Generic Programming Galore Using D

Transcription

1 Generic Programming Galore Using D Andrei Alexandrescu, PhD Research Scientist Facebook c 2011 Andrei Alexandrescu 1 / 33

2 37% off c 2011 Andrei Alexandrescu 2 / 33

3 Generic Programming c 2011 Andrei Alexandrescu 3 / 33

4 What is Generic Programming? c 2011 Andrei Alexandrescu 4 / 33

5 What is Generic Programming? Find most general algorithm representation Narrowest requirements Widest guarantees Big-O() encapsulation should be a crime No need to regress to hand-written code c 2011 Andrei Alexandrescu 4 / 33

6 What is Generic Programming? Find most general algorithm representation Narrowest requirements Widest guarantees Big-O() encapsulation should be a crime No need to regress to hand-written code Define types to implement said requirements c 2011 Andrei Alexandrescu 4 / 33

7 What is Generic Programming? Find most general algorithm representation Narrowest requirements Widest guarantees Big-O() encapsulation should be a crime No need to regress to hand-written code Define types to implement said requirements Leverage the algorithm for ultimate reuse c 2011 Andrei Alexandrescu 4 / 33

8 What is Generic Programming? Find most general algorithm representation Narrowest requirements Widest guarantees Big-O() encapsulation should be a crime No need to regress to hand-written code Define types to implement said requirements Leverage the algorithm for ultimate reuse... Profit! c 2011 Andrei Alexandrescu 4 / 33

9 What is Generic Programming? Find most general algorithm representation Narrowest requirements Widest guarantees Big-O() encapsulation should be a crime No need to regress to hand-written code Define types to implement said requirements Leverage the algorithm for ultimate reuse... Profit! Arguably one of the noblest endeavors of our métier c 2011 Andrei Alexandrescu 4 / 33

10 Generic Programming Write once, instantiate anywhere Specialized implementations welcome Prefers static type information and static expansion (macro style) Fosters strong mathematical underpinnings Minimizing assumptions common theme in math Tenuous relationship with binary interfaces Starts with algorithms, not interfaces or objects Premature encapsulation is the root of some derangement c 2011 Andrei Alexandrescu 5 / 33

11 Warmup: the dream min Should work at efficiency comparable to hand-written code Take variadic arguments: min(a, b,...) Avoid nonsensical calls min() and min(a) Work for all ordered types and conversions Decline incompatible types, without prejudice c 2011 Andrei Alexandrescu 6 / 33

12 First prototype auto min(l, R)(L lhs, R rhs) { return rhs < lhs? rhs : lhs; } auto a = min(x, y); This function is as efficient as any specialized, handwritten version (true genericity) Deduction for argument types and result type c 2011 Andrei Alexandrescu 7 / 33

13 Variadic arguments auto min(t...)(t x) { static if (x.length > 2) return min(min(x[0], x[1]), x[2.. $]); else return x[0] > x[1]? x[1] : x[0]; }... auto m = min(a + b, 100, c); x is not an array This is not classic recursion c 2011 Andrei Alexandrescu 8 / 33

14 Reject nonsensical calls auto min(t...)(t x) if (x.length > 1) {... } Rejects calls with 0 or 1 arguments Allows other overloads to take over, e.g. min element over a collection More work needed Only accept types with a valid intersection Only accept types comparable with < c 2011 Andrei Alexandrescu 9 / 33

15 Common type Task: Given a list of types, find the common type of all template CommonType(T...) { static if (T.length == 1) alias T[0] CommonType; else static if (is(typeof(1? T[0].init : T[1].init) U)) alias CommonType!(U, T[2.. $]) CommonType; else alias void CommonType; } // Usage static assert(is(commontype!(int, short, long) == long)); c 2011 Andrei Alexandrescu 10 / 33

16 Using CommonType auto min(t...)(t x) if (x.length > 1 && is(typeof(commontype!t.init < CommonType!T.init) == bool)) { static if (x.length > 2) return min(min(x[0], x[1]), x[2.. $]); else return x[0] > x[1]? x[1] : x[0]; } c 2011 Andrei Alexandrescu 11 / 33

17 How about min over many elements? auto min(r)(r range) if (isinputrange!r && is(typeof(range.front < range.front) == bool)) { auto result = range.front; range.popfront(); foreach (e; range) { if (e < result) result = e; } return result; } auto m = [ 1, 5, 2, 0, 7, 9 ].min(); Works over anything that can be iterated c 2011 Andrei Alexandrescu 12 / 33

18 How about argmin now? auto argmin(alias fun, R)(R r) if (isinputrange!r && is(typeof(fun(r.front) < fun(r.front)) == bool)) { auto result = r.front; auto cache = fun(result); r.popfront(); foreach (e; r) { auto cand = fun(e); if (cand > cache) continue; result = e; cache = cand; } return result; } c 2011 Andrei Alexandrescu 13 / 33

19 argmin auto s = ["abc", "a", "xz"]; auto m = s.argmin!((x) => x.length); Works on anything iterable and any predicate Predicate is passed by alias No loss of efficiency c 2011 Andrei Alexandrescu 14 / 33

20 The Generative Connection c 2011 Andrei Alexandrescu 15 / 33

21 Generative programming In brief: code that generates code Generic programming often requires algorithm specialization Specification often present in a DSL c 2011 Andrei Alexandrescu 16 / 33

22 Embedded DSLs Force into host language s syntax? c 2011 Andrei Alexandrescu 17 / 33

23 Embedded DSLs Formatted printing? c 2011 Andrei Alexandrescu 18 / 33

24 Embedded DSLs Formatted printing? Regular expressions? c 2011 Andrei Alexandrescu 18 / 33

25 Embedded DSLs Formatted printing? Regular expressions? EBNF? c 2011 Andrei Alexandrescu 18 / 33

26 Embedded DSLs Formatted printing? Regular expressions? EBNF? PEG? c 2011 Andrei Alexandrescu 18 / 33

27 Embedded DSLs Formatted printing? Regular expressions? EBNF? PEG? SQL? c 2011 Andrei Alexandrescu 18 / 33

28 Embedded DSLs Formatted printing? Regular expressions? EBNF? PEG? SQL?... Pasta for everyone! c 2011 Andrei Alexandrescu 18 / 33

29 Embedded DSLs Here: use with native grammar Process during compilation Generate D code accordingly c 2011 Andrei Alexandrescu 19 / 33

30 Compile-Time Evaluation A large subset of D available for compile-time evaluation ulong factorial(uint n) { ulong result = 1; foreach (i; 2.. n) result *= i; return result; }... auto f1 = factorial(10); // run-time static f2 = factorial(10); // compile-time c 2011 Andrei Alexandrescu 20 / 33

31 Code injection with mixin mixin("writeln(\"hello, world\");"); mixin(generatesomecode()); Not as glamorous as AST manipulation but darn effective Easy to understand and debug Now we have compile-time evaluation ANDmixin... c 2011 Andrei Alexandrescu 21 / 33

32 Wait a minute! c 2011 Andrei Alexandrescu 22 / 33

33 Example: bitfields in library struct A { int a; mixin(bitfields!( uint, "x", 2, int, "y", 3, uint, "z", 2, bool, "flag", 1)); } A obj; obj.x = 2; obj.z = obj.x; c 2011 Andrei Alexandrescu 23 / 33

34 Parser import pegged.grammar; // by Philippe Sigaud mixin(grammar(" Expr < Factor AddExpr* AddExpr < ( + / - ) Factor Factor < Primary MulExpr* MulExpr < Primary < Parens < ( Expr ) Number <~ [0-9]+ Variable <- Identifier ")); ( * / / ) Primary Parens / Number / Variable / - Primary c 2011 Andrei Alexandrescu 24 / 33

35 Usage // Parsing at compile-time: static parsetree1 = Expr.parse( " (3*x-5)*6"); pragma(msg, parsetree1.capture); // Parsing at run-time: auto parsetree2 = Expr.parse(readln()); writeln(parsetree2.capture); c 2011 Andrei Alexandrescu 25 / 33

36 Scaling up 1000 lines of D grammar 3000 lines D parser c 2011 Andrei Alexandrescu 26 / 33

37 Highly integrated lex+yacc c 2011 Andrei Alexandrescu 27 / 33

38 What about regexen? c 2011 Andrei Alexandrescu 28 / 33

39 Compile-time regular expressions GSoC project by Dmitry Olshansky: FReD Fully UTF capable, no special casing for ASCII Two modes sharing the same backend: auto r1 = regex("^.*/([^/]+)/?$"); static r2 = ctregex!("^.*/([^/]+)/?$"); Run-time version uses intrinsics in a few places Static version generates specialized automaton, then compiles it c 2011 Andrei Alexandrescu 29 / 33

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41 Summary c 2011 Andrei Alexandrescu 31 / 33

42 Summary Generic/generative programming long unattained promise D s angle Powerful base language Type manipulation Compile-time evaluation Code generation c 2011 Andrei Alexandrescu 32 / 33

43 Grill the Speaker! More about ranges? Thought the talk was boring Intriguing! He lost me atmixin Awesome Went over my head Meh Iwonder how I can implement binary search What s for dinner? Accent is bothersome Real programmers use C Must... control... fist... of... death... c 2011 Andrei Alexandrescu 33 / 33