Mojo struct

Variant

struct Variant[*Ts: Movable]

A union that can hold a runtime-variant value from a set of predefined types.

Variant is a discriminated union type, similar to std::variant in C++ or enum in Rust. It can store exactly one value that can be any of the specified types, determined at runtime.

The key feature is that the actual type stored in a Variant is determined at runtime, not compile time. This allows you to change what type a variant holds during program execution. Memory-wise, a variant only uses the space needed for the largest possible type plus a small discriminant field to track which type is currently active.

Tips:

• use isa[T]() to check what type a variant is
• use unsafe_take[T]() to take a value from the variant
• use [T] to get a value out of a variant
  • This currently does an extra copy/move until we have origins
  • It also temporarily requires the value to be mutable
• use set[T](var new_value: T) to reset the variant to a new value
• use is_type_supported[T] to check if the variant permits the type T

Note: Currently, variant operations require the variant to be mutable (mut), even for read operations.

Example:

from std.utils import Variant
import std.random as random

comptime IntOrString = Variant[Int, String]

def to_string(mut x: IntOrString) -> String:
    if x.isa[String]():
        return x[String]
    return String(x[Int])

var an_int = IntOrString(4)
var a_string = IntOrString("I'm a string!")
var who_knows = IntOrString(0)
# Randomly change who_knows to a string
random.seed()
if random.random_ui64(0, 1):
    who_knows.set[String]("I'm also a string!")

print(a_string[String])      # => I'm a string!
print(an_int[Int])           # => 4
print(to_string(who_knows))  # Either 0 or "I'm also a string!"

if who_knows.isa[String]():
    print("It's a String!")

Example usage for error handling:

comptime Result = Variant[String, Error]

def process_data(data: String) -> Result:
    if len(data) == 0:
        return Result(Error("Empty data"))
    return Result(String("Processed: ", data))

var result = process_data("Hello")
if result.isa[String]():
    print("Success:", result[String])
else:
    print("Error:", result[Error])

Example usage in a List to create a heterogeneous list:

comptime MixedType = Variant[Int, Float64, String, Bool]

var mixed_list = List[MixedType]()
mixed_list.append(MixedType(42))
mixed_list.append(MixedType(3.14))
mixed_list.append(MixedType("hello"))
mixed_list.append(MixedType(True))

for item in mixed_list:
    if item.isa[String]():
        print("String:", item[String])
    elif item.isa[Int]():
        print("Integer:", item[Int])
    elif item.isa[Float64]():
        print("Float:", item[Float64])
    elif item.isa[Bool]():
        print("Boolean:", item[Bool])

Layout

The layout of Variant is not guaranteed and may change at any time. The implementation may apply niche optimizations (for example, encoding the discriminant inside spare bits of one of the types in Ts) that alter the resulting layout. Do not rely on size_of[Variant[...]]() or align_of[Variant[...]]() being stable across language versions. The only guarantee is that the size and alignment will be at least as large as those of the largest type among Ts.

If you need to inspect the current size or alignment, use size_of and align_of, but treat the results as non-stable implementation details.

Parameters

• ​*Ts (Movable): The possible types that this variant can hold. Types that implement Copyable enable copy semantics for the variant.

Implemented traits

AnyType, Copyable, Equatable, Hashable, ImplicitlyCopyable, ImplicitlyDestructible, Movable, RegisterPassable, Writable

Methods

__init__

@implicit __init__[T: Movable](out self, var value: T)

Create a variant with one of the types.

Parameters:

• ​T (Movable): The type to initialize the variant to. Generally this should be able to be inferred from the call type, eg. Variant[Int, String](4).

Args:

• ​value (T): The value to initialize the variant with.

__init__(out self, *, copy: Self)

Copy-initialize this variant from another variant of the same type.

Args:

• ​copy (Self): The variant to copy from.

__init__(out self, *, deinit take: Self)

Move-initialize this variant from another variant of the same type.

Args:

• ​take (Self): The variant to move from.

__del__

__del__(deinit self)

Destroy the variant, running the destructor of the currently held value.

Constraints:

All types in Ts must conform to ImplicitlyDestructible.

__eq__

__eq__(self, other: Self) -> Bool where #kgen.variadic.reduce(Ts, base=True, reducer=[PrevV: Variadic[Bool], VA: Variadic[AnyType], idx: __mlir_type.index] PrevV[0] if conforms_to(VA[idx], AnyType & ImplicitlyDestructible & Equatable) else conforms_to(VA[idx], AnyType & ImplicitlyDestructible & Equatable))[0]

Compares two variants for equality.

Two variants are equal if they hold the same type and the held values are equal.

Args:

• ​other (Self): The other variant to compare against.

Returns:

Bool: True if the variants hold the same type and equal values.

__ne__

__ne__(self, other: Self) -> Bool where #kgen.variadic.reduce(Ts, base=True, reducer=[PrevV: Variadic[Bool], VA: Variadic[AnyType], idx: __mlir_type.index] PrevV[0] if conforms_to(VA[idx], AnyType & ImplicitlyDestructible & Equatable) else conforms_to(VA[idx], AnyType & ImplicitlyDestructible & Equatable))[0]

Compares two variants for inequality.

Args:

• ​other (Self): The other variant to compare against.

Returns:

Bool: True if the variants hold different types or unequal values.

__getitem_param__

__getitem_param__[T: AnyType](ref self) -> ref[self_is_mut] T

Get the value out of the variant as a type-checked type.

This explicitly check that your value is of that type! If you haven't verified the type correctness at runtime, the program will abort!

For now this has the limitations that it - requires the variant value to be mutable

Parameters:

• ​T (AnyType): The type of the value to get out.

Returns:

ref: A reference to the internal data.

__hash__

__hash__(self, mut hasher: T) where #kgen.variadic.reduce(Ts, base=True, reducer=[PrevV: Variadic[Bool], VA: Variadic[AnyType], idx: __mlir_type.index] PrevV[0] if conforms_to(VA[idx], AnyType & Hashable) else conforms_to(VA[idx], AnyType & Hashable))[0]

Hashes the variant using the given hasher.

The hash incorporates both the type discriminant and the held value's hash, so variants holding different types are unlikely to collide.

Args:

• ​hasher (T): The hasher instance.

write_to

write_to(self, mut writer: T) where #kgen.variadic.reduce(Ts, base=True, reducer=[PrevV: Variadic[Bool], VA: Variadic[AnyType], idx: __mlir_type.index] PrevV[0] if conforms_to(VA[idx], AnyType & ImplicitlyDestructible & Writable) else conforms_to(VA[idx], AnyType & ImplicitlyDestructible & Writable))[0]

Writes the currently held variant value to the provided Writer.

Args:

• ​writer (T): The object to write to.

write_repr_to

write_repr_to(self, mut writer: T) where #kgen.variadic.reduce(Ts, base=True, reducer=[PrevV: Variadic[Bool], VA: Variadic[AnyType], idx: __mlir_type.index] PrevV[0] if conforms_to(VA[idx], AnyType & ImplicitlyDestructible & Writable) else conforms_to(VA[idx], AnyType & ImplicitlyDestructible & Writable))[0]

Write the string representation of the Variant.

Args:

• ​writer (T): The object to write to.

take

take[T: Movable](deinit self) -> T

Take the current value of the variant with the provided type.

The caller takes ownership of the underlying value.

This explicitly check that your value is of that type! If you haven't verified the type correctness at runtime, the program will abort!

Parameters:

• ​T (Movable): The type to take out.

Returns:

T: The underlying data to be taken out as an owned value.

unsafe_take

unsafe_take[T: Movable](deinit self) -> T

Unsafely take the current value of the variant with the provided type.

The caller takes ownership of the underlying value.

This doesn't explicitly check that your value is of that type! If you haven't verified the type correctness at runtime, you'll get a type that looks like your type, but has potentially unsafe and garbage member data.

Parameters:

• ​T (Movable): The type to take out.

Returns:

T: The underlying data to be taken out as an owned value.

replace

replace[Tin: Movable & ImplicitlyDestructible, Tout: Movable](mut self, var value: Tin) -> Tout

Replace the current value of the variant with the provided type.

The caller takes ownership of the underlying value.

This explicitly check that your value is of that type! If you haven't verified the type correctness at runtime, the program will abort!

Parameters:

• ​Tin (Movable & ImplicitlyDestructible): The type to put in.
• ​Tout (Movable): The type to take out.

Args:

• ​value (Tin): The value to put in.

Returns:

Tout: The underlying data to be taken out as an owned value.

unsafe_replace

unsafe_replace[Tin: Movable, Tout: Movable](mut self, var value: Tin) -> Tout

Unsafely replace the current value of the variant with the provided type.

The caller takes ownership of the underlying value.

This doesn't explicitly check that your value is of that type! If you haven't verified the type correctness at runtime, you'll get a type that looks like your type, but has potentially unsafe and garbage member data.

Parameters:

• ​Tin (Movable): The type to put in.
• ​Tout (Movable): The type to take out.

Args:

• ​value (Tin): The value to put in.

Returns:

Tout: The underlying data to be taken out as an owned value.

set

set[T: Movable](mut self, var value: T)

Set the variant value.

This will call the destructor on the old value, and update the variant's internal type and data to the new value.

Parameters:

• ​T (Movable): The new variant type. Must be one of the Variant's type arguments.

Args:

• ​value (T): The new value to set the variant to.

isa

isa[T: AnyType](self) -> Bool

Check if the variant contains the required type.

Parameters:

• ​T (AnyType): The type to check.

Returns:

Bool: True if the variant contains the requested type.

unsafe_get

unsafe_get[T: AnyType](ref self) -> ref[self_is_mut] T

Get the value out of the variant as a type-checked type.

This doesn't explicitly check that your value is of that type! If you haven't verified the type correctness at runtime, you'll get a type that looks like your type, but has potentially unsafe and garbage member data.

For now this has the limitations that it - requires the variant value to be mutable

Parameters:

• ​T (AnyType): The type of the value to get out.

Returns:

ref: The internal data represented as a Pointer[T].

is_type_supported

static is_type_supported[T: AnyType]() -> Bool

Check if a type can be used by the Variant.

Example:

from std.utils import Variant

def takes_variant(mut arg: Variant) raises:
    if arg.is_type_supported[Float64]():
        arg = Float64(1.5)

def main() raises:
    var x = Variant[Int, Float64](1)
    takes_variant(x)
    if x.isa[Float64]():
        print(x[Float64]) # 1.5

For example, the Variant[Int, Bool] permits Int and Bool.

Parameters:

• ​T (AnyType): The type of the value to check support for.

Returns:

Bool: True if type T is supported by the Variant.

destroy_with

destroy_with[T: Movable](deinit self, destroy_func: def(var T) -> None)

Destroy a value contained in this Variant in-place using a caller provided destructor function.

This method can be used to destroy linear types in a Variant in-place, without requiring that they be Movable.

This method will abort if this variant does not current contain an element of the specified type T.

Parameters:

• ​T (Movable): The element type the variant is expected to currently contain, and which will be destroyed by destroy_func.

Args:

• ​destroy_func (def(var T) -> None): Caller-provided destructor function for destroying an instance of T.

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