Algebraic data type restricted to a closed set of possible types. It's an alias for VariantN with an appropriately-constructed maximum size. Algebraic is useful when it is desirable to restrict what a discriminated type could hold to the end of defining simpler and more efficient manipulation.
auto v = Algebraic!(int, double, string)(5); assert(v.peek!(int)); v = 3.14; assert(v.peek!(double)); // auto x = v.peek!(long); // won't compile, type long not allowed // v = '1'; // won't compile, type char not allowed
A useful and popular use of algebraic data structures is for defining $(LUCKY self-referential data structures), i.e. structures that embed references to values of their own type within.
This is achieved with Algebraic by using This as a placeholder whenever a reference to the type being defined is needed. The Algebraic instantiation will perform alpha renaming on its constituent types, replacing This with the self-referenced type. The structure of the type involving This may be arbitrarily complex.
1 import std.typecons : Tuple, tuple; 2 3 // A tree is either a leaf or a branch of two other trees 4 alias Tree(Leaf) = Algebraic!(Leaf, Tuple!(This*, This*)); 5 Tree!int tree = tuple(new Tree!int(42), new Tree!int(43)); 6 Tree!int* right = tree.get!1; 7 assert(*right == 43); 8 9 // An object is a double, a string, or a hash of objects 10 alias Obj = Algebraic!(double, string, This[string]); 11 Obj obj = "hello"; 12 assert(obj.get!1 == "hello"); 13 obj = 42.0; 14 assert(obj.get!0 == 42); 15 obj = ["customer": Obj("John"), "paid": Obj(23.95)]; 16 assert(obj.get!2["customer"] == "John");