Ur (programming language)

<h2 id="function">Function</h2>
Ur supports a powerful kind of <a href="/facts/Metaprogramming/ujbjjJTY">metaprogramming</a> based on row <a href="/facts/Data_type/fcx5hgVL">data types</a>.<a class="footnote-ref" id="fnref:5" href="#fn:5">5</a>
Ur/Web is Ur plus a special standard <a href="/facts/Library_(computing)/xRzL3P7q">library</a> and associated rules for <a href="/facts/Parsing/nRKB01S3">parsing</a> and optimizing. Ur/Web supports construction of <a href="/facts/Dynamic_web_page/psjqGKBx">dynamic web pages</a> and applications backed by <a href="/facts/SQL/UjQLJcYm">SQL</a> databases. The signature of the standard library is such that well-typed Ur/Web programs "don't go wrong" in a very broad sense. They do not crash during particular page generations, and may not:<a class="footnote-ref" id="fnref:6" href="#fn:6">6</a>

<ul><li>Suffer from any kinds of <a href="/facts/Code_injection/6gPRDpd1">code injection</a> attacks</li>
<li>Return invalid <a href="/facts/HTML/sSc3fT78">HTML</a></li>
<li>Contain dead intra-application links</li>
<li>Have mismatches between <a href="/facts/Form_(HTML)/9Ivp2peo">HTML forms</a> and the fields expected by their handlers</li>
<li>Include client-side code that makes incorrect assumptions about the "<a href="/facts/Ajax_(programming)/cS0P0whT">Ajax</a>"-style services that the remote web server provides</li>
<li>Attempt invalid <a href="/facts/SQL/UjQLJcYm">SQL</a> queries</li>
<li>Use improper <a href="/facts/Marshalling_(computer_science)/NhcdrmhR">marshaling</a> or unmarshaling in communication with <a href="/facts/SQL_database/UjQLJcYm">SQL databases</a> or between <a href="/facts/Web_browser/2pfyymLs">web browsers</a> and <a href="/facts/Web_server/A4GS6thA">web servers</a></li></ul>
This <a href="/facts/Type_safety/WxmEKq8R">type safety</a> is just the foundation of the Ur/Web methodology. It is also possible to use metaprogramming to build significant application pieces by analysis of type structure.<a class="footnote-ref" id="fnref:7" href="#fn:7">7</a>
The Ur/Web compiler also produces very efficient object code that does not use <a href="/facts/Garbage_collection_(computer_science)/j2vlIu7g">garbage collection</a>.<a class="footnote-ref" id="fnref:8" href="#fn:8">8</a>
<a href="/facts/SQL/UjQLJcYm">SQL</a> syntax templates embedded in the language facilitate the handling of tables.
Although the syntax is based on <a href="/facts/Standard_ML/4LaEq9tA">Standard ML</a> the language includes concepts from <a href="/facts/Haskell/ydEsHuGy">Haskell</a> with added type manipulation.
<a href="/facts/Ajax_(programming)/cS0P0whT">Ajax</a> call/response is serialized through a <a href="/facts/Monad_(functional_programming)/kYazRoF1">monad</a> called transaction (corresponds to Haskell's <a href="/facts/Input/output/WQTE8Jas">input/output</a> (IO)) and its marshalling and decoding is encapsulated in the rpc function.
The browser client side includes <a href="/facts/Functional_programming/cdxqPEIM">functional</a> <a href="/facts/Reactive_programming/Jj8CsdKY">reactive programming</a> facilities using the (source a) type and a signal <a href="/facts/Monad_(functional_programming)/kYazRoF1">monad</a>.

<h2 id="example-program">Example program</h2>
This is a demo program showing client, server and database code with <a href="/facts/Ajax_(programming)/cS0P0whT">Ajax</a> communication, from the web demos,<a class="footnote-ref" id="fnref:9" href="#fn:9">9</a> with extra comments to outline each of the components:
Interface file (<a href="/facts/Standard_ML/4LaEq9tA">ML</a>-like signature) with .urs extension:

(* the environment monad is called transaction, corresponds to Haskell's IO monad *)
val main : unit -> transaction page

Implementation file (.ur extension):

datatype list t = Nil | Cons of t * list t

table t : { Id : int, A : string }
  PRIMARY KEY Id

(* server side database access, called through AJAX XmlHttpRequest
                    encapsulated as ''rpc'' function (remote procedure call) *)
fun add id s =
    (* sql dml template with {[expression]} *)
    dml (INSERT INTO t (Id, A) VALUES ({[id]}, {[s]}))

fun del id =
    dml (DELETE FROM t WHERE t.Id = {[id]})

fun lookup id =
 (* haskell style monadic code *)
 ro <- oneOrNoRows (SELECT t.A FROM t WHERE t.Id = {[id]});
 case ro of
 None => return None (* return is the ''monad'' lifting function *)
 | Some r => return (Some r.T.A)

(* ''check'' called by client side onClick event handler,
 so it will be compiled to JavaScript as page embedded client script *)
fun check ls =
 case ls of
 Nil => return ()
 | Cons (id, ls') =>
 ao <- rpc (lookup id); (* Ajax call to server side *)
 alert (case ao of
 None => "Nada"
 | Some a => a
 );
 check ls'

fun main () =
 idAdd <- source "";
 aAdd <- source "";

idDel <- source "";

(* generates web page with JavaScript inclusions *)
 return <xml><body>
 <button value="Check values of 1, 2, and 3"
 onclick={fn _ => let val mylist = 1 :: 2 :: 3 :: []
 in
 check mylist
 end
 }/> 
 
 <button value="Add"
 onclick={fn _ => id <- get idAdd;
 a <- get aAdd;
 rpc (add (readError id) a) (* Ajax call to server side *)
 }/>
 <ctextbox source={idAdd}/>
 <ctextbox source={aAdd}/> 
 
 <button value="Delete"
 onclick={fn _ => id <- get idDel;
 rpc (del (readError id)) (* Ajax call to server side *)
 }/>
 <ctextbox source={idDel}/>
 </body></xml>

Project file (.urp extension), must contain an optional directive list followed by a listing of project modules:<a class="footnote-ref" id="fnref:10" href="#fn:10">10</a>

# hash prefixed line comments
rewrite url Module1/main # set root URL to Module1/main function
exe myexename
database dbname=test # database attrib. and parameters
sql noisy.sql

$/list # stdlib modules prefixed with "$/"
module2 # if used by module1 it must precede it
module1 # main module

<ul><li>server side, page retrieving functions with no side effects (http GET method) are accessible through a URL as /ModulePath/functionName; they should have type (unit -> transaction page).</li>
<li>To export a page which may cause side effects, accessible only via HTTP POST, include one argument of the page handler of type Basis.postBody.<a class="footnote-ref" id="fnref:11" href="#fn:11">11</a></li></ul>
Compile:

urweb module1 # looks for module1.urp

Execute as a web server (other modes are <a href="/facts/Common_Gateway_Interface/zWsLFXGJ">CGI</a>, <a href="/facts/FastCGI/hy2It6dA">FastCGI</a>, ...):

./module1.exe -p 8081 # -h : RTS options help

<h2 id="libraries">Libraries</h2>
<ul><li><a href="https://github.com/urweb/urweb/blob/master/lib/ur/basis.urs">The predefined API</a></li>
<li><a href="https://github.com/urweb/urweb/tree/master/lib/ur">The standard library</a></li>
<li><a href="https://github.com/urweb/urweb/tree/master/tests">Per feature tests</a></li>
<li><a href="http://www.impredicative.com/wiki/index.php/Libraries_and_FFI_bindings">Ur wiki - Libraries and FFI bindings</a></li></ul>
<h2 id="special-features-and-problems">Special features and problems</h2>
<ul><li>Record updating</li></ul>
datatype mystruc k v = Empty | Node of {Key: k, Value: v}

fun setKey [k][v] (* type polymorphism *)
 (_: ord k) (* implicit instance of class ord *)
 (callerErrNote: string) (k1: k) (my: mystruc k v) : mystruc k v =
 if k1 < kmin
 then error <xml>setKey: illegal k1 {[callerErrNote]}</xml>
 else case my of
 Node r => Node (r -- #Key ++ {Key = k1})
 | _ => error <xml>setKey: not a Node {[callerErrNote]}</xml>

corresponding signature (kind annotations (:::) implicit; (::) explicit):

con mystruc :: Type -> Type -> Type (* two param. type constructor *)

val setKey : k ::: Type -> v ::: Type -> ord k -> string -> k -> mystruc k v -> mystruc k v

<ul><li>Record fields ellipsis</li></ul>
 case my of
 Node {Key = k, ...} => doWhatever k
 | _ => ....

<ul><li>Error "Substitution in constructor is blocked by a too-deep unification variable"<a class="footnote-ref" id="fnref:12" href="#fn:12">12</a></li></ul>
This error happens with types of <a href="/facts/Arity/zSVuwHvP">arity</a> > 0 in nested case or let clauses and disappears by type annotating the variables in the nested clauses.

<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Opa_(programming_language)/5EYoYQR6">Opa</a>, a language for combined frontend-backend development</li></ul>

<h2 id="external-links">External links</h2>
<ul><li><a href="http://impredicative.com/ur">Official website</a></li>
<li><a href="https://github.com/urweb">Ur/Web</a> on <a href="/facts/GitHub/n1hNCsc3">GitHub</a></li>
<li><a href="http://www.impredicative.com/wiki/index.php/Special:AllPages">Ur wiki</a></li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1">Chlipala, Adam (January 2015). "Ur/Web: A Simple Model for Programming the Web". MIT / Association for Computing Machinery (ACM). Retrieved 5 January 2015. <a href="https://dspace.mit.edu/handle/1721.1/92321" target="_blank">https://dspace.mit.edu/handle/1721.1/92321</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></li>
<li id="fn:2">"The Ur Programming Language Family". Impredicative.com/ur. Retrieved 3 April 2016. <a href="http://impredicative.com/ur/" target="_blank">http://impredicative.com/ur/</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></li>
<li id="fn:3">Chlipala, Adam (2006). "The Laconic programming language family". SourceForge. <a href="https://laconic.sourceforge.net/" target="_blank">https://laconic.sourceforge.net/</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></li>
<li id="fn:4">Chlipala, Adam (2006). "Scrap Your Web Application Boilerplate, or Metaprogramming with Row Types". Adam.Chlipala.net. <a href="http://adam.chlipala.net/papers/LaconicTR/" target="_blank">http://adam.chlipala.net/papers/LaconicTR/</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></li>
<li id="fn:5">"The Ur Programming Language Family". Impredicative.com/ur. Retrieved 3 April 2016. <a href="http://impredicative.com/ur/" target="_blank">http://impredicative.com/ur/</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></li>
<li id="fn:6">"The Ur Programming Language Family". Impredicative.com/ur. Retrieved 3 April 2016. <a href="http://impredicative.com/ur/" target="_blank">http://impredicative.com/ur/</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></li>
<li id="fn:7">"The Ur Programming Language Family". Impredicative.com/ur. Retrieved 3 April 2016. <a href="http://impredicative.com/ur/" target="_blank">http://impredicative.com/ur/</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></li>
<li id="fn:8">"The Ur Programming Language Family". Impredicative.com/ur. Retrieved 3 April 2016. <a href="http://impredicative.com/ur/" target="_blank">http://impredicative.com/ur/</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></li>
<li id="fn:9">Ur language demo programs <a href="http://impredicative.com/ur/demo/" target="_blank">http://impredicative.com/ur/demo/</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></li>
<li id="fn:10">Chlipala, Adam (January 2015). "The Ur/Web Manual – Project files". GitHub. Retrieved 8 January 2015. <a href="https://enn.github.io/urweb-doc/node5.html" target="_blank">https://enn.github.io/urweb-doc/node5.html</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></li>
<li id="fn:11">The Ur/Web Manual - The Structure of Web Applications <a href="https://enn.github.io/urweb-doc/node54.html" target="_blank">https://enn.github.io/urweb-doc/node54.html</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></li>
<li id="fn:12">Unexpected type error: "Substitution in constructor is blocked by a too-deep unification variable" <a href="http://www.impredicative.com/pipermail/ur/2011-January/000498.html" target="_blank">http://www.impredicative.com/pipermail/ur/2011-January/000498.html</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></li>
</ol>

Ur (programming language) open-in-new

Ur (programming language)