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.

template Algebraic (
) {}


auto v = Algebraic!(int, double, string)(5);
v = 3.14;
// auto x = v.peek!(long); // won't compile, type long not allowed
// v = '1'; // won't compile, type char not allowed

Self-Referential Types

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;
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[1];
7 assert(*right == 43);
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");


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