mir.algebraic

enum bool isVariant(T);

Checks if the type is instance of Algebraic.

Examples:

static assert(isVariant!(Variant!(int, string)));
static assert(isVariant!(const Variant!(int[], string)));
static assert(isVariant!(Nullable!(int, string)));
static assert(!isVariant!int);

enum bool isTaggedVariant(T);

Checks if the type is instance of tagged Algebraic.

Tagged algebraics can be defined with TaggedVariant.

Examples:

static assert(!isTaggedVariant!int);
static assert(!isTaggedVariant!(Variant!(int, string)));
static assert(isTaggedVariant!(TaggedVariant!(["integer", "string"], int, string)));

enum bool isNullable(T);

Checks if the type is instance of Algebraic with a self TypeSet that contains typeof(null).

Examples:

static assert(isNullable!(const Nullable!(int, string)));
static assert(isNullable!(Nullable!()));

static assert(!isNullable!(Variant!()));
static assert(!isNullable!(Variant!string));
static assert(!isNullable!int);
static assert(!isNullable!string);

template ValueTypeOfNullable(T : Algebraic!(typeof(null), Types), Types...)

Gets type of

.Algebraic.get.2

method.

Examples:

static assert(is(ValueTypeOfNullable!(const Nullable!(int, string)) == Algebraic!(int, string)));
static assert(is(ValueTypeOfNullable!(Nullable!()) == Algebraic!()));
static assert(is(typeof(Nullable!().get()) == Algebraic!()));

struct This;

Dummy type for Variant and Nullable self-referencing.

struct TaggedType(T, string name) if (name.length);

Dummy type used to associate tags with a type.

enum auto isTaggedType(T);

Checks if T is TaggedType instance.

template getTaggedTypeUnderlying(T : TaggedType!(I, name), I, string name)

Gets TaggedType underlying type.

enum auto getTaggedTypeName(T : TaggedType!(I, name), I, string name);

Gets TaggedType tag name.

Examples:

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 Variant by using This as a placeholder whenever a reference to the type being defined is needed. The Variant 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 mir.functional: Tuple = RefTuple;

// A tree is either a leaf or a branch of two others
alias Tree(Leaf) = Variant!(Leaf, Tuple!(This*, This*));
alias Leafs = Tuple!(Tree!int*, Tree!int*);

Tree!int tree = Leafs(new Tree!int(41), new Tree!int(43));
Tree!int* right = tree.get!Leafs[1];
assert(*right == 43);

Examples:

// An object is a double, a string, or a hash of objects
alias Obj = Variant!(double, string, This[string], This[]);
alias Map = Obj[string];

Obj obj = "hello";
assert(obj._is!string);
assert(obj.trustedGet!string == "hello");
obj = 42.0;
assert(obj.get!double == 42);
obj = ["customer": Obj("John"), "paid": Obj(23.95)];
assert(obj.get!Map["customer"] == "John");

template TypeSet(T...)

Type set resolution template used to construct Algebraic .

template TaggedTypeSet(string[] tagNames, T...)

Type set for tagged Variants self-referencing.

enum bool isTypeSet(T...);

Checks if the type list is TypeSet.

Examples:

static assert(isTypeSet!(TypeSet!()));
static assert(isTypeSet!(TypeSet!void));
static assert(isTypeSet!(TypeSet!(void, int, typeof(null))));

template Variant(T...)

Variant Type (aka Algebraic Type).

The impllementation is defined as

alias Variant(T...) = Algebraic!(TypeSet!T);

Compatible with BetterC mode.

Examples:

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

Examples:

Single argument Variant

static struct S
{
    int n;
    this(ref return scope inout S rhs) inout
    {
        this.n = rhs.n + 1;
    }
}

Variant!S a = S();
auto b = a;

import mir.conv;
assert(a.get!S.n == 0);
assert(b.n == 1); //direct access of a member in case of all algebraic types has this member

Examples:

Empty type set

Variant!() a;
auto b = a;
assert(a.toHash == 0);
assert(a == b);
assert(a <= b && b >= a);
static assert(typeof(a).sizeof == 1);

Examples:

Small types

struct S { ubyte d; }
static assert(Nullable!(byte, char, S).sizeof == 2);

template TaggedVariant(string[] tags, T...)

template TaggedVariant(T) if (is(T == union))

Tagged Variant Type (aka Tagged Algebraic Type).

Compatible with BetterC mode.

Template has two declarations:

alias TaggedVariant(string[] tags, T...) = Variant!(applyTags!(tags, T));
// and
template TaggedVariant(T)
    if (is(T == union))
{
    ...
}

See Also:

Variant, isTaggedVariant.

Examples:

Json Value

static union JsonUnion
{
    long integer;
    double floating;
    bool boolean;
    typeof(null) null_;
    string string_;
    This[] array;
    This[string] object;
}

alias JsonValue = TaggedVariant!JsonUnion;

// typeof(null) has priority
static assert(JsonValue.Kind.init == JsonValue.Kind.null_);
static assert(JsonValue.Kind.null_ == 0);

// Kind and AllowedTypes has the same order
static assert (is(JsonValue.AllowedTypes[JsonValue.Kind.array] == JsonValue[]));
static assert (is(JsonValue.AllowedTypes[JsonValue.Kind.boolean] == bool));
static assert (is(JsonValue.AllowedTypes[JsonValue.Kind.floating] == double));
static assert (is(JsonValue.AllowedTypes[JsonValue.Kind.integer] == long));
static assert (is(JsonValue.AllowedTypes[JsonValue.Kind.null_] == typeof(null)));
static assert (is(JsonValue.AllowedTypes[JsonValue.Kind.object] == JsonValue[string]));

JsonValue v;
assert(v.kind == JsonValue.Kind.null_);

v = 1;
assert(v.kind == JsonValue.Kind.integer);
assert(v == 1);
v = JsonValue(1);
assert(v == 1);
v = v.get!(long, double);

v = "Tagged!";
assert(v.get       !string                  == "Tagged!");
assert(v.trustedGet!string                  == "Tagged!");

assert(v.kind == JsonValue.Kind.string_);

assert(v.get       !(JsonValue.Kind.string_) == "Tagged!"); // Kind-based get
assert(v.trustedGet!(JsonValue.Kind.string_) == "Tagged!"); // Kind-based trustedGet

v = [JsonValue("str"), JsonValue(4.3)];

assert(v.kind == JsonValue.Kind.array);
assert(v.trustedGet!(JsonValue[])[1].kind == JsonValue.Kind.floating);

v = null;
assert(v.kind == JsonValue.Kind.null_);

Examples:

Wrapped algebraic with propogated primitives

static struct Response
{
    alias Union = TaggedVariant!(
        ["double_", "string", "array", "table"],
        double,
        string,
        Response[],
        Response[string],
    );

    Union data;
    alias Tag = Union.Kind;
    // propogates opEquals, opAssign, and other primitives
    alias data this;

    static foreach (T; Union.AllowedTypes)
        this(T v) @safe pure nothrow @nogc { data = v; }
}

Response v = 3.0;
assert(v.kind == Response.Tag.double_);
v = "str";
assert(v == "str");

template Nullable(T...)

Nullable!T nullable(T)(T t);

Nullable Variant Type (aka Algebraic Type).

The impllementation is defined as

alias Nullable(T...) = Variant!(typeof(null), T);

In additional to common algebraic API the following members can be accesssed:

• .Algebraic.isNull
• .Algebraic.nullify
• .Algebraic.get.2

Compatible with BetterC mode.

Examples:

Single type Nullable

static assert(is(Nullable!int == Variant!(typeof(null), int)));

Nullable!int a = 5;
assert(a.get!int == 5);

a.nullify;
assert(a.isNull);

a = 4;
assert(!a.isNull);
assert(a.get == 4);
assert(a == 4);
a = 4;

a = null;
assert(a == null);

Examples:

Empty nullable type set support

Nullable!() a;
auto b = a;
assert(a.toHash == 0);
assert(a == b);
assert(a <= b && b >= a);
static assert(typeof(a).sizeof == 1);

struct Algebraic(_Types...);

Algebraic implementation. For more portable code, it is higly recommeded to don't use this template directly. Instead, please use of Variant and Nullable, which sort types.

Examples:

Constructor and methods propagation.

static struct Base
{
    double d;
}

static class C
{
    // alias this members are supported 
    Base base;
    alias base this;

    int a;
    private string _b;

@safe pure nothrow @nogc:

    string b() const @property { return _b; }
    void b(string b) @property { _b = b; }

    int retArg(int v) { return v; }

    this(int a, string b)
    {
        this.a = a;
        this._b = b;
    }
}

static struct S
{
    string b;
    int a;

    double retArg(double v) { return v; }

    // alias this members are supported 
    Base base;
    alias base this;
}

static void inc(ref int a) { a++; }

alias V = Nullable!(C, S); // or Variant!

auto v = V(2, "str");
assert(v._is!C);
assert(v.a == 2);
assert(v.b == "str");
// members are returned by reference if possible
inc(v.a);
assert(v.a == 3);
v.b = "s";
assert(v.b == "s");
// alias this members are supported 
v.d = 10;
assert(v.d == 10);
// method call support
assert(v.retArg(100)._is!int);
assert(v.retArg(100) == 100);

v = V("S", 5);
assert(v._is!S);
assert(v.a == 5);
assert(v.b == "S");
// members are returned by reference if possible
inc(v.a);
assert(v.a == 6);
v.b = "s";
assert(v.b == "s");
// alias this members are supported 
v.d = 15;
assert(v.d == 15);
// method call support
assert(v.retArg(300)._is!double);
assert(v.retArg(300) == 300.0);

alias AllowedTypes = AliasSeq!(ReplaceTypeUnless!(isVariant, This, Algebraic!_Types, _UntaggedThisTypeSetList));

Allowed types list

See Also:

TypeSet

Examples:

import std.meta: AliasSeq;

alias V = Nullable!
(
    This*,
    string,
    double,
    bool,
);

static assert(is(V.AllowedTypes == TypeSet!(
    typeof(null),
    bool,
    string,
    double,
    V*)));

enum Kind;

Algebraic Kind.

Defined as enum for tagged algebraics and as unsigned for common algebraics.

The Kind enum contains the members defined using tag names.

If the algebraic type is Nullable then the default Kind enum member has zero value and corresponds to typeof(null).

See Also:

TaggedVariant.

const pure nothrow @nogc @property @safe Kind kind();

Returns:

.Algebraic.Kind.

Defined as enum for tagged algebraics and as unsigned for common algebraics.

See Also:

TaggedVariant.

this(RhsTypes...)(Algebraic!RhsTypes rhs)
if (allSatisfy!(Contains!AllowedTypes, Algebraic!RhsTypes.AllowedTypes));

Construct an algebraic type from its subset.

Examples:

alias Float = Variant!(float, double);
alias Int = Variant!(long, int);
alias Number = Variant!(Float.AllowedTypes, Int.AllowedTypes);

Float fp = 3.0;
Number number = fp; // constructor call
assert(number == 3.0);

Int integer = 12L;
number = Number(integer);
assert(number == 12L);

const nothrow @trusted size_t toHash();

const @trusted bool opEquals()(auto ref const typeof(this) rhs);

const @trusted auto opCmp()(auto ref const typeof(this) rhs);

const string toString()();

const void toString(W)(ref scope W w);

Requires mir-algorithm package

const bool opCast(C)()
if (is(C == bool));

const Algebraic opCast(C)()
if (is(C == Algebraic));

const @property bool isNull();

void nullify();

auto get()()
if (allSatisfy!(isCopyable, AllowedTypes[1 .. $]) && (AllowedTypes.length != 2));

Defined if the first type is typeof(null)

inout ref inout(AllowedTypes[1]) get() return;

inout @property ref inout(AllowedTypes[1]) get()(auto ref inout(AllowedTypes[1]) fallback) return;

Gets the value if not null. If this is in the null state, and the optional parameter fallback was provided, it will be returned. Without fallback, calling get with a null state is invalid.

When the fallback type is different from the Nullable type, get(T) returns the common type.

Parameters:

inout(AllowedTypes[1]) fallback

the value to return in case the Nullable is null.

Returns:

The value held internally by this Nullable.

Examples:

enum E { a = "a", b = "b" }
Nullable!E f = E.a;
auto e = f.get();
static assert(is(typeof(e) == E), Nullable!E.AllowedTypes.stringof);
assert(e == E.a);

assert(f.get(E.b) == E.a);

f = null;
assert(f.get(E.b) == E.b);

const pure nothrow @nogc @property @safe bool _is(R : Algebraic!RetTypes, RetTypes...)()
if (allSatisfy!(Contains!AllowedTypes, Algebraic!RetTypes.AllowedTypes));

const pure nothrow @nogc @property @safe bool _is(RetTypes...)()
if (RetTypes.length > 1);

Checks if the underlaying type is an element of a user provided type set.

@property ref auto trustedGet(R : Algebraic!RetTypes, this This, RetTypes...)() return
if (allSatisfy!(Contains!AllowedTypes, Algebraic!RetTypes.AllowedTypes));

template trustedGet(RetTypes...) if (RetTypes.length > 1)

nothrow .Algebraic.get alternative that returns an algebraic subset.

Examples:

alias Float = Variant!(float, double);
alias Int = Variant!(long, int);
alias Number = Variant!(Float.AllowedTypes, Int.AllowedTypes);

Number number = 3.0;
assert(number._is!Float);
auto fp = number.trustedGet!Float;
static assert(is(typeof(fp) == Float));
assert(fp == 3.0);

// type list overload
number = 12L;
assert(number._is!(int, long));
auto integer = number.trustedGet!(int, long);
static assert(is(typeof(integer) == Int));
assert(integer == 12L);

template trustedGet(Kind kind)

trustedGet overload that accept .Algebraic.Kind.

@property ref auto get(R : Algebraic!RetTypes, this This, RetTypes...)() return
if (allSatisfy!(Contains!AllowedTypes, Algebraic!RetTypes.AllowedTypes));

template get(RetTypes...) if (RetTypes.length > 1)

Gets an algebraic subset.

Throws:

Exception if the storage contains value of the type that isn't represented in the allowed type set of the requested algebraic.

Examples:

alias Float = Variant!(float, double);
alias Int = Variant!(long, int);
alias Number = Variant!(Float.AllowedTypes, Int.AllowedTypes);

Number number = 3.0;
auto fp = number.get!Float;
static assert(is(typeof(fp) == Float));
assert(fp == 3.0);

// type list overload
number = 12L;
auto integer = number.get!(int, long);
static assert(is(typeof(integer) == Int));
assert(integer == 12L);

template get(Kind kind)

get overload that accept .Algebraic.Kind.

ref @trusted auto opAssign(RhsTypes...)(Algebraic!RhsTypes rhs) return
if (RhsTypes.length < AllowedTypes.length && allSatisfy!(Contains!AllowedTypes, Algebraic!RhsTypes.AllowedTypes));

template visit(visitors...)

Applies a delegate or function to the given Variant depending on the held type, ensuring that all types are handled by the visiting functions.

Examples:

alias Number = Variant!(int, double);

Number x = 23;
Number y = 1.0;

assert(x.visit!((int v) => true, (float v) => false));
assert(y.visit!((int v) => false, (float v) => true));

Examples:

alias Number = Nullable!(int, double);

Number z = null; // default
Number x = 23;
Number y = 1.0;

() nothrow {
    assert(x.visit!((int v) => true, (float v) => false));
    assert(y.visit!((int v) => false, (v) => true));
    assert(z.visit!((typeof(null) v) => true, (v) => false));
} ();

auto xx = x.get;
static assert (is(typeof(xx) == Variant!(int, double)));
assert(xx.visit!((int v) => v, (float v) => 0) == 23);
assert(xx.visit!((ref v) => v) == 23);

x = null;
y.nullify;

assert(x.isNull);
assert(y.isNull);
assert(z.isNull);
assert(z == y);

Examples:

Checks .Algebraic.toString and void Algerbraic.toString requries mir-algorithm package

import mir.conv: to;
enum MIR_ALGORITHM = __traits(compiles, { import mir.format; });

alias visitorHandler = visit!(
    (typeof(null)) => "NULL",
    () => "VOID",
    (ref r) {r += 1;}, // returns void
);

alias secondOrderVisitorHandler = visit!(
    () => "SO VOID", // void => to "RV VOID"
    (str) => str, // string to => it self
);

alias V = Nullable!(void, int);
static assert(is(V == Variant!(typeof(null), void, int)));

V variant;

assert(secondOrderVisitorHandler(visitorHandler(variant)) == "NULL");
assert(variant.to!string == "null");

variant = V._void;
assert(variant._is!void);
assert(is(typeof(variant.get!void()) == void));

assert(secondOrderVisitorHandler(visitorHandler(variant)) == "VOID");
assert(variant.to!string == "void");

variant = 5;

assert(secondOrderVisitorHandler(visitorHandler(variant)) == "SO VOID");
assert(variant == 6);
assert(variant.to!string == (MIR_ALGORITHM ? "6" : "int"));

template tryVisit(visitors...)

Behaves as visit but doesn't enforce at compile time that all types can be handled by the visiting functions.

Throws:

Exception if naryFun!visitors can't be called with provided arguments

Examples:

alias Number = Variant!(int, double);

Number x = 23;

assert(x.tryVisit!((int v) => true));

template optionalVisit(visitors...)

Behaves as visit but doesn't enforce at compile time that all types can be handled by the visiting functions.

Returns:

nullable variant, null value is used if naryFun!visitors can't be called with provided arguments.

Examples:

static struct S { int a; }

Variant!(S, double) variant;

alias optionalVisitInst = optionalVisit!((ref value) => value + 0);

// do nothing because of variant isn't initialized
Nullable!double result = optionalVisitInst(variant);
assert(result.isNull);

variant = S(2);
// do nothing because of lambda can't compile
result = optionalVisitInst(variant);
assert(result == null);

variant = 3.0;
result = optionalVisitInst(variant);
assert (result == 3.0);

template autoVisit(visitors...)

Behaves as visit but doesn't enforce at compile time that all types can be handled by the visiting functions.

Returns:

optionally nullable type, null value is used if naryFun!visitors can't be called with provided arguments.

template match(visitors...)

Applies a delegate or function to the given arguments depending on the held type, ensuring that all types are handled by the visiting functions.

The handler supports multiple dispatch or multimethods: a feature of handler in which a function or method can be dynamically dispatched based on the run time (dynamic) type or, in the more general case, some other attribute of more than one of its arguments.

Fuses algebraic types on return.

See Also:

Multiple dispatch

Examples:

struct Asteroid { uint size; }
struct Spaceship { uint size; }
alias SpaceObject = Variant!(Asteroid, Spaceship);

alias collideWith = match!(
    (Asteroid x, Asteroid y) => "a/a",
    (Asteroid x, Spaceship y) => "a/s",
    (Spaceship x, Asteroid y) => "s/a",
    (Spaceship x, Spaceship y) => "s/s",
);

import mir.utility: min;

// Direct access of a member in case of all algebraic types has this member
alias oops = (a, b) => (a.size + b.size) > 3 && min(a.size, b.size) > 1;

alias collide = (x, y) => oops(x, y) ? "big-boom" : collideWith(x, y);

auto ea = Asteroid(1);
auto es = Spaceship(2);
auto oa = SpaceObject(ea);
auto os = SpaceObject(es);

// Asteroid-Asteroid
assert(collide(ea, ea) == "a/a");
assert(collide(ea, oa) == "a/a");
assert(collide(oa, ea) == "a/a");
assert(collide(oa, oa) == "a/a");

// Asteroid-Spaceship
assert(collide(ea, es) == "a/s");
assert(collide(ea, os) == "a/s");
assert(collide(oa, es) == "a/s");
assert(collide(oa, os) == "a/s");

// Spaceship-Asteroid
assert(collide(es, ea) == "s/a");
assert(collide(es, oa) == "s/a");
assert(collide(os, ea) == "s/a");
assert(collide(os, oa) == "s/a");

// Spaceship-Spaceship
assert(collide(es, es) == "big-boom");
assert(collide(es, os) == "big-boom");
assert(collide(os, es) == "big-boom");
assert(collide(os, os) == "big-boom");

template tryMatch(visitors...)

Behaves as match but doesn't enforce at compile time that all types can be handled by the visiting functions.

Throws:

Exception if naryFun!visitors can't be called with provided arguments

Fuses algebraic types on return.

Examples:

import std.exception: assertThrown;
struct Asteroid { uint size; }
struct Spaceship { uint size; }
alias SpaceObject = Variant!(Asteroid, Spaceship);

alias collideWith = tryMatch!(
    (Asteroid x, Asteroid y) => "a/a",
    // No visitor for A/S pair 
    // (Asteroid x, Spaceship y) => "a/s",
    (Spaceship x, Asteroid y) => "s/a",
    (Spaceship x, Spaceship y) => "s/s",
);

import mir.utility: min;
// Direct access of a member in case of all algebraic types has this member
alias oops = (a, b) => (a.size + b.size) > 3 && min(a.size, b.size) > 1;

alias collide = (x, y) => oops(x, y) ? "big-boom" : collideWith(x, y);

auto ea = Asteroid(1);
auto es = Spaceship(2);
auto oa = SpaceObject(ea);
auto os = SpaceObject(es);

// Asteroid-Asteroid
assert(collide(ea, ea) == "a/a");
assert(collide(ea, oa) == "a/a");
assert(collide(oa, ea) == "a/a");
assert(collide(oa, oa) == "a/a");

// Asteroid-Spaceship
assertThrown!Exception(collide(ea, es));
assertThrown!Exception(collide(ea, os));
assertThrown!Exception(collide(oa, es));
assertThrown!Exception(collide(oa, os));

// can deduce the type based on other return values
static assert(is(typeof(collide(ea, os)) == string));
static assert(is(typeof(collide(oa, es)) == string));
static assert(is(typeof(collide(oa, os)) == string));

// Also allows newer compilers to detect combinations which always throw an exception
static if (is(typeof(collideWith(ea, es)) == noreturn))
{
    static assert(is(typeof(collide(ea, es)) == string));
}
else
{
    // not enough information to deduce the type from (ea, es) pair
    static assert(is(typeof(collide(ea, es)) == void));
}

// Spaceship-Asteroid
assert(collide(es, ea) == "s/a");
assert(collide(es, oa) == "s/a");
assert(collide(os, ea) == "s/a");
assert(collide(os, oa) == "s/a");

// Spaceship-Spaceship
assert(collide(es, es) == "big-boom");
assert(collide(es, os) == "big-boom");
assert(collide(os, es) == "big-boom");
assert(collide(os, os) == "big-boom");

template optionalMatch(visitors...)

Behaves as match but doesn't enforce at compile time that all types can be handled by the visiting functions.

Returns:

nullable variant, null value is used if naryFun!visitors can't be called with provided arguments.

Fuses algebraic types on return.

Examples:

struct Asteroid { uint size; }
struct Spaceship { uint size; }
alias SpaceObject = Variant!(Asteroid, Spaceship);

alias collideWith = optionalMatch!(
    (Asteroid x, Asteroid y) => "a/a",
    // No visitor for A/S pair 
    // (Asteroid x, Spaceship y) => "a/s",
    (Spaceship x, Asteroid y) => "s/a",
    (Spaceship x, Spaceship y) => "s/s",
);

import mir.utility: min;
// Direct access of a member in case of all algebraic types has this member
alias oops = (a, b) => (a.size + b.size) > 3 && min(a.size, b.size) > 1;

alias collide = (x, y) => oops(x, y) ? "big-boom".nullable : collideWith(x, y);

auto ea = Asteroid(1);
auto es = Spaceship(2);
auto oa = SpaceObject(ea);
auto os = SpaceObject(es);

// Asteroid-Asteroid
assert(collide(ea, ea) == "a/a");
assert(collide(ea, oa) == "a/a");
assert(collide(oa, ea) == "a/a");
assert(collide(oa, oa) == "a/a");

// Asteroid-Spaceship
// assert(collide(ea, es).isNull);  // Compiler error: incompatible types
assert(collideWith(ea, es).isNull); // OK
assert(collide(ea, os).isNull);
assert(collide(oa, es).isNull);
assert(collide(oa, os).isNull);


// Spaceship-Asteroid
assert(collide(es, ea) == "s/a");
assert(collide(es, oa) == "s/a");
assert(collide(os, ea) == "s/a");
assert(collide(os, oa) == "s/a");

// Spaceship-Spaceship
assert(collide(es, es) == "big-boom");
assert(collide(es, os) == "big-boom");
assert(collide(os, es) == "big-boom");
assert(collide(os, os) == "big-boom");

// check types  

static assert(!__traits(compiles, collide(Asteroid.init, Spaceship.init)));
static assert(is(typeof(collideWith(Asteroid.init, Spaceship.init)) == Nullable!()));

static assert(is(typeof(collide(Asteroid.init, Asteroid.init)) == Nullable!string));
static assert(is(typeof(collide(Asteroid.init, SpaceObject.init)) == Nullable!string));
static assert(is(typeof(collide(SpaceObject.init, Asteroid.init)) == Nullable!string));
static assert(is(typeof(collide(SpaceObject.init, SpaceObject.init)) == Nullable!string));
static assert(is(typeof(collide(SpaceObject.init, Spaceship.init)) == Nullable!string));
static assert(is(typeof(collide(Spaceship.init, Asteroid.init)) == Nullable!string));
static assert(is(typeof(collide(Spaceship.init, SpaceObject.init)) == Nullable!string));
static assert(is(typeof(collide(Spaceship.init, Spaceship.init)) == Nullable!string));

template autoMatch(visitors...)

Behaves as match but doesn't enforce at compile time that all types can be handled by the visiting functions.

Returns:

optionally nullable type, null value is used if naryFun!visitors can't be called with provided arguments.

Fuses algebraic types on return.

Examples:

struct Asteroid { uint size; }
struct Spaceship { uint size; }
alias SpaceObject = Variant!(Asteroid, Spaceship);

alias collideWith = autoMatch!(
    (Asteroid x, Asteroid y) => "a/a",
    // No visitor for A/S pair 
    // (Asteroid x, Spaceship y) => "a/s",
    (Spaceship x, Asteroid y) => "s/a",
    (Spaceship x, Spaceship y) => "s/s",
);

import mir.utility: min;
// Direct access of a member in case of all algebraic types has this member
alias oops = (a, b) => (a.size + b.size) > 3 && min(a.size, b.size) > 1;

import mir.conv: to;
alias collide = (x, y) => oops(x, y) ? "big-boom".to!(typeof(collideWith(x, y))) : collideWith(x, y);

auto ea = Asteroid(1);
auto es = Spaceship(2);
auto oa = SpaceObject(ea);
auto os = SpaceObject(es);

// Asteroid-Asteroid
assert(collide(ea, ea) == "a/a");
assert(collide(ea, oa) == "a/a");
assert(collide(oa, ea) == "a/a");
assert(collide(oa, oa) == "a/a");

// Asteroid-Spaceship
// assert(collide(ea, es).isNull);  // Compiler error: incompatible types
assert(collideWith(ea, es).isNull); // OK
assert(collide(ea, os).isNull);
assert(collide(oa, es).isNull);
assert(collide(oa, os).isNull);

// Spaceship-Asteroid
assert(collide(es, ea) == "s/a");
assert(collide(es, oa) == "s/a");
assert(collide(os, ea) == "s/a");
assert(collide(os, oa) == "s/a");

// Spaceship-Spaceship
assert(collide(es, es) == "big-boom");
assert(collide(es, os) == "big-boom");
assert(collide(os, es) == "big-boom");
assert(collide(os, os) == "big-boom");

// check types  

static assert(!__traits(compiles, collide(Asteroid.init, Spaceship.init)));
static assert(is(typeof(collideWith(Asteroid.init, Spaceship.init)) == Nullable!()));

static assert(is(typeof(collide(Asteroid.init, Asteroid.init)) == string));
static assert(is(typeof(collide(SpaceObject.init, Asteroid.init)) == string));
static assert(is(typeof(collide(Spaceship.init, Asteroid.init)) == string));
static assert(is(typeof(collide(Spaceship.init, SpaceObject.init)) == string));
static assert(is(typeof(collide(Spaceship.init, Spaceship.init)) == string));

static assert(is(typeof(collide(Asteroid.init, SpaceObject.init)) == Nullable!string));
static assert(is(typeof(collide(SpaceObject.init, SpaceObject.init)) == Nullable!string));
static assert(is(typeof(collide(SpaceObject.init, Spaceship.init)) == Nullable!string));

template getMember(string member)

Applies a member handler to the given Variant depending on the held type, ensuring that all types are handled by the visiting handler.

Examples:

static struct S { auto bar(int a) { return a; } enum boolean = true; }
static struct C { alias bar = (double a) => a * 2; enum boolean = false; }

alias V = Variant!(S, C);

V x = S();
V y = C();

static assert(is(typeof(x.getMember!"bar"(2)) == Variant!(int, double)));
assert(x.getMember!"bar"(2) == 2);
assert(y.getMember!"bar"(2) != 4);
assert(y.getMember!"bar"(2) == 4.0);

// direct implementation
assert(x.bar(2) == 2);
assert(y.bar(2) != 4);
assert(y.bar(2) == 4.0);
assert(x.boolean);
assert(!y.boolean);

template matchMember(string member)

Applies a member handler to the given Variant depending on the held type, ensuring that all types are handled by the visiting handler.

Fuses algebraic types on return.

Examples:

static struct S
{
    Nullable!int m;
}

static struct C
{
    Variant!(float, double) m;
}

alias V = Variant!(S, C);

V x = S(2.nullable);
V y = C(Variant!(float, double)(4.0));

// getMember returns an algebraic of algebaics
static assert(is(typeof(x.getMember!"m") == Variant!(Variant!(float, double), Nullable!int)));
// matchMember returns a fused algebraic
static assert(is(typeof(x.matchMember!"m") == Nullable!(int, float, double)));
assert(x.matchMember!"m" == 2);
assert(y.matchMember!"m" != 4);
assert(y.matchMember!"m" == 4.0);

template tryGetMember(string member)

Behaves as getMember but doesn't enforce at compile time that all types can be handled by the member visitor.

Throws:

Exception if member can't be accessed with provided arguments

Examples:

static struct S { int bar(int a) { return a; }}
static struct C { alias Bar = (double a) => a * 2; }

alias V = Variant!(S, C);

V x = S();
V y = C();

static assert(is(typeof(x.tryGetMember!"bar"(2)) == int));
static assert(is(typeof(y.tryGetMember!"Bar"(2)) == double));
assert(x.tryGetMember!"bar"(2) == 2);
assert(y.tryGetMember!"Bar"(2) == 4.0);

Examples:

alias Number = Variant!(int, double);

Number x = Number(23);
Number y = Number(1.0);

assert(x.visit!((int v) => true, (float v) => false));
assert(y.visit!((int v) => false, (float v) => true));

template tryMatchMember(string member)

Behaves as matchMember but doesn't enforce at compile time that all types can be handled by the member visitor.

Throws:

Exception if member can't be accessed with provided arguments

Fuses algebraic types on return.

template optionalGetMember(string member)

Behaves as getMember but doesn't enforce at compile time that all types can be handled by the member visitor.

Returns:

nullable variant, null value is used if the member can't be called with provided arguments.

template optionalMatchMember(string member)

Behaves as matchMember but doesn't enforce at compile time that all types can be handled by the member visitor.

Returns:

nullable variant, null value is used if the member can't be called with provided arguments.

Fuses algebraic types on return.

template autoGetMember(string member)

Behaves as getMember but doesn't enforce at compile time that all types can be handled by the member visitor.

Returns:

optionally nullable type, null value is used if the member can't be called with provided arguments.

template autoMatchMember(string member)

Behaves as matchMember but doesn't enforce at compile time that all types can be handled by the member visitor.

Returns:

optionally nullable type, null value is used if the member can't be called with provided arguments.

Fuses algebraic types on return.