The history of programming languages shows a progressive development from low-level programming languages close to the machine, to high-level languages close to the problems being solved with software. Domain-specific languages take this a step further than general purpose programming languages by making assumptions about the class of applications for which the language is intended. Complete applications typically require programs in multiple (technical) domains, which can be catered for by separate domain-specific languages. While such separation of concerns is beneficial for domain expressivity, it often leads to loose coupling and lack of static verification. Hence, the design of individual DSLs needs to be complemented with their linguistic integration. In this talk, I illustrate these ideas with the design of WebDSL, a domain-specific language for data centric web applications. WebDSL linguistically integrates the definition of data models, user interfaces, actions, access control rules, data validation rules, styling rules, and workflow definitions. While maintaining separation between these concerns through specialized sub-languages, linguistic integration ensures static consistency checking and correct code generation. The language allows developers to concentrate on the essential design of web applications, abstracting from accidental complexity, such as the details of data persistence. The combination of high-level and low-level constructs ensures high expressivity, while supporting customization to application requirements.

1. Composing
Domain-Specific Languages
Eelco Visser
Delft University of Technology
http://eelcovisser.org
12/11/2009
Technical University Eindhoven

2. Software Engineering
implement
Problem Solution
Domain Domain
validate

3. Programming Languages

4. Abstraction Scala
Programming Languages
machine

5. Abstraction Scala
garbage collection
objects
Programming Languages
expressions structured
control-flow
procedures
machine

6. Problem
GPL Machine
Domain

7. The Next Level of Abstraction?
Formal Methods

8. Spec
Problem
GPL Machine
Domain

9. Spec
Problem
GPL Machine
Domain

10. The Next Level of Abstraction?
Domain-Specific Languages

11. The Next Level of Abstraction?
SQL
Domain-Specific Languages
CFG (BNF)
RegExp

12. Problem
DSL GPL Machine
Domain
Domain-Specific Languages: Specialization

13. Software System
Composition of Specialized Sub-domains

14. Software System
Separation of Concerns
Composition of Specialized Sub-domains

15. Loose Coupling
Lack of consistency checks

16. Thesis
Separation of Concerns
requires
Linguistic Integration

17. Case
Study
World Wide Web

19. where does that link point?
is that URL unique?
who may edit this entry?
where is data stored?

20. request
Client Server
response
Architecture

21. quest Convert Request
Access Control Validate Forms
Parameters
Render Page Or
Handle Actions
Redirect
ponse
Update
Database

22. Access Data
Navigation
quest Convert Request
Control Validation
Access Control Validate Forms
Parameters
Forms Data Model Actions
Render Page Or
Handle Actions
Redirect
ponse
Object
Page
Data Binding Relational
Templates
Mapping
Update
Database

23. Access Data
Navigation
Control Validation
Forms Data Model Actions
Object
Page
Data Binding Relational
Templates
Mapping
(Some) Concerns in Web Applications

24. Access Data
Navigation
Control Validation
Data
Forms Actions
Model
Object
Page Data
Relational
Templates Binding
Mapping
data model entity class classes
orm JPA active record
actions Java/Seam Ruby
data validation
navigation servlets controllers
page templates JavaServer Faces
forms JSF tags embedded html
data binding JSF + Seam
access control Acegi

25. Access Data
Navigation
Control Validation
Data
Forms Actions
Model
Object
Page Data
Relational
Templates Binding
Mapping
<s:link view="/entry.xhtml"
value="#{entry.title}"
propagation="none">
<f:param name="entry" value="#{entry.id}"/>
</s:link>
@Name("entry")
public class EntryBean implements EntryBeanInterface {
@RequestParameter("entry")
private Long entryId;
}

26. Access Data
Navigation
Control Validation
Data
Forms Actions
Model
Object
Page Data
Relational
Templates Binding
Mapping
map.root :controller => "homme" # should be "home"
http://zef.me/2308/when-rails-fails

27. Access Data
Navigation
Control Validation
Data
Forms Actions
Model
Object
Page Data
Relational
Templates Binding
Mapping
data model entity class classes
orm JPA active record
actions Java/Seam
boilerplate code Ruby
data validation
navigation loose coupling
servlets controllers
page templates JavaServer Faces
forms no/little consistency checking
JSF tags embedded html
data binding JSF + Seam
access control Acegi

28. Access Data
Navigation
Control Validation
Data
Forms Actions
Model
Object
Page Data
Relational
Templates Binding
Mapping
- separate languages for separate concerns
- linguistically integrated
- compile-time consistency checking
- 100% code generation (no scaffolding)

29. Access Data
Navigation
Control Validation
Forms
Data
Model
Actions entity Blog {
name :: String (id)
Object
Page
Templates
Data
Binding
Relational entries -> Set<Entry>
Mapping
authors -> Set<User>
}
entity Entry {
url :: String (id)
title :: String (name)
text :: WikiText
blog -> Blog (inverse=Blog.entries)
author -> User (inverse=User.entries)
created :: DateTime
}
entity User {
username :: String
password :: Secret
blogs -> Set<Blog> (inverse=Blog.authors)
entries -> Set<Entry> (inverse=Entry.author)
}

30. Access Data
Navigation
Control Validation
Data
Forms Actions
Model
Object
Page Data
Relational
Templates Binding
Mapping
define page entry(e : Entry) {
header{output(e.title)}
output(e.text)
“Posted by “ output(e.author)
“ on “ output(e.created)
navigate(editentry(e)){“Edit”}
}

31. Access Data
Navigation
Control Validation
Data
Forms Actions
Model
define page editentry(e : Entry) {
Object
Page Data
Relational
Templates Binding
Mapping
form{
input(e.url)
input(e.title)
input(e.text)
input(e.author)
input(e.created)
action(“Save”, action{
return entry(e);
})
}
}

32. Access Data
Navigation
Control Validation
Data
Forms Actions
Model
Object
Page Data
Relational
Templates Binding
Mapping
entity Entry {
url validate(isUniqueEntry(this),
“Entry with that URL already exists”)
}

33. Access Data
Navigation
Control Validation
Forms
Data
Actions
access control rules
Model
Page Data
Object
Relational
principal is User
Templates Binding
Mapping with credentials username, password
rule page entry(e : Entry) {
!e.private
|| principal in e.blog.authors
}
rule page newentry(b : Blog) {
principal in e.blog.authors
}
rule page editentry(e : Entry) {
e.author == principal
}

34. Access Data
Navigation
Control Validation
Data
Forms Actions
Model
Object
Page Data
Relational
Templates Binding
Mapping
- separate languages for separate concerns
- linguistically integrated
- compile-time consistency checking
- 100% code generation (no scaffolding)
- order of magnitude decrease in code size

35. http://researchr.org

36. Access Data
Navigation
Control Validation
Data
Forms Actions
Model
Object Stratego/XT
Page Data
Relational
Templates Binding
Mapping
Domain-Specific Language Engineering

37. Access Data
Navigation
Control Validation
Data
Forms Actions
Model
Object Stratego/XT
Page Data
Relational
Templates Binding
Mapping
WebDSL Java

38. Access Data
Navigation
Control Validation
Data
Forms Actions
Model
Object Stratego/XT
Page Data
Relational
Templates Binding
Mapping
Core
WebDSL Java
WebDSL

39. Access Data
Navigation
Control Validation
Data
Forms Actions
Model
Object Stratego/XT
Page Data
Relational
Templates Binding
Mapping
Python
Core
WebDSL Java
WebDSL
C#

40. Access Data
Navigation
Control Validation
Data
Forms Actions
Model
Object Stratego/XT
Page Data
Relational
Templates Binding
Mapping
Python
Core
WebDSL PIL Java
WebDSL
C#

41. Access Data
Navigation
Control Validation
Data
Forms Actions
Model
Object Stratego/XT
Page Data
Relational
Templates Binding
Mapping
Component-Based Language Engineering
DM PIL
Python
UI PIL
AC PIL
+ WebDSL
Core
WebDSL
PIL Java
C#
DV PIL

42. Access Data
Navigation
Control Validation
Data
Forms Actions
Model
Page Data
Object
Relational - separate languages for separate concerns
Templates Binding
Mapping - linguistically integrated
- compile-time consistency checking
- 100% code generation (no scaffolding)
- order of magnitude decrease in code size
http://webdsl.org
http://strategoxt.org http://researchr.org