Intermediate languages are used in compiler construction to simplify retargeting compilers to multiple machine architectures. In the implementation of \emph{domain-specific languages} (DSLs), compilers typically generate high-level source code, rather than low-level machine instructions. DSL compilers target a software platform, i.e. a programming language with a set of libraries, deployable on one or more operating systems. DSLs enable targeting \emph{multiple} software platforms if its abstractions are platform independent. While transformations from DSL to each targeted platform are often conceptually very similar, there is little reuse between transformations due to syntactic and API differences of the target platforms, making supporting multiple platforms expensive. In this paper, we discuss the design and implementation of PIL, a Platform Independent Language, an intermediate language providing a layer of abstraction between DSL and target platform code, abstracting from syntactic and API differences between platforms, thereby removing the need for platform-specific transformations. We discuss the use of PIL in an implemementation of WebDSL, a DSL for building web applications.

1. PIL: A Platform
Independent Language
for Retargetable DSLs
Zef Hemel, Eelco Visser
Delft University of Techology, The Netherlands

2. domain-specific languages
Stratego/XT

3. domain-specific languages
Stratego/XT

4. compiler

5. “traditional” compilers

6. “traditional” compilers
✘ low-level
✘ complex to generate
✘ machine-specific

7. DSL compilers

8. DSL compilers
✔ high-level
✔ easier to generate
✔ machine-independent

9. DSL generators
✔ high-level
✔ easier to generate
✔ machine-independent

10. DSL generators
✔ high-level
✔ easier to generate
✔ machine-independent

11. DSL generators
✔ high-level
✔ easier to generate
✔ machine-independent

12. DSL generators
✔ high-level
✔ easier to generate
✔ machine-independent

13. software platforms

16. data model user interface
logic access control
workflow data validation

17. define page conference(c : Conference) {
header { “Accepted papers” }
list {
for(p : Paper in c.papers where p.accepted) {
listitem { output(p) }
}
}
}

20. http://tweetview.net/ev/sle09

21. WebDSL compiler

22. WebDSL compiler

23. java

24. java python

25. ANTLR

26. ANTLR
perl 5 python
objective-c action script
c c#
javascript c# 3
java delphi

28. perl 5 objective-c python action script c
javascript java c# c# 3 delphi

29. ANTLR back-end support
as of 4th of september 2009
language status
Java in sync with ANTLR3
JavaScript in sync with ANTLR3
in sync with ANTLR3 (except no
C# 3 support for -debug and -profile)
currently catching up with
Objective-C release version
last working version is v3.0ea7.
Ruby Catching up with v3.0b3

31. DSL compiler architecture

32. DSL
(text)
parse
DSL
front-end
(model)
typecheck
DSL
(checked)
desugar
DSL
(core)
generate code
Platform
back-end
pretty-print
Code

33. DSL
(text)
parse
DSL
front-end
(model)
typecheck
DSL
(checked)
desugar
DSL
(core)
generate code
Platform
back-end
pretty-print
Code

34. retargeting a DSL compiler

35. DSL
(core)
Platform

36. DSL
(core)
Platform 1 Platform 2

37. DSL
(core)
large semantic gap
Platform 1 Platform 2
front-end 25,000
back-ends 10,000 each

38. Java
page-elem-to-java:
|[ list { elem* } ]| ->
|[ out.print(“<ul>”);
stat_elem*
out.print(“</ul>”); ]|
with stat_elem* := <map(elem-to-java)> elem*

39. Java
page-elem-to-java:
|[ list { elem* } ]| ->
|[ out.print(“<ul>”);
stat_elem*
out.print(“</ul>”); ]|
with stat_elem* := <map(elem-to-java)> elem*

41. servlets

42. servlets
flexibility control performance

43. developer platform != code generator platform

44. developer platforms
OOP
Java C# Python Ruby

45. developer platforms
dynamic typing
lambda functions
ORM framework
event support
flexible syntax
inner classes
anonymous classes
iterators
annotations
OOP
Java C# Python Ruby

46. fancy platform features

47. Java
page-elem-to-java:
|[ list { elem* } ]| ->
|[ out.print(“<ul>”);
stat_elem*
out.print(“</ul>”); ]|
with stat_elem* := <map(elem-to-java)> elem*

48. Python
page-elem-to-python:
|[ list { elem* } ]| ->
|[ out.print(“<ul>”)
stat_elem*
out.print(“</ul>”) ]|
with stat_elem* := <map(elem-to-python)> elem*

49. language translation
DSL
(core)
Platform 1 Platform 2

50. language translation
DSL
(core)
Platform 1 Platform 2

51. Microsoft™ Java™ language conversion assistant
for Visual Studio™

52. Microsoft™ Java™ language conversion assistant
for Visual Studio™
incomplete

53. DSL
(core)
C
JRuby

54. DSL
(core)
C
JRuby

55. compatibility

56. compatibility
performance

57. compatibility
performance
maintenance

58. compatibility
performance
maintenance
just for JVM and .NET

59. code generation
OOP
Java C# Python Ruby
just subset of platforms

60. DSL
(core)
Platform 1 Platform 2 Platform n

62. DSL
(core)
IL
Platform 1 Platform 2 Platform n

63. UNCOL
Universal Computer Oriented Language
Melvin E. Conway, 1958

64. C--
O-code
UNCOL
Universal Computer Oriented Language
P-code
Melvin E. Conway, 1958
gcc’s RTL

65. traditional ILs High-level IL
machine
software platform
architecture
low-level high-level
familiar
abstract syntax
concrete syntax

66. abstract from platform
familiar concrete syntax
cheap to port
easy DSL compiler integration
platform interaction

67. PIL
Platform Independent Language

68. DSL
(core)
PIL
Java Python ...

69. abstract from platform
familiar concrete syntax
cheap to port
easy DSL compiler integration
platform interaction

70. ✔ abstract from platform
familiar concrete syntax
cheap to port
easy DSL compiler integration
platform interaction

71. simplified Java
✘ visibility modifiers
✘ abstract classes
✘ interfaces
✘ inner/anonymous classes
✘ import com.somelib.*;
✘ checked exceptions
✘ primitive types
✘ one class per file
✘ static class members

72. .

73. a.b

74. a.b
a = package, class, field,
variable?

75. PIL
a::b
a.b
this.a.b

76. Java-like syntax
class page::Home extends webdsl::web::Page {
String title = null;
String name = null;
void init(pil::web::Request request) {
this.name = request.getParameter(“name”);
}
void render(pil::util::PrintWriter out) {
out.print(“<html><head><title>” + title +
“</title></head><body>”);
out.print(“<h1>Welcome “ + name + “</h1>”);
out.print(“</body></html>”);
}
}

77. Object
Bool
Int
Float
Char
Byte
String
MutableString
Array<T>
List<T>
Set<T>
Map<K,V>
Exception

78. 1,200 lines Stratego/platform

79. ✔ abstract from platform
familiar concrete syntax
cheap to port
easy DSL compiler integration
platform interaction

80. ✔ abstract from platform
✔ familiar concrete syntax
cheap to port
easy DSL compiler integration
platform interaction

81. ✔ abstract from platform
✔ familiar concrete syntax
✔ cheap to port
easy DSL compiler integration
platform interaction

82. Stratego/XT library
imports
libpil-front
libpil-java-backend
libpil-python-backend

83. stand-alone compiler
$ pilc -i application.pil --java
$ pilc -i application.pil --python

84. ✔ abstract from platform
✔ familiar concrete syntax
✔ cheap to port
easy DSL compiler integration
platform interaction

85. ✔ abstract from platform
✔ familiar concrete syntax
✔ cheap to port
✔ easy DSL compiler integration
platform interaction

86. platform interaction

87. external class pil::web::Request {
String getParameter(String name);
...
}
platform-specific implementations

88. Java implementation
package pil.web;
import javax.servlet.http.*;
public class Request {
private HttpServletResponse r;
public Response(HttpServletResponse r) {
this.r = r;
}
public String getParameter(String name) {
return r.getParameter(name);
}
...
}

89. Python implementation
import cgi
class Request(object):
def __init__(self):
self.fs = cgi.FieldStorage()
def getParameter(self, name):
return self.fs[name]
...

91. NHibernate

92. NHibernate

93. NHibernate

94. external class db::DatabaseSession {
void beginTransaction();
void rollback();
void commit();
List<Object> getAll(Class cls);
void persist(Object o);
void delete(Object o);
}

95. WebDSL
ORM
PIL
Interface
Hibernate Java Python SQLAlchemy

96. query API/language?

97. query API/language?
persistence behavior?

98. WebDSL
PIL/ORM PIL
Java Python

99. ✔ abstract from platform
✔ familiar concrete syntax
✔ cheap to port
✔ easy DSL compiler integration
✔ code-generation specific abstractions
platform interaction

100. ✔ abstract from platform
✔ familiar concrete syntax
✔ cheap to port
✔ easy DSL compiler integration
✔ code-generation specific abstractions
✔ platform interaction

101. conclusion

102. retargetability problem:
software platforms

103. retargetability problem:
software platforms
maintaining platform back-ends is expensive

104. retargetability problem:
software platforms
maintaining platform back-ends is expensive
PIL

105. retargetability problem:
software platforms
maintaining platform back-ends is expensive
PIL
PIL/G

106. Java & Python back-ends

107. Java & Python back-ends
Stratego/XT

108. Java & Python back-ends
Stratego/XT

109. pil-lang.org
webdsl.org
twitter.com/zef