adamc@421 191
adamc@421 192 <p>Looking at <tt>crud1.ur</tt>, 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.</p>
adamc@421 193
adamc@643 194 <p>We won't go into detail on the implementation of <tt>Crud.Make</tt>. The types of the functions used there can be found in the signatures of the built-in <tt>Basis</tt> module and the <tt>Top</tt> module from the standard library. The signature of the first and the signature and implementation of the second can be found in the <tt>lib/ur</tt> directory of the Ur/Web distribution.</p>
adamc@422 195
adamc@422 196 crud2.urp
adamc@422 197
adamc@422 198 <p>This example shows another application of <tt>Crud.Make</tt>. We mix one standard column with one customized column. We write an underscore for the <tt>Inject</tt> field of meta-data, since the type class facility can infer that witness.</p>
adamc@644 199
adamc@644 200 alert.urp
adamc@644 201
adamc@644 202 <p>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.</p>
adamc@644 203
adamc@644 204 <p>Here's an example of a button that, when clicked, opens an alert dialog on the client.</p>
adamc@645 205
adamc@645 206 react.urp
adamc@645 207
adamc@645 208 <p>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.</p>
adamc@645 209
adamc@645 210 <p>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 <tt>dyn</tt>, whose <tt>signal</tt> attribute specifies one of these pure functions over mutable sources. A source containing data of type <tt>t</tt> has type <tt>source t</tt> and is created with the <tt>source</tt> operation within the <tt>transaction</tt> monad. Functions over sources are represented in the monad <tt>signal</tt>. Like in Haskell, we overload monad notations, so that the same return and bind operators can be used to write signals and transactions. The <tt>signal</tt> function coerces a source to a signal.</p>
adamc@647 211
adamc@647 212 listEdit.urp
adamc@647 213
adamc@647 214 <p>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 <tt>ListShop</tt> example, but with all editing happening on the client side.</p>
adamc@647 215
adamc@647 216 <p>The central data structure is the <tt>rlist</tt>, 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.</p>
adamc@647 217
adamc@647 218 <p>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 <tt>head</tt>, which points to the first list element; and a source <tt>tailP</tt>, which points to a <tt>source rlist</tt> where we should place the next appended node.</p>
adamc@648 219
adamc@648 220 increment.urp
adamc@648 221
adamc@648 222 <p>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.</p>
adamc@649 223
adamc@651 224 noisy.urp
adamc@651 225
adamc@651 226 <p>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.</p>
adamc@651 227
adamc@649 228 batch.urp
adamc@649 229
adamc@649 230 <p>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.</p>
adamc@650 231
adamc@650 232 batchG.urp
adamc@650 233
adamc@650 234 <p>We can redo the last example with a generic component, like we did in the <tt>Crud</tt> examples. The module <tt>BatchFun</tt> is analogous to the <tt>Crud</tt> module. It contains a functor that builds a batching editor, when given a suitable description of a table.</p>
adamc@650 235
adamc@650 236 <p>The signature of the functor is the same as for <tt>Crud</tt>. We change the definition of <tt>colMeta</tt> 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 <i>local state</i> to be used in a reactive widget for inputing that column.</p>
adamc@650 237
adamc@650 238 <p>The first three fields of a <tt>colMeta</tt> record are the same as for <tt>Crud</tt>. The rest of the fields are:</p>
adamc@650 239 <ol>
adamc@773 240 <li><tt>NewState</tt>, which allocates some new widget local state</li>
adamc@773 241 <li><tt>Widget</tt>, which produces a reactive widget from some state</li>
adamc@773 242 <li><tt>ReadState</tt>, which reads the current value of some state to determine which SQL value it encodes</li>
adamc@650 243 </ol>
adamc@650 244
adamc@650 245 <p><tt>BatchFun.Make</tt> handles the plumbing of allocating the local state, using it to create widgets, and reading the state values when the user clicks "Batch it."</p>
adamc@650 246
adamc@650 247 <p><tt>batchG.ur</tt> contains an example instantiation, which is just as easy to write as in the <tt>Crud1</tt> example.</p>
adamc@697 248
adamc@697 249 threads.urp
adamc@697 250
adamc@697 251 <p>Ur/Web makes it easy to write multi-threaded client-side code. This example demonstrates two threads writing to a page at once.</p>
adamc@697 252
adamc@697 253 <p>First, we define a useful component for sections of pages that can have lines of text added to them dynamically. This is the <tt>Buffer</tt> 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.</p>
adamc@697 254
adamc@697 255 <p>The entry point to the main module <tt>Threads</tt> begins by creating a buffer. The function <tt>loop</tt> implements writing to that buffer periodically, incrementing a counter each time. The arguments to <tt>loop</tt> specify a prefix for the messages and the number of milliseconds to wait between writes.</p>
adamc@697 256
adamc@697 257 <p>We specify some client-side code to run on page load using the <tt>onload</tt> attribute of <tt>&lt;body&gt;</tt>. The <tt>onload</tt> code in this example spawns two separate threads running the <tt>loop</tt> code with different prefixes, update intervals, and starting counters.</p>
adamc@697 258
adamc@697 259 <p>Old hands at concurrent programming may be worried at the lack of synchronization in this program. Ur/Web uses <i>cooperative multi-threading</i>, not the more common <i>preemptive</i> multi-threading. Only one thread runs at a time, and only particular function calls can trigger context switches. In this example, <tt>sleep</tt> is the only such function that appears.</p>
adamc@698 260
adamc@698 261 roundTrip.urp
adamc@698 262
adamc@698 263 <p>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.</p>
adamc@698 264
adamc@698 265 <p>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 <tt>channels</tt> stores a mapping from client IDs to message channels. The abstract type <tt>client</tt> holds unique client IDs, which Ur/Web generates automatically as needed. A <tt>channel <i>T</i></tt> is a channel to which messages of type <tt><i>T</i></tt> 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 <tt>option client</tt>s (and likewise for channels) in databases, in which case such columns are merely nulled out when they refer to dead clients.</p>
adamc@698 266
adamc@698 267 <p>The <tt>main</tt> 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 <tt>onload</tt> attribute starts two loops running in parallel. In contrast to in the last example, here we only use <tt>spawn</tt> with the call to the first loop, since every client-side event handler is implicitly started in a new thread.</tt>
adamc@698 268
adamc@698 269 <p>The first loop, <tt>receiver</tt>, repeatedly reads messages from the channel and writes them to the buffer. The second loop, <tt>sender</tt>, 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 <tt>writeBack</tt> instead keys off of the client ID, looking up the corresponding channel in the database.</p>
adamc@699 270
adamc@699 271 chat.urp
adamc@699 272
adamc@699 273 <p>This example provides a simple anonymous online chatting system, with multiple named channels.</p>
adamc@699 274
adamc@699 275 <p>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 <tt>Broadcast</tt> 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.</p>
adamc@699 276
adamc@699 277 <p>The main <tt>Chat</tt> application includes some standard management of a table of named channels. All of the interesting client-server work is done with the <tt>recv</tt> function and with the functions provided by <tt>Broadcast</tt>.</p>