Struct TaggedAlgebraic

This struct takes a union or struct declaration as an input and builds an algebraic data type from its fields, using an automatically generated Kind enumeration to identify which field of the union is currently used. Multiple fields with the same value are supported.

All operators and methods are transparently forwarded to the contained value. The caller has to make sure that the contained value supports the requested operation. Failure to do so will result in an assertion failure.

The return value of forwarded operations is determined as follows:

• If the type can be uniquely determined, it is used as the return value
• If there are multiple possible return values and all of them match the unique types defined in the TaggedAlgebraic, a TaggedAlgebraic is returned.
• If there are multiple return values and none of them is a Variant, an Algebraic of the set of possible return types is returned.
• If any of the possible operations returns a Variant, this is used as the return value.

Constructors

Fields

Properties

Methods

Aliases

Example

import taggedalgebraic;

struct Foo {
	string name;
	void bar() @safe {}
}

union Base {
	int i;
	string str;
	Foo foo;
}

alias Tagged = TaggedAlgebraic!Base;

// Instantiate
Tagged taggedInt = 5;
Tagged taggedString = "Hello";
Tagged taggedFoo = Foo();
Tagged taggedAny = taggedInt;
taggedAny = taggedString;
taggedAny = taggedFoo;

// Check type: Tagged.Kind is an enum
assert(taggedInt.kind == Tagged.Kind.i);
assert(taggedString.kind == Tagged.Kind.str);
assert(taggedFoo.kind == Tagged.Kind.foo);
assert(taggedAny.kind == Tagged.Kind.foo);

// In most cases, can simply use as-is
auto num = 4 + taggedInt;
auto msg = taggedString ~ " World!";
taggedFoo.bar();
if (taggedAny.kind == Tagged.Kind.foo) // Make sure to check type first!
	taggedAny.bar();
//taggedString.bar(); // AssertError: Not a Foo!

// Convert back by casting
auto i   = cast(int)    taggedInt;
auto str = cast(string) taggedString;
auto foo = cast(Foo)    taggedFoo;
if (taggedAny.kind == Tagged.Kind.foo) // Make sure to check type first!
	auto foo2 = cast(Foo) taggedAny;
//cast(Foo) taggedString; // AssertError!

// Kind is an enum, so final switch is supported:
final switch (taggedAny.kind) {
	case Tagged.Kind.i:
		// It's "int i"
		break;

	case Tagged.Kind.str:
		// It's "string str"
		break;

	case Tagged.Kind.foo:
		// It's "Foo foo"
		break;
}

Example

Operators and methods of the contained type can be used transparently.

static struct S {
	int v;
	int test() { return v / 2; }
}

static union Test {
	typeof(null) null_;
	int integer;
	string text;
	string[string] dictionary;
	S custom;
}

alias TA = TaggedAlgebraic!Test;

TA ta;
assert(ta.kind == TA.Kind.null_);

ta = 12;
assert(ta.kind == TA.Kind.integer);
assert(ta == 12);
assert(cast(int)ta == 12);
assert(cast(long)ta == 12);
assert(cast(short)ta == 12);

ta += 12;
assert(ta == 24);
assert(ta - 10 == 14);

ta = ["foo" : "bar"];
assert(ta.kind == TA.Kind.dictionary);
assert(ta["foo"] == "bar");

ta["foo"] = "baz";
assert(ta["foo"] == "baz");

ta = S(8);
assert(ta.test() == 4);

Example

Multiple fields are allowed to have the same type, in which case the type ID enum is used to disambiguate.

static union Test {
	typeof(null) null_;
	int count;
	int difference;
}

alias TA = TaggedAlgebraic!Test;

TA ta = TA(12, TA.Kind.count);
assert(ta.kind == TA.Kind.count);
assert(ta == 12);

ta = null;
assert(ta.kind == TA.Kind.null_);