digitalmars.D.bugs - [Issue 15300] New: Better support for operator overloading in
- via Digitalmars-d-bugs (83/83) Nov 08 2015 https://issues.dlang.org/show_bug.cgi?id=15300
https://issues.dlang.org/show_bug.cgi?id=15300 Issue ID: 15300 Summary: Better support for operator overloading in std.variant.Algebraic Product: D Version: D2 Hardware: x86_64 OS: Linux Status: NEW Severity: enhancement Priority: P1 Component: phobos Assignee: nobody puremagic.com Reporter: tobias pankrath.net Code --- import std.algorithm, std.container; import std.variant, std.stdio; alias Doc = Algebraic!(P, Q); struct Q { Q opBinary(string s)(const Q rhs) { P result; result.curColumn = this.curColumn + rhs.curColumn; return result; } int curColumn = 0; } struct P { P opBinary(string s)(const Doc rhs) { P result; return result; } P opBinary(string s)(const P rhs) { P result; result.curColumn = this.curColumn + rhs.curColumn; return result; } int curColumn = 0; } void main() { auto a = Doc(P(2)); auto b = P(3); // works (1) auto c = Doc(P(3)); // runtime failure (2) auto d = a + b; writeln(d); } --- Currently you can use operators/operator overloading with std.variant.Algebraic, if * the operand is a buildin arithmetic datatype like int * both operands have the same type But not if the operand is the algebraic datatype itself. This could be improved. (A) In case (2) if P does have an opBinary overload for Doc, it should use it. (B) If there is no specific overload, it should try to do some kind of double dispatch on the types: Algebraic!(A,B,C) opBinary(string op)(const Algebraic!(A,B,C) rhs) { return this.visit!( (A lhsA) { return rhs.visit!( (A rhsA) { return DoOrFail!op(lhsA, rhsA); }, (B rhsB) { return DoOrFail!op(lhsA, rhsB); }, (...) (B lhsB) { return rhs.visit!( (A rhsA) { return DoOrFail!op(lhsB, rhsA); }, (B rhsB) { return DoOrFail!op(lhsB, rhsB); }, (...) ); } Where DoOrFail is Algebraic!(A,B,C) DoOrFail(L,R, string op)(L l, R r) { static if(__compiles(l.opBinary!(op)(r))) return Algebraic!(A, B, C)( l op r) else throw VariantError ... } (A) is especially useful with self referential types. --
Nov 08 2015