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Alias std.variant.Algebraic

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.

alias Algebraic(T...) = VariantN!(maxSize!T,T);

Warning: Algebraic is outdated and not recommended for use in new code. Instead, use SumType.


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 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.

import std.typecons : Tuple, tuple;

// A tree is either a leaf or a branch of two other trees
alias Tree(Leaf) = Algebraic!(Leaf, Tuple!(This*, This*));
Tree!int tree = tuple(new Tree!int(42), new Tree!int(43));
Tree!int* right = tree.get!1[1];
writeln(*right); // 43

// An object is a double, a string, or a hash of objects
alias Obj = Algebraic!(double, string, This[string]);
Obj obj = "hello";
writeln(obj.get!1); // "hello"
obj = 42.0;
writeln(obj.get!0); // 42
obj = ["customer": Obj("John"), "paid": Obj(23.95)];
writeln(obj.get!2["customer"]); // "John"


Andrei Alexandrescu


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