Ur/Web: A Statically-Typed Language for
Building Web Applications from
Components
Adam Chlipala
Emerging Languages Camp 2010

Web Applications: A Steaming Pile
of Text
HTML, CSS, JavaScript,
SQL, URLs, JSON, ...

Web App Developer

Strings, strings,
strings, ...

Compiler/Interpreter

Executive Summary
Ur/Web is a domain-specific language
with a fancy static type system
with first-class support for Web app architecture
including strong encapsulation
and statically-checked metaprogramming
with security by construction

Strong Encapsulation

Insert
Lookup

Hash Table

Module Interface

Client Code
4

Web 1.0 Encapsulation

Users Database Table

Login Cookie

Module Interface

Client Code
5

Web 2.0 Encapsulation

Subtree of
Dynamic Page
Structure
Module Interface

Client Code
6

Automatic Admin Interface
Id

A

B

C

D
Metaprogram

SQL Table Schema

7

In-Browser Spreadsheet

8

Metaprogramming in a Nutshell
Compile-time Programming Language

(AKA, type system)
(not Turing complete, but includes basic lambda calculus)

Crucial feature: type-level records!
Included in
Run-time Programming Language
(Turing complete)
Key Property 1:
These types are expressive
enough to guarantee absence of
code injection, abstraction
violation, etc..

Metadata
controlling
code
generation
options

Metaprogram

Sub-Web

Key Property 2:
Effective static checking that the
metaprogram really has the
claimed type

Metadata's Type-level program
type

Sub-Web's
type

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An Example Application
Id

A

B

C

D
Metaprogram

SQL Table Schema
table t1 : {Id : int, A : int, B : string,
C : float, D : bool}
PRIMARY KEY Id
open Crud.Make(struct
val tab = t1
val title = "Crud1"
val cols = {A
B
C
D
end)

=
=
=
=

Crud.int "A",
Crud.string "B",
Crud.float "C",
Crud.bool "D"}

10

Nice and Speedy

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Low Memory Footprint

.....
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Ur/Web Available At:
http://www.impredicative.com/ur/

Including online demos with syntax-highlighted source code

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