http://d.puremagic.com/issues/show_bug.cgi?id=5660
Summary: yield syntax sugar Product: D Version: D2 Platform: All OS/Version: All Status: NEW Severity: enhancement Priority: P2 Component: DMD AssignedTo: nob...@puremagic.com ReportedBy: bearophile_h...@eml.cc --- Comment #0 from bearophile_h...@eml.cc 2011-02-27 17:55:27 PST --- Ranges are flexible and useful, but my practice with Python has shown me that many times you just want something simple that yields items lazily. To do this in D there is opApply() (that currently doesn't play well with most Phobos. Only std.array.array() and few other things are able to use them), but its syntax is awful, and even after years of usage I can't remember it and I need to look for an example of opApply() usage to copy & modify. opApply() requires a significant amount of boilerplate code that makes code quite longer, obfuscates the purpose of the code, and is bug-prone. This is a simple example (D2 code, works with dmd 2.052): // Program #1 import std.stdio; /// Sequence of moves to solve Towers of Hanoi OEIS A001511 immutable final class hanoiTower { static opCall() { return new typeof(this)(); } int opApply(int delegate(ref int) dg) { int result; int y = 1; result = dg(y); if (result) return result; foreach (x; hanoiTower()) { y = x + 1; result = dg(y); if (result) return result; y = 1; result = dg(y); if (result) return result; } return result; } int opApply(int delegate(ref int, ref int) dg) { int result; int i, y = 1; result = dg(i, y); i++; if (result) return result; foreach (x; hanoiTower()) { y = x + 1; result = dg(i, y); i++; if (result) return result; y = 1; result = dg(i, y); i++; if (result) return result; } return result; } } class genChar { char stop; this(char stop_) { stop = stop_; } static opCall(char stop_) { return new typeof(this)(stop_); } int opApply(int delegate(ref char) dg) { int result; for (char c = 'a'; c < stop; c++) { result = dg(c); if (result) return result; } return result; } int opApply(int delegate(ref int, ref char) dg) { int result, i; for (char c = 'a'; c < stop; c++) { result = dg(i, c); i++; if (result) return result; } return result; } } void main() { foreach (i, move; hanoiTower()) { writeln(i, " ", move); if (i >= 20) break; } writeln(); foreach (i, c; genChar('h')) writeln(i, " ", c); } So I suggest a syntax like this, that's just syntax sugar for the precedent code: // Program #2 import std.stdio; yield(int) hanoiTower() { yield(1); foreach (x; hanoiTower()) { yield(x + 1); yield(1); } } yield(auto) genChar(char stop) { for (char c = 'a'; c < stop; c++) yield(c); } void main() { foreach (i, move; hanoiTower()) { writeln(i, " ", move); if (i >= 20) break; } writeln(); foreach (i, c; genChar('h')) writeln(i, " ", c); } Different good modern used languages like Python, C# and Scala have a clean yield syntax, they show the way (if necessary I may translate this program #2 to those other three languages.) The normal syntax of opApply() is of course kept in D, this is just an additive change, that acts at syntax level (so just a lowering is needed to implement it). I suggest the generators with yield to be classes that define a "static opCall()" because their semantics is more flexible than structs. This causes a heap allocation, but situations where performance is so important are less common, and in such situation the programmer may just write the normal struct code with opApply(), or use a struct with more inlinable range protocol methods, or use something more lower level. The syntax uses yield(...) as return type to denote such iterable classes, and yield(auto) too supported, and it acts like auto return type for functions (all yields in a generator must yield the same type). Some disadvantages: - "yield" may need to become a kind of keyword. - Currently opApply() doesn't work well with std.algorithm and Phobos in general. This is true for the normal current usage of opApply(), but this proposal makes their usage more attractive and probably more widespread. -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------