1. Generating Pieces of Web Applications with Type-Level Programming
Adam Chlipala
DTP 2010

2. Verification Types Programmer Effort Strength of Guarantee Ur/Web,
Dependent Types
Ur/Web,
a domain-specific language for metaprogramming Web applications
Type-Level Computation
Web Applications
Programmer Effort
Strength of Guarantee

3. Object-Relational Mapping
SQL Table Schema
Metaprogram
Object-Oriented Interface
Add row
Modify row
Search
...

4. Automatic Admin Interface
SQL Table Schema Id A B C D
Metaprogram

5. In-Browser Spreadsheet

6. Ad-Hoc Code Generation?
Edit some source files to customize....
Now change the database schema....
app
models
post.rb
views
posts
index.html.erb
show.html.erb
config
routes.rb
.... ?
SQL Table Schema Id A B C D
Metaprogram
script/generate scaffold T Id:int A:string B:float C:string D:int

7. Library uses reflection to behave differently based on object metadata
Run-Time Reflection?
Library uses reflection to behave differently based on object metadata
How do we know that this library is correct?
How can we be sure that the library never produces code vulnerable to code injection attacks or other failures of abstraction?
SQL Table Schema Id A B C D

8. A Solution Inspired by Dependent Typing
Metaprogram
(Static type guarantees correct operation!)
Uses type-level computation
Code-level iteration over types
SQL Table Schema Id A B C D
Guaranteed free of injection vulnerabilities, for any metaprogram input

9. Core Language Features
 ::= Type |   
| Name | {}
 ::=    |  | ∀ :: .  |   |  :: . 
| #n | [] | [ = ] |  ++  | $ | map | [ ~ ]  
e ::= x | e e | x : . e | e [] |  :: . e
| {} | { = e} | e. | e –  | e ++ e | [ ~ ]  e | e !
Convert
“type-level record” to
“record type”
Apply a function to every field of a record
Guarded type: require that two type-level records share no field names
Literal field name
Empty record
Singleton record
Record concatenation
For type-level record field names
For type-level records
Empty record
Singleton record
Field projection
Record concatenation
Field removal
Guarded expression abstraction: require that two type-level records share no field names
Guard elimination

10. The Type-Checker is Clever
Algebraic laws are applied automatically:
1 ++ 2 = 2 ++ 1
1 ++ (2 ++ 3) = (1 ++ 2) ++ 3
map (. )  = 
map f (1 ++ 2) = map f 1 ++ map f 2
map f (map g ) = map (f ○ g) 

11. Ur/Web Available At: http://www.impredicative.com/ur/
Including online demos with syntax-highlighted source code