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 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. Ur has its start and roots in a superseded progenitor language named Laconic/Web, in 2006.

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 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. Ur has its start and roots in a superseded progenitor language named Laconic/Web, in 2006. (* 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 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 "Nada" | Some a => a ); check ls' fun main () = idAdd let val mylist = 1 :: 2 :: 3 :: [] in check mylist end }/> id id Project file (.urp extension), must contain an optional directive list followed by a listing of project modules: # 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 server side, page retrieving functions with no side effects (http GET method) are accessible through a URL as ; they should have type (unit -> transaction page). 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. 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 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 setKey: illegal k1 {[callerErrNote]} else case my of Node r => Node (r -- #Key ++ {Key = k1}) | _ => error setKey: not a Node {[callerErrNote]} 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 Record fields ellipsis case my of Node {Key = k, ...} => doWhatever k | _ => .... Error "Substitution in constructor is blocked by a too-deep unification variable" This error happens with types of arity > 0 in nested case or let clauses and disappears by type annotating the variables in the nested clauses.

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