Ur (programming language)
Ur, also called Ur/Web, is a multi-paradigm, high-level, pure, strict, functional programming language. It is a dialect of the language ML, designed for web development, created by Adam Chlipala at the Massachusetts Institute of Technology[3] that one program can emit code for a server, web browser client, and SQL specific to a given database backend. The full implementation is free and open-source software released under an MIT License.[2] Ur has its start and roots in a superseded progenitor language named Laconic/Web,[4] in 2006.[5] FunctionUr supports a powerful kind of metaprogramming based on row data types.[2] Ur/Web is Ur plus a special standard library and associated rules for parsing and optimizing. Ur/Web supports construction of dynamic web pages and applications backed by SQL 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:[2]
This type safety 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.[2] The Ur/Web compiler also produces very efficient object code that does not use garbage collection.[2] SQL syntax templates embedded in the language facilitate the handling of tables. Although the syntax is based on Standard ML the language includes concepts from Haskell with added type manipulation. Ajax call/response is serialized through a monad called transaction (corresponds to Haskell's input/output (IO)) and its marshalling and decoding is encapsulated in the rpc function. The browser client side includes functional reactive programming facilities using the
Example programThis is a demo program showing client, server and database code with Ajax communication, from the web demos,[7] with extra comments to outline each of the components: Interface file (ML-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
}/><br/>
<br/>
<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}/><br/>
<br/>
<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:[8] # 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
Compile: urweb module1 # looks for module1.urp Execute as a web server (other modes are CGI, FastCGI, ...): ./module1.exe -p 8081 # -h : RTS options help LibrariesSpecial features and problems
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
case my of
Node {Key = k, ...} => doWhatever k
| _ => ....
This error happens with types of arity See also
References
External links
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