| Ur, Ur/Web | |
|---|---|
| Paradigms | functional,reactive |
| Family | ML,Haskell |
| Designed by | Adam Chlipala |
| First appeared | December 2014; 10 years ago (2014-12)[1] |
| Stable release | 20200209 / February 9, 2020; 5 years ago (2020-02-09) |
| Typing discipline | static, row |
| Platform | x86-64 |
| OS | POSIX |
| License | MIT |
| Filename extensions | .ur, .urs, .urp |
| Website | impredicative |
| Influenced by | |
| ML,Haskell[2] | |
Ur, also calledUr/Web, is amulti-paradigm,high-level,pure,strict,functionalprogramming language. It is adialect of the languageML, designed forweb development, created by Adam Chlipala at theMassachusetts Institute of Technology[3] that one program can emit code for aserver,web browser client, andSQL specific to a givendatabasebackend. The full implementation isfree and open-source software released under anMIT License.[2]
Ur has its start and roots in a superseded progenitor language namedLaconic/Web,[4] in 2006.[5]
Ur supports a powerful kind ofmetaprogramming based onrowdata types.[2]
Ur/Web is Ur plus a special standardlibrary and associated rules forparsing and optimizing. Ur/Web supports construction ofdynamic web pages and applications backed bySQL 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]
Thistype 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]
TheUr/Web compiler also produces very efficient object code that does not usegarbage collection.[2]
SQL syntax templates embedded in the language facilitate the handling of tables.
Although the syntax is based onStandard ML the language includes concepts fromHaskell with added type manipulation.
Ajax call/response is serialized through amonad calledtransaction (corresponds to Haskell'sinput/output (IO)) and its marshalling and decoding is encapsulated in therpc function.
The browser client side includesfunctionalreactive programming facilities using the(source a) type and asignalmonad.
Ur/Web not only makes web applications easier to write, it also makes them more secure.
"Let's say you want to have a calendar widget on your web page, and you're going to use a library that provides the calendar widget, and on the same page there's also an advertisement box that's based on code that's provided by the ad network," Chlipala said.
"What you don't want is for the ad network to be able to change how the calendar works or the author of the calendar code to be able to interfere with delivering the ads."
This is a demo program showing client, server and database code withAjax 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 *)valmain:unit->transactionpage
Implementation file (.ur extension):
datatypelistt=Nil|Consoft*listttablet:{Id:int,A:string}PRIMARYKEYId(* server side database access, called through AJAX XmlHttpRequest encapsulated as ''rpc'' function (remote procedure call) *)funaddids=(* sql dml template with {[expression]} *)dml(INSERTINTOt(Id,A)VALUES({[id]},{[s]}))fundelid=dml(DELETEFROMtWHEREt.Id={[id]})funlookupid=(* haskell style monadic code *)ro<-oneOrNoRows(SELECTt.AFROMtWHEREt.Id={[id]});caseroofNone=>returnNone(* return is the ''monad'' lifting function *)|Somer=>return(Somer.T.A)(* ''check'' called by client side onClick event handler, so it will be compiled to JavaScript as page embedded client script *)funcheckls=caselsofNil=>return()|Cons(id,ls')=>ao<-rpc(lookupid);(* Ajax call to server side *)alert(caseaoofNone=>"Nada"|Somea=>a);checkls'funmain()=idAdd<-source"";aAdd<-source"";idDel<-source"";(* generates web page with JavaScript inclusions *)return<xml><body><buttonvalue="Check values of 1, 2, and 3"onclick={fn_=>letvalmylist=1::2::3::[]incheckmylistend}/><br/><br/><buttonvalue="Add"onclick={fn_=>id<-getidAdd;a<-getaAdd;rpc(add(readErrorid)a)(* Ajax call to server side *)}/><ctextboxsource={idAdd}/><ctextboxsource={aAdd}/><br/><br/><buttonvalue="Delete"onclick={fn_=>id<-getidDel;rpc(del(readErrorid))(* Ajax call to server side *)}/><ctextboxsource={idDel}/></body></xml>
Project file (.urp extension), must contain an optional directive list followed by a listing of project modules:[8]
# hash prefixed line commentsrewrite url Module1/main# set root URL to Module1/main functionexe myexenamedatabase dbname=test# database attrib. and parameterssql noisy.sql
$/list# stdlib modules prefixed with "$/"module2# if used by module1 it must precede itmodule1# main module
Compile:
urweb module1# looks for module1.urp
Execute as a web server (other modes areCGI,FastCGI, ...):
./module1.exe -p 8081# -h : RTS options help
datatypemystruckv=Empty|Nodeof{Key:k,Value:v}funsetKey[k][v](* type polymorphism *)(_:ordk)(* implicit instance of class ord *)(callerErrNote:string)(k1:k)(my:mystruckv):mystruckv=ifk1<kminthenerror<xml>setKey:illegalk1{[callerErrNote]}</xml>elsecasemyofNoder=>Node(r--#Key++{Key=k1})|_=>error<xml>setKey:notaNode{[callerErrNote]}</xml>
corresponding signature (kind annotations (:::) implicit; (::) explicit):
conmystruc::Type->Type->Type(* two param. type constructor *)valsetKey:k:::Type->v:::Type->ordk->string->k->mystruckv->mystruckv
casemyofNode{Key=k,...}=>doWhateverk|_=>....
This error happens with types ofarity> 0 in nestedcase orlet clauses and disappears by type annotating the variables in the nested clauses.
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