Puneet Varma (Editor)

Ur (programming language)

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Paradigms
  
functional, reactive

Designed by
  
Adam Chlipala

Platform
  
POSIX

Family
  
ML

First appeared
  
December 2014

Stable release
  
20160306 release / March 6, 2016; 12 months ago (2016-03-06)

Ur also called Ur/Web is a Free and Open source functional programming language specific for web development, created by Adam Chlipala at the Massachusetts Institute of Technology that from a single program produces server code, browser client code and SQL code specific for the chosen database backend.

Contents

Ur supports a powerful kind of metaprogramming based on row types.

Ur/Web is Ur plus a special standard library and associated rules for parsing and optimization. Ur/Web supports construction of dynamic web 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. Not only do they not crash during particular page generations, but they also may not:

  • Suffer from any kinds of code injection attacks
  • Return invalid HTML
  • Contain dead intra-application links
  • Have mismatches between HTML forms and the fields expected by their handlers
  • Include client-side code that makes incorrect assumptions about the "AJAX"-style services that the remote web server provides
  • Attempt invalid SQL queries
  • Use improper marshaling or unmarshaling in communication with SQL databases or between browsers and web servers

    • 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.

    The Ur/Web compiler also produces very efficient object code that does not use garbage collection.

    The implementation of all this is open source.

    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 additional type manipulation.

    Ajax call/response is serialized through a monad called transaction (corresponds to Haskell's IO) and its marshalling and decoding is encapsulated in the rpc function.

    The browser client side includes functional reactive programming facilities using the (source a) type and a signal monad.

    Example program

    This is a demo program showing client, server and database code with Ajax communication, from the web demos, with extra comments to outline each of the components:

    Interface file (ML-like signature) with .urs extension:

    Implementation file (.ur extension):

    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 /ModulePath/functionName, 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

    Libraries

  • The predefined API
  • The standard library
  • Per feature tests
  • Ur wiki - Libraries and FFI bindings

    • Special features and problems

  • Record updating

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

  • Record fields ellipsis
  • 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.

    References

    Ur (programming language) Wikipedia
    (Text) CC BY-SA