AutoImplement

1 interface PackageSupplier
2 {
3     int foo();
4     int bar();
5 }
6 
7 static abstract class AbstractFallbackPackageSupplier : PackageSupplier
8 {
9     protected PackageSupplier default_, fallback;
10 
11     this(PackageSupplier default_, PackageSupplier fallback)
12     {
13         this.default_ = default_;
14         this.fallback = fallback;
15     }
16 
17     abstract int foo();
18     abstract int bar();
19 }
20 
21 template fallback(T, alias func)
22 {
23     import std.format : format;
24     // for all implemented methods:
25     // - try default first
26     // - only on a failure run & return fallback
27     enum fallback = q{
28         scope (failure) return fallback.%1$s(args);
29         return default_.%1$s(args);
30     }.format(__traits(identifier, func));
31 }
32 
33 // combines two classes and use the second one as fallback
34 alias FallbackPackageSupplier = AutoImplement!(AbstractFallbackPackageSupplier, fallback);
35 
36 class FailingPackageSupplier : PackageSupplier
37 {
38     int foo(){ throw new Exception("failure"); }
39     int bar(){ return 2;}
40 }
41 
42 class BackupPackageSupplier : PackageSupplier
43 {
44     int foo(){ return -1; }
45     int bar(){ return -1;}
46 }
47 
48 auto registry = new FallbackPackageSupplier(new FailingPackageSupplier(), new BackupPackageSupplier());
49 
50 assert(registry.foo() == -1);
51 assert(registry.bar() == 2);