1. Metaprogramming from University to Industry Zoltán Porkoláb gsd@elte.hu http://gsd.web.elte.hu Dept. of Programming Languages and Compilers, Faculty of Informatics Eötvös Loránd University, Budapest

2. 2OTS 2007 Maribor Agenda I always knew C++ templates were the work of the Devil, and now I'm sure... - Cliff Click cited by Todd Veldhuisen Parameterized types C++ Template Metaprograms Power of generative metaprograms Sample usages Open questions

3. 3OTS 2007 Maribor Parameterized types (Generics) Widely used in modern programming languages: ADA generics Eiffel generics C++ templates Java generics: Pizza, GJ, Java 1.5 C# generics Clean, Generic Haskell, other functional langs.

4. 4OTS 2007 Maribor Why generics? int max( int a, int b) { if ( a > b ) return a; else return b; } double max( double a, double b) { if ( a > b ) return a; else return b; if ( a > b ) return a; else return b;}//… class date { /* … */ }; date d = max ( d1, d2); Conventional techniques are working only for complete types:

5. 5OTS 2007 Maribor Preprocessor Macro #define MAX(a,b) a > b ? a : b Works, because a macro is typeless. Processed not by the compiler, therefore there are a number of "secondary effects": MAX( x, y)*2 -> x > y ? x : y*2 MAX( ++x, y) -> ++x > y ? ++x : y

6. 6OTS 2007 Maribor Macro – the limits void swap( int& x, int& y) void swap( int& x, int& y) { int temp = x; x = y; y = temp; int temp = x; x = y; y = temp; } Does not work with macro: a macro is typeless. We need a facility to use type parameters.

7. 7OTS 2007 MariborTemplates template template void swap( T& x, T& y) void swap( T& x, T& y){ T temp = x; x = y; y = temp; } Ada generics: "a form of context-sensitive macro" C++ templates: "a clever kind of macro that obeys the scope, naming, and type rules of C++" Does not require different types used as arguments to be explicitly related. In particular, the argument types used as a template need not be from a single inheritance hierarchy.

8. 8OTS 2007 Maribor C++ Function Templates Template is not a single function A schema to instantiate functions on request Parameter deduction Parameter deduction Template instantiation Template instantiation Compilaton time template T max( T a, T b) { if ( a > b ) return a; if ( a > b ) return a; else return b; else return b;} int i = 3, j = 6, k; double x = 3.14, y = 4.15, z; k = max(i,j); z = max(x,y);

9. 9OTS 2007 Maribor C++ Function Templates 2. Strong type system rules are applied int i = 3; double y = 3.14, z; z = max(i,y); // error template template T max( T a, S b) { if ( a > b ) return a; if ( a > b ) return a; else return b; else return b;} z == 3.0; No deduction on return type No runtime information could be used

10. 10OTS 2007 Maribor Explicit specialization int i = 3; double y = 3.14, z; template template R max( T a, S b) { if ( a > b ) return a; if ( a > b ) return a; else return b; else return b;} z = max (i,y);

11. 11OTS 2007 Maribor User specialization const char *s1 = “Hello”, *s2 = “world”; const char *s = max(s1,s2); template <> const char *max(const char *a, const char *b) { if ( strcmp(a,b) < 0 ) return a; if ( strcmp(a,b) < 0 ) return a; else return b; else return b;}

12. 12OTS 2007 Maribor Template overloading You can provide overloadd template definitions The compiler selects the most specific template cout << max (4,5); cout (3.14,’6’); cout << max (“this”, “greater”); R max(S,T) char *max(char *,char *) T max(T, T)

13. 13OTS 2007 Maribor C++ Class Templates 1. Similar way template classes could be defined template class matrix {public: matrix( int i, int j ); matrix( int i, int j ); matrix& operator=( const matrix &other); matrix& operator=( const matrix &other); T at(int i, int j); T at(int i, int j); //… //…private: vector v; vector v;}; Created always with explicit specialisation matrix m(4,5);

14. 14OTS 2007 Maribor C++ Class Templates 2. User specialization template <> class matrix template <> class matrix {public: matrix( int i, int j ); matrix( int i, int j ); matrix( const matrix &other) matrix( const matrix &other) bool at(int i, int j); bool at(int i, int j);private: aBetterRepresentation v; aBetterRepresentation v;}; Used the same way matrix m(4,5);

15. 15OTS 2007 Maribor C++ Templates The C++ templates were first implemented in the early ’90s Accepted as part of the ANSI/ISO in 1994 Erwin Unruh: 1994 unruh.cpp 30: conversion from enum to D requested

16. 16OTS 2007 Maribor Power of C++ templates The Compiler “executes” template metaprograms The result is a non-templated program executed in “run-time” executed in “run-time” In 1966 Böhm and Jacopini proved: Turing machine implementation conditional and looping constructions Turing machine implementation conditional and looping constructions The C++ templates are Turing-complete in compilation time in compilation time

17. 17OTS 2007 Maribor The Factorial example The run-time solution int factorial( int n) { if ( n == 1 ) return 1; if ( n == 1 ) return 1; else return n*factorial(n-1); else return n*factorial(n-1);} int main() { cout << factorial(15) << endl; cout << factorial(15) << endl; return 0; return 0;}

18. 18OTS 2007 Maribor The Factorial example The metaprogram solution template struct Factorial { enum { value = N * Factorial ::value }; enum { value = N * Factorial ::value };}; template <> struct Factorial template <> struct Factorial { enum { value = 1 }; enum { value = 1 };}; int main() { const int fact15 = Factorial ::value; const int fact15 = Factorial ::value; std::cout << fact15 << endl; std::cout << fact15 << endl; return 0; return 0;}

19. 19OTS 2007 Maribor Conditional statement template template struct IF { typedef Then RET; typedef Then RET;}; template template struct IF struct IF { typedef Else RET; typedef Else RET;}; template template IF ::RET max(T t, S s) { if (t > s) return t; if (t > s) return t; else return s; else return s;}

20. 20OTS 2007 Maribor (Run-time) Programs vs. Metaprograms Function (runtime) Data VariableConditionLoopAssignmentClass Type and constant Symbolic names Type selection Recursion No assignment

21. 21OTS 2007 Maribor Data in Template Metaprograms Referential transparency: No assignment Still possible to store, modify, and retrieve data Typelist struct NullType {}; typedef Typelist< char, Typelist<signed char, Typelist > > Charlist; Typelist > > Charlist; char signed char NullType unsigned char

22. 22OTS 2007 Maribor Data handling template struct Length; template <> struct Length struct Length { enum { value = 0 }; enum { value = 0 };}; template template struct Length > { enum { value = 1 + Length ::value }; enum { value = 1 + Length ::value };};

23. 23OTS 2007 MariborMotivation int main() { const unsigned int di = 12; const unsigned int di = 12; const unsigned int oi = 014; const unsigned int oi = 014; const unsigned int hi = 0xc; const unsigned int hi = 0xc; const unsigned int bi0 = binary_value("1101"); const unsigned int bi0 = binary_value("1101"); const unsigned int bi1 = binary ::value; const unsigned int bi1 = binary ::value;} template struct binary { // prepend higher bits to lowest bit // prepend higher bits to lowest bit static const int value=binary ::value*2+N%10; static const int value=binary ::value*2+N%10;}; template<> struct binary // specialization { static unsigned const value = 0; static unsigned const value = 0;};

24. 24OTS 2007 MariborMotivation Constant expression – array size, case label, etc… Better code compiled Faster in run-time Syntactically checked – the language semantic is extended t Design timeCompilation timeRun-time No change in type system Change in run-time values Template metaprograms apply, automatic config of the program Decisions on Stategies, policies

25. 25OTS 2007 Maribor Generative Metaprograms metaprogramming: writing programs that represent and manipulate other programs or themselves (ie:reflection). Metaprograms are programs about programs. introspection: the ability of a program to observe its own state the ability of a program to observe its own stateintercession: the ability to modify its own state the ability to modify its own state Open compilers: transformations on AST Hygenic macros (Scheme) Two level languages: AspectJ, Template Haskell

26. 26OTS 2007 Maribor Areas of Template Metaprogramming Expression templates Blitz+, PETE Blitz+, PETE Static interface checking Early catch of syntactical/semantical errors Early catch of syntactical/semantical errors Extending the C++ type system Introspection Code adaption/optimalization Language embedding

27. 27OTS 2007 Maribor Expression templates Array a, b, c, d, e; // Object-oriented way of a = b + c + d + e double* _t1 = new double[N]; for ( int i=0; i<N; ++i) _t1[i] = b[i] + c[i]; double* _t2 = new double[N]; for ( int i=0; i<N; ++i) _t2[i] = _t1[i] + d[i]; double* _t3 = new double[N*M]; for ( int i=0; i<N; ++i) _t3[i] = _t2[i] + e[i]; for ( int i=0; i<N; ++i) a[i] = _t3[i]; delete [] _t3; delete [] _t2; delete [] _t1; // Fortran like solution: for ( int i=0; i<N; ++i) a[i] = b[i] + c[i] + d[i] + e[i];

28. 28OTS 2007 Maribor Language embedding SQL Gil, et.al.: AraRat Gil, et.al.: AraRat XML parsing Jarvi, et.al.: type-safe XML library Jarvi, et.al.: type-safe XML library Regular expressions Boost::Xpressive Boost::Xpressive Compiler embedding boost::spirit boost::spirit

29. 29OTS 2007 Maribor Language embedding SQL example string s = ”select form ”+tName + ” where 1=1” ; if ( cond1 ) s += ” and fName1 = ” + field1; if ( cond2 ) s += ” and fName2 < ” + field2; if ( cond3 ) s += ” and fName3 = ” + field3; Run-time errors Injection attacks Conversion problems

30. 30OTS 2007 Maribor Language embedding SQL example void f() { const string s = ( (tName / (fName1 == field1 && fName2 < field2) ) [ fName1, fName3 ] ).asSQL(); }

31. 31OTS 2007 Maribor Language embedding Boost::Xpressive example std::string hello( "hello world!" ); sregex rex1 = sregex::compile( "(\\w+)(\\w+)!" ); sregex rex2 = (s1= +_w) >> ' ' >> (s2= +_w) >> '!'; smatch what; if( regex_match( hello, what, rex1 ) ) { std::cout << what[0] << '\n'; // whole match std::cout << what[0] << '\n'; // whole match std::cout << what[1] << '\n'; // first capture std::cout << what[1] << '\n'; // first capture std::cout << what[2] << '\n'; // second capture std::cout << what[2] << '\n'; // second capture}

32. 32OTS 2007 Maribor Embedding alternatives If you are (accidently) not a C++ programmer.NET platform, VB, C#: LINQ project LINQ project Java platform: Eric Van Wijk et.al.: Attribute Grammar-based Language Extensions for Java Eric Van Wijk et.al.: Attribute Grammar-based Language Extensions for JavaStratego TU Delft TU Delft Charles Simonyi: Intentional programming

33. 33OTS 2007 Maribor Intentional programming Charles Simonyi XEROX Palo Alto Bravo XEROX Palo Alto Bravo MicrosoftWord, Excel MicrosoftWord, Excel Intentional softwarehttp://www.intentsoft.com Intentional softwarehttp://www.intentsoft.com

34. 34OTS 2007 Maribor Open questions The real expressive power Standard tools but: Loki from Andrei Alexandrescu but: Loki from Andrei Alexandrescu boost::mpl boost::mpl Garantees (number of instantiations, etc…) How to design? How to debug?

35. Metaprogramming from University to Industry Zoltán Porkoláb gsd@elte.hu http://gsd.web.elte.hu Dept. of Programming Languages and Compilers, Faculty of Informatics Eötvös Loránd University, Budapest Thank you! Questions?