Parameterization: compile-time metaprogramming

Many languages have facilities for metaprogramming: that is, for writing code that generates or modifies code. Python has facilities for dynamic metaprogramming: features like decorators, metaclasses, and many more. These features make Python very flexible and productive, but since they're dynamic, they come with runtime overhead. Other languages have static or compile-time metaprogramming features, like C preprocessor macros and C++ templates. These can be limiting and hard to use.

To support Modular's work in AI, Mojo aims to provide powerful, easy-to-use metaprogramming with zero runtime cost. This compile-time metaprogramming uses the same language as runtime programs, so you don't have to learn a new language—just a few new features.

The main new feature is parameters. You can think of a parameter as a compile-time variable that becomes a runtime constant. This usage of "parameter" is probably different from what you're used to from other languages, where "parameter" and "argument" are often used interchangeably. In Mojo, "parameter" and "parameter expression" refer to compile-time values, and "argument" and "expression" refer to runtime values.

In Mojo, you can add parameters to a struct or function. You can also define named parameter expressions—aliases—that you can use as runtime constants.

Parameterized functions

To define a parameterized function, add parameters in square brackets ahead of the argument list. Each parameter is formatted just like an argument: a parameter name, followed by a colon and a type (which is required). In the following example, the function has a single parameter, count of type Int.

fn repeat[count: Int](msg: String):
    @parameter
    for i in range(count):
        print(msg)

fn repeat[count: Int](msg: String):
    @parameter
    for i in range(count):
        print(msg)

The @parameter directive shown here causes the for loop to be evaluated at compile time. The directive only works if the loop limits are compile-time constants. Since count is a parameter, range(count) can be calculated at compile time.

Calling a parameterized function, you provide values for the parameters, just like function arguments:

repeat[3]("Hello")

repeat[3]("Hello")

Hello
Hello
Hello

Hello
Hello
Hello

The compiler resolves the parameter values during compilation, and creates a concrete version of the repeat[]() function for each unique parameter value. After resolving the parameter values and unrolling the loop, the repeat[3]() function would be roughly equivalent to this:

fn repeat_3(msg: String):
    print(msg)
    print(msg)
    print(msg)

fn repeat_3(msg: String):
    print(msg)
    print(msg)
    print(msg)

This doesn't represent actual code generated by the compiler. By the time parameters are resolved, Mojo code has already been transformed to an intermediate representation in MLIR.

If the compiler can't resolve all parameter values to constant values, compilation fails.

Parameters and generics

"Generics" refers to functions that can act on multiple types of values, or containers that can hold multiple types of values. For example, List, can hold different types of values, so you can have a list of Int values, or a list of String values).

In Mojo, generics use parameters to specify types. For example, List takes a type parameter, so a vector of integers is written List[Int]. So all generics use parameters, but not everything that uses parameters is a generic.

For example, the repeat[]() function in the previous section includes parameter of type Int, and an argument of type String. It's parameterized, but not generic. A generic function or struct is parameterized on type. For example, we could rewrite repeat[]() to take any type of argument that conforms to the Stringable trait:

fn repeat[MsgType: Stringable, count: Int](msg: MsgType):
    @parameter
    for i in range(count):
        print(str(msg))

# Must use keyword parameter for `count`
repeat[count=2](42)

fn repeat[MsgType: Stringable, count: Int](msg: MsgType):
    @parameter
    for i in range(count):
        print(str(msg))

# Must use keyword parameter for `count`
repeat[count=2](42)

42
42

42
42

This updated function takes any Stringable type, so you can pass it an Int, String, or Bool value.

You can't pass the count as a positional keyword without also specifying MsgType. You can put // after MsgType to specify that it's always inferred by the argument. Now you can pass the following parameter count positionally:

fn repeat[MsgType: Stringable, //, count: Int](msg: MsgType):
    @parameter
    for i in range(count):
        print(str(msg))

# MsgType is always inferred, so first positional keyword `2` is passed to `count`
repeat[2](42)

fn repeat[MsgType: Stringable, //, count: Int](msg: MsgType):
    @parameter
    for i in range(count):
        print(str(msg))

# MsgType is always inferred, so first positional keyword `2` is passed to `count`
repeat[2](42)

42
42

42
42

Mojo's support for generics is still early. You can write generic functions like this using traits and parameters. You can also write generic collections like List and Dict. If you're interested in learning how these types work, you can find the source code for the standard library collection types on GitHub.

Parameterized structs

You can also add parameters to structs. You can use parameterized structs to build generic collections. For example, a generic array type might include code like this:

from memory import UnsafePointer

struct GenericArray[ElementType: CollectionElement]:
    var data: UnsafePointer[ElementType]
    var size: Int

    fn __init__(out self, *elements: ElementType):
        self.size = len(elements)
        self.data = UnsafePointer[ElementType].alloc(self.size)
        for i in range(self.size):
            (self.data + i).init_pointee_move(elements[i])

    fn __del__(owned self):
        for i in range(self.size):
            (self.data + i).destroy_pointee()
        self.data.free()

    fn __getitem__(self, i: Int) raises -> ref [self] ElementType:
        if (i < self.size):
            return self.data[i]
        else:
            raise Error("Out of bounds")

from memory import UnsafePointer

struct GenericArray[ElementType: CollectionElement]:
    var data: UnsafePointer[ElementType]
    var size: Int

    fn __init__(out self, *elements: ElementType):
        self.size = len(elements)
        self.data = UnsafePointer[ElementType].alloc(self.size)
        for i in range(self.size):
            (self.data + i).init_pointee_move(elements[i])

    fn __del__(owned self):
        for i in range(self.size):
            (self.data + i).destroy_pointee()
        self.data.free()

    fn __getitem__(self, i: Int) raises -> ref [self] ElementType:
        if (i < self.size):
            return self.data[i]
        else:
            raise Error("Out of bounds")

This struct has a single parameter, ElementType, which is a placeholder for the data type you want to store in the array, sometimes called a type parameter. ElementType is typed as CollectionElement, which is a trait representing any type that can be copied and moved.

As with parameterized functions, you need to pass in parameter values when you use a parameterized struct. In this case, when you create an instance of GenericArray, you need to specify the type you want to store, like Int, or Float64. (This is a little confusing, because the parameter value you're passing in this case is a type. That's OK: a Mojo type is a valid compile-time value.)

You'll see that ElementType is used throughout the struct where you'd usually see a type name. For example, as the formal type for the elements in the constructor, and the return type of the __getitem__() method.

Here's an example of using GenericArray:

var array = GenericArray[Int](1, 2, 3, 4)
for i in range(array.size):
    print(array[i], end=" ")

var array = GenericArray[Int](1, 2, 3, 4)
for i in range(array.size):
    print(array[i], end=" ")

1 2 3 4

1 2 3 4

A parameterized struct can use the Self type to represent a concrete instance of the struct (that is, with all its parameters specified). For example, you could add a static factory method to GenericArray with the following signature:

struct GenericArray[ElementType: CollectionElement]:
    ...

    @staticmethod
    fn splat(count: Int, value: ElementType) -> Self:
        # Create a new array with count instances of the given value

struct GenericArray[ElementType: CollectionElement]:
    ...

    @staticmethod
    fn splat(count: Int, value: ElementType) -> Self:
        # Create a new array with count instances of the given value

Here, Self is equivalent to writing GenericArray[ElementType]. That is, you can call the splat() method like this:

GenericArray[Float64].splat(8, 0)

GenericArray[Float64].splat(8, 0)

The method returns an instance of GenericArray[Float64].

Conditional conformance

When creating a generic struct, you might want to define some methods that require extra features. For example, consider a collection like GenericArray that holds instances of CollectionElement. The CollectionElement trait only requires that the stored data type be copyable and movable. This imposes a lot of limitations: you can't implement a sort() method because you can't guarantee that the stored type supports the comparison operators; you can't write a useful __str__() or __repr__() dunder method because you can't guarantee that the stored type supports conversion to a string.

The answer to these issues is conditional conformance, which lets you define a method that requires additional features. You do this by defining the self value that has a more specific bound on one or more of its parameters.

For example, the following code defines a Container type that holds an instance of CollectionElement. It also defines a __str__() method that can only be called if the stored ElementType conforms to StringableCollectionElement:

@value
struct Container[ElementType: CollectionElement]:
    var element: ElementType

    def __str__[StrElementType: StringableCollectionElement, //](
            self: Container[StrElementType]) -> String:
        return str(self.element)

def use_container():
    float_container = Container(5)
    string_container = Container("Hello")
    print(float_container.__str__())
    print(string_container.__str__())

use_container()

@value
struct Container[ElementType: CollectionElement]:
    var element: ElementType

    def __str__[StrElementType: StringableCollectionElement, //](
            self: Container[StrElementType]) -> String:
        return str(self.element)

def use_container():
    float_container = Container(5)
    string_container = Container("Hello")
    print(float_container.__str__())
    print(string_container.__str__())

use_container()

5
Hello

5
Hello

Note the signature of the __str__() method, which declares the self argument with a more specific type. Specifically, it declares that it takes a Container with an ElementType that conforms to the StringableCollectionElement trait.

def __str__[StrElementType: StringableCollectionElement, //](
        self: Container[StrElementType]) -> String:

def __str__[StrElementType: StringableCollectionElement, //](
        self: Container[StrElementType]) -> String:

This trait must be a superset of ElementType's original trait: for example, StringableCollectionElement inherits from CollectionElement, so it includes all of requirements of the original trait.

Note that the use_container() function calls the __str__() method directly, rather than calling str(float_container). One current limitation of conditional conformance is that Mojo can't recognize the struct Container[Int] as conforming to Stringable, even though the __str__() method is implemented for any ElementType that's also Stringable.

Case study: the SIMD type

For a real-world example of a parameterized type, let's look at the SIMD type from Mojo's standard library.

Single instruction, multiple data (SIMD) is a parallel processing technology built into many modern CPUs, GPUs, and custom accelerators. SIMD allows you to perform a single operation on multiple pieces of data at once. For example, if you want to take the square root of each element in an array, you can use SIMD to parallelize the work.

Processors implement SIMD using low-level vector registers in hardware that hold multiple instances of a scalar data type. In order to use the SIMD instructions on these processors, the data must be shaped into the proper SIMD width (data type) and length (vector size). Processors may support 512-bit or longer SIMD vectors, and support many data types from 8-bit integers to 64-bit floating point numbers, so it's not practical to define all of the possible SIMD variations.

Mojo's SIMD type (defined as a struct) exposes the common SIMD operations through its methods, and makes the SIMD data type and size values parametric. This allows you to directly map your data to the SIMD vectors on any hardware.

Here's a cut-down (non-functional) version of Mojo's SIMD type definition:

struct SIMD[type: DType, size: Int]:
    var value: … # Some low-level MLIR stuff here

    # Create a new SIMD from a number of scalars
    fn __init__(out self, *elems: SIMD[type, 1]):  ...

    # Fill a SIMD with a duplicated scalar value.
    @staticmethod
    fn splat(x: SIMD[type, 1]) -> SIMD[type, size]: ...

    # Cast the elements of the SIMD to a different elt type.
    fn cast[target: DType](self) -> SIMD[target, size]: ...

    # Many standard operators are supported.
    fn __add__(self, rhs: Self) -> Self: ...

struct SIMD[type: DType, size: Int]:
    var value: … # Some low-level MLIR stuff here

    # Create a new SIMD from a number of scalars
    fn __init__(out self, *elems: SIMD[type, 1]):  ...

    # Fill a SIMD with a duplicated scalar value.
    @staticmethod
    fn splat(x: SIMD[type, 1]) -> SIMD[type, size]: ...

    # Cast the elements of the SIMD to a different elt type.
    fn cast[target: DType](self) -> SIMD[target, size]: ...

    # Many standard operators are supported.
    fn __add__(self, rhs: Self) -> Self: ...

So you can create and use a SIMD vector like this:

var vector = SIMD[DType.int16, 4](1, 2, 3, 4)
vector = vector * vector
for i in range(4):
    print(vector[i], end=" ")

var vector = SIMD[DType.int16, 4](1, 2, 3, 4)
vector = vector * vector
for i in range(4):
    print(vector[i], end=" ")

1 4 9 16

1 4 9 16

As you can see, a simple arithmetic operator like * applied to a pair of SIMD vector operates on the corresponding elements in each vector.

Defining each SIMD variant with parameters is great for code reuse because the SIMD type can express all the different vector variants statically, instead of requiring the language to pre-define every variant.

Because SIMD is a parameterized type, the self argument in its functions carries those parameters—the full type name is SIMD[type, size]. Although it's valid to write this out (as shown in the return type of splat()), this can be verbose, so we recommend using the Self type (from PEP673) like the __add__ example does.

Overloading on parameters

Functions and methods can be overloaded on their parameter signatures. The overload resolution logic filters for candidates according to the following rules, in order of precedence:

1. Candidates with the minimal number of implicit conversions (in both arguments and parameters).
2. Candidates without variadic arguments.
3. Candidates without variadic parameters.
4. Candidates with the shortest parameter signature.
5. Non-@staticmethod candidates (over @staticmethod ones, if available).

If there is more than one candidate after applying these rules, the overload resolution fails. For example:

@register_passable("trivial")
struct MyInt:
    """A type that is implicitly convertible to `Int`."""
    var value: Int

    @implicit
    fn __init__(out self, _a: Int):
        self.value = _a

fn foo[x: MyInt, a: Int]():
    print("foo[x: MyInt, a: Int]()")

fn foo[x: MyInt, y: MyInt]():
    print("foo[x: MyInt, y: MyInt]()")

fn bar[a: Int](b: Int):
    print("bar[a: Int](b: Int)")

fn bar[a: Int](*b: Int):
    print("bar[a: Int](*b: Int)")

fn bar[*a: Int](b: Int):
    print("bar[*a: Int](b: Int)")

fn parameter_overloads[a: Int, b: Int, x: MyInt]():
    # `foo[x: MyInt, a: Int]()` is called because it requires no implicit
    # conversions, whereas `foo[x: MyInt, y: MyInt]()` requires one.
    foo[x, a]()

    # `bar[a: Int](b: Int)` is called because it does not have variadic
    # arguments or parameters.
    bar[a](b)

    # `bar[*a: Int](b: Int)` is called because it has variadic parameters.
    bar[a, a, a](b)

parameter_overloads[1, 2, MyInt(3)]()

struct MyStruct:
    fn __init__(out self):
        pass

    fn foo(mut self):
        print("calling instance method")

    @staticmethod
    fn foo():
        print("calling static method")

fn test_static_overload():
    var a = MyStruct()
    # `foo(mut self)` takes precedence over a static method.
    a.foo()

@register_passable("trivial")
struct MyInt:
    """A type that is implicitly convertible to `Int`."""
    var value: Int

    @implicit
    fn __init__(out self, _a: Int):
        self.value = _a

fn foo[x: MyInt, a: Int]():
    print("foo[x: MyInt, a: Int]()")

fn foo[x: MyInt, y: MyInt]():
    print("foo[x: MyInt, y: MyInt]()")

fn bar[a: Int](b: Int):
    print("bar[a: Int](b: Int)")

fn bar[a: Int](*b: Int):
    print("bar[a: Int](*b: Int)")

fn bar[*a: Int](b: Int):
    print("bar[*a: Int](b: Int)")

fn parameter_overloads[a: Int, b: Int, x: MyInt]():
    # `foo[x: MyInt, a: Int]()` is called because it requires no implicit
    # conversions, whereas `foo[x: MyInt, y: MyInt]()` requires one.
    foo[x, a]()

    # `bar[a: Int](b: Int)` is called because it does not have variadic
    # arguments or parameters.
    bar[a](b)

    # `bar[*a: Int](b: Int)` is called because it has variadic parameters.
    bar[a, a, a](b)

parameter_overloads[1, 2, MyInt(3)]()

struct MyStruct:
    fn __init__(out self):
        pass

    fn foo(mut self):
        print("calling instance method")

    @staticmethod
    fn foo():
        print("calling static method")

fn test_static_overload():
    var a = MyStruct()
    # `foo(mut self)` takes precedence over a static method.
    a.foo()

foo[x: MyInt, a: Int]()
bar[a: Int](b: Int)
bar[*a: Int](b: Int)

foo[x: MyInt, a: Int]()
bar[a: Int](b: Int)
bar[*a: Int](b: Int)

Using parameterized types and functions

You can use parametric types and functions by passing values to the parameters in square brackets. For example, for the SIMD type above, type specifies the data type and size specifies the length of the SIMD vector (it must be a power of 2):

# Make a vector of 4 floats.
var small_vec = SIMD[DType.float32, 4](1.0, 2.0, 3.0, 4.0)

# Make a big vector containing 1.0 in float16 format.
var big_vec = SIMD[DType.float16, 32](1.0)

# Do some math and convert the elements to float32.
var bigger_vec = (big_vec+big_vec).cast[DType.float32]()

# You can write types out explicitly if you want of course.
var bigger_vec2 : SIMD[DType.float32, 32] = bigger_vec

print('small_vec type:', small_vec.element_type, 'length:', len(small_vec))
print('bigger_vec2 type:', bigger_vec2.element_type, 'length:', len(bigger_vec2))

# Make a vector of 4 floats.
var small_vec = SIMD[DType.float32, 4](1.0, 2.0, 3.0, 4.0)

# Make a big vector containing 1.0 in float16 format.
var big_vec = SIMD[DType.float16, 32](1.0)

# Do some math and convert the elements to float32.
var bigger_vec = (big_vec+big_vec).cast[DType.float32]()

# You can write types out explicitly if you want of course.
var bigger_vec2 : SIMD[DType.float32, 32] = bigger_vec

print('small_vec type:', small_vec.element_type, 'length:', len(small_vec))
print('bigger_vec2 type:', bigger_vec2.element_type, 'length:', len(bigger_vec2))

small_vec type: float32 length: 4
bigger_vec2 type: float32 length: 32

small_vec type: float32 length: 4
bigger_vec2 type: float32 length: 32

Note that the cast() method also needs a parameter to specify the type you want from the cast (the method definition above expects a target parametric value). Thus, just as the SIMD struct is a generic type definition, the cast() method is a generic method definition. At compile time, the compiler creates a concrete version of the cast() method with the target parameter bound to DType.float32.

The code above shows the use of concrete types (that is, the parameters are all bound to known values). But the major power of parameters comes from the ability to define parametric algorithms and types (code that uses the parameter values). For example, here's how to define a parametric algorithm with SIMD that is type- and width-agnostic:

from math import sqrt

fn rsqrt[dt: DType, width: Int](x: SIMD[dt, width]) -> SIMD[dt, width]:
    return 1 / sqrt(x)

var v = SIMD[DType.float16, 4](42)
print(rsqrt(v))

from math import sqrt

fn rsqrt[dt: DType, width: Int](x: SIMD[dt, width]) -> SIMD[dt, width]:
    return 1 / sqrt(x)

var v = SIMD[DType.float16, 4](42)
print(rsqrt(v))

[0.154296875, 0.154296875, 0.154296875, 0.154296875]

[0.154296875, 0.154296875, 0.154296875, 0.154296875]

Notice that the x argument is actually a SIMD type based on the function parameters. The runtime program can use the value of the parameters, because the parameters are resolved at compile-time before they are needed by the runtime program (but compile-time parameter expressions cannot use runtime values).

Parameter inference

The Mojo compiler can often infer parameter values, so you don't always have to specify them. For example, you can call the rsqrt() function defined above without any parameters:

var v = SIMD[DType.float16, 4](33)
print(rsqrt(v))

var v = SIMD[DType.float16, 4](33)
print(rsqrt(v))

[0.174072265625, 0.174072265625, 0.174072265625, 0.174072265625]

[0.174072265625, 0.174072265625, 0.174072265625, 0.174072265625]

The compiler infers its parameters based on the parametric v value passed into it, as if you wrote rsqrt[DType.float16, 4](v) explicitly.

Mojo can also infer the values of struct parameters from the arguments passed to a constructor or static method.

For example, consider the following struct:

@value
struct One[Type: StringableCollectionElement]:
    var value: Type

    fn __init__(out self, value: Type):
        self.value = value

def use_one():
    s1 = One(123)
    s2 = One("Hello")

@value
struct One[Type: StringableCollectionElement]:
    var value: Type

    fn __init__(out self, value: Type):
        self.value = value

def use_one():
    s1 = One(123)
    s2 = One("Hello")

Note that you can create an instance of One without specifying the Type parameter—Mojo can infer it from the value argument.

You can also infer parameters from a parameterized type passed to a constructor or static method:

struct Two[Type: StringableCollectionElement]:
    var val1: Type
    var val2: Type

    fn __init__(out self, one: One[Type], another: One[Type]):
        self.val1 = one.value
        self.val2 = another.value
        print(str(self.val1), str(self.val2))

    @staticmethod
    fn fire(thing1: One[Type], thing2: One[Type]):
        print("🔥", str(thing1.value), str(thing2.value))

def use_two():
    s3 = Two(One("infer"), One("me"))
    Two.fire(One(1), One(2))

use_two()

struct Two[Type: StringableCollectionElement]:
    var val1: Type
    var val2: Type

    fn __init__(out self, one: One[Type], another: One[Type]):
        self.val1 = one.value
        self.val2 = another.value
        print(str(self.val1), str(self.val2))

    @staticmethod
    fn fire(thing1: One[Type], thing2: One[Type]):
        print("🔥", str(thing1.value), str(thing2.value))

def use_two():
    s3 = Two(One("infer"), One("me"))
    Two.fire(One(1), One(2))

use_two()

infer me
🔥 1 2

infer me
🔥 1 2

Two takes a Type parameter, and its constructor takes values of type One[Type]. When constructing an instance of Two, you don't need to specify the Type parameter, since it can be inferred from the arguments.

Similarly, the static fire() method takes values of type One[Type], so Mojo can infer the Type value at compile time.

If you're familiar with C++, you may recognize this as similar to Class Template Argument Deduction (CTAD).

Optional parameters and keyword parameters

Just as you can specify optional arguments in function signatures, you can also define an optional parameter by giving it a default value.

You can also pass parameters by keyword, just like you can use keyword arguments. For a function or struct with multiple optional parameters, using keywords allows you to pass only the parameters you want to specify, regardless of their position in the function signature.

For example, here's a function with two parameters, each with a default value:

fn speak[a: Int = 3, msg: StringLiteral = "woof"]():
    print(msg, a)

fn use_defaults() raises:
    speak()             # prints 'woof 3'
    speak[5]()          # prints 'woof 5'
    speak[7, "meow"]()  # prints 'meow 7'
    speak[msg="baaa"]() # prints 'baaa 3'

fn speak[a: Int = 3, msg: StringLiteral = "woof"]():
    print(msg, a)

fn use_defaults() raises:
    speak()             # prints 'woof 3'
    speak[5]()          # prints 'woof 5'
    speak[7, "meow"]()  # prints 'meow 7'
    speak[msg="baaa"]() # prints 'baaa 3'

Recall that when a parametric function is called, Mojo can infer the parameter values. That is, it can use the parameter values attached to an argument value (see the sqrt[]() example above). If the parametric function also has a default value defined, then the inferred parameter type takes precedence.

For example, in the following code, we update the parametric speak[]() function to take an argument with a parametric type. Although the function has a default parameter value for a, Mojo instead uses the inferred a parameter value from the bar argument (as written, the default a value can never be used, but this is just for demonstration purposes):

@value
struct Bar[v: Int]:
    pass

fn speak[a: Int = 3, msg: StringLiteral = "woof"](bar: Bar[a]):
    print(msg, a)

fn use_inferred():
    speak(Bar[9]())  # prints 'woof 9'

@value
struct Bar[v: Int]:
    pass

fn speak[a: Int = 3, msg: StringLiteral = "woof"](bar: Bar[a]):
    print(msg, a)

fn use_inferred():
    speak(Bar[9]())  # prints 'woof 9'

As mentioned above, you can also use optional parameters and keyword parameters in a struct:

struct KwParamStruct[greeting: String = "Hello", name: String = "🔥mojo🔥"]:
    fn __init__(out self):
        print(greeting, name)

fn use_kw_params():
    var a = KwParamStruct[]()                 # prints 'Hello 🔥mojo🔥'
    var b = KwParamStruct[name="World"]()     # prints 'Hello World'
    var c = KwParamStruct[greeting="Hola"]()  # prints 'Hola 🔥mojo🔥'

struct KwParamStruct[greeting: String = "Hello", name: String = "🔥mojo🔥"]:
    fn __init__(out self):
        print(greeting, name)

fn use_kw_params():
    var a = KwParamStruct[]()                 # prints 'Hello 🔥mojo🔥'
    var b = KwParamStruct[name="World"]()     # prints 'Hello World'
    var c = KwParamStruct[greeting="Hola"]()  # prints 'Hola 🔥mojo🔥'

Mojo supports positional-only and keyword-only parameters, following the same rules as positional-only and keyword-only arguments.

Infer-only parameters

Sometimes you need to declare functions where parameters depend on other parameters. Because the signature is processed left to right, a parameter can only depend on a parameter earlier in the parameter list. For example:

fn dependent_type[dtype: DType, value: Scalar[dtype]]():
    print("Value: ", value)
    print("Value is floating-point: ", dtype.is_floating_point())

dependent_type[DType.float64, Float64(2.2)]()

fn dependent_type[dtype: DType, value: Scalar[dtype]]():
    print("Value: ", value)
    print("Value is floating-point: ", dtype.is_floating_point())

dependent_type[DType.float64, Float64(2.2)]()

Value:  2.2000000000000002
Value is floating-point:  True

Value:  2.2000000000000002
Value is floating-point:  True

You can't reverse the position of the dtype and value parameters, because value depends on dtype. However, because dtype is a required parameter, you can't leave it out of the parameter list and let Mojo infer it from value:

dependent_type[Float64(2.2)]() # Error!

dependent_type[Float64(2.2)]() # Error!

Infer-only parameters are a special class of parameters that are always either inferred from context or specified by keyword. Infer-only parameters are placed at the beginning of the parameter list, set off from other parameters by the // sigil:

fn example[type: CollectionElement, //, list: List[type]]()

fn example[type: CollectionElement, //, list: List[type]]()

Transforming dtype into an infer-only parameter solves this problem:

fn dependent_type[dtype: DType, //, value: Scalar[dtype]]():
    print("Value: ", value)
    print("Value is floating-point: ", dtype.is_floating_point())

dependent_type[Float64(2.2)]()

fn dependent_type[dtype: DType, //, value: Scalar[dtype]]():
    print("Value: ", value)
    print("Value is floating-point: ", dtype.is_floating_point())

dependent_type[Float64(2.2)]()

Value:  2.2000000000000002
Value is floating-point:  True

Value:  2.2000000000000002
Value is floating-point:  True

Because infer-only parameters are declared at the beginning of the parameter list, other parameters can depend on them, and the compiler will always attempt to infer the infer-only values from bound parameters or arguments.

There are sometimes cases where it's useful to specify an infer-only parameter by keyword. For example, the StringSlice type is parametric on origin:

struct StringSlice[is_mutable: Bool, //, origin: Origin[is_mutable]]: ...

struct StringSlice[is_mutable: Bool, //, origin: Origin[is_mutable]]: ...

Here, the StringSlice is_mutable parameter is infer-only. The value is usually inferred when you create an instance of StringSlice. Binding the is_mutable parameter by keyword lets you define a new type that's constrained to an immutable origin:

alias ImmutableStringSlice = StringSlice[is_mutable=False]

alias ImmutableStringSlice = StringSlice[is_mutable=False]

If the compiler can't infer the value of an infer-only parameter, and it's not specified by keyword, compilation fails.

Variadic parameters

Mojo also supports variadic parameters, similar to Variadic arguments:

struct MyTensor[*dimensions: Int]:
    pass

struct MyTensor[*dimensions: Int]:
    pass

Variadic parameters currently have some limitations that variadic arguments don't have:

• Variadic parameters must be homogeneous—that is, all the values must be the same type.

• The parameter type must be register-passable.

• The parameter values aren't automatically projected into a VariadicList, so you need to construct the list explicitly:

fn sum_params[*values: Int]() -> Int:
    alias list = VariadicList(values)
    var sum = 0
    for v in list:
        sum += v
    return sum

fn sum_params[*values: Int]() -> Int:
    alias list = VariadicList(values)
    var sum = 0
    for v in list:
        sum += v
    return sum

Variadic keyword parameters (for example, **kwparams) are not supported yet.

Parameter expressions are just Mojo code

A parameter expression is any code expression (such as a+b) that occurs where a parameter is expected. Parameter expressions support operators and function calls, just like runtime code, and all parameter types use the same type system as the runtime program (such as Int and DType).

Because parameter expressions use the same grammar and types as runtime Mojo code, you can use many "dependent type" features. For example, you might want to define a helper function to concatenate two SIMD vectors:

fn concat[ty: DType, len1: Int, len2: Int](
        lhs: SIMD[ty, len1], rhs: SIMD[ty, len2]) -> SIMD[ty, len1+len2]:

    var result = SIMD[ty, len1 + len2]()
    for i in range(len1):
        result[i] = SIMD[ty, 1](lhs[i])
    for j in range(len2):
        result[len1 + j] = SIMD[ty, 1](rhs[j])
    return result

var a = SIMD[DType.float32, 2](1, 2)
var x = concat(a, a)

print('result type:', x.element_type, 'length:', len(x))

fn concat[ty: DType, len1: Int, len2: Int](
        lhs: SIMD[ty, len1], rhs: SIMD[ty, len2]) -> SIMD[ty, len1+len2]:

    var result = SIMD[ty, len1 + len2]()
    for i in range(len1):
        result[i] = SIMD[ty, 1](lhs[i])
    for j in range(len2):
        result[len1 + j] = SIMD[ty, 1](rhs[j])
    return result

var a = SIMD[DType.float32, 2](1, 2)
var x = concat(a, a)

print('result type:', x.element_type, 'length:', len(x))

result type: float32 length: 4

result type: float32 length: 4

Note that the resulting length is the sum of the input vector lengths, and this is expressed with a simple + operation.

Powerful compile-time programming

While simple expressions are useful, sometimes you want to write imperative compile-time logic with control flow. You can even do compile-time recursion. For instance, here is an example "tree reduction" algorithm that sums all elements of a vector recursively into a scalar:

fn slice[ty: DType, new_size: Int, size: Int](
        x: SIMD[ty, size], offset: Int) -> SIMD[ty, new_size]:
    var result = SIMD[ty, new_size]()
    for i in range(new_size):
        result[i] = SIMD[ty, 1](x[i + offset])
    return result

fn reduce_add[ty: DType, size: Int](x: SIMD[ty, size]) -> Int:
    @parameter
    if size == 1:
        return int(x[0])
    elif size == 2:
        return int(x[0]) + int(x[1])

    # Extract the top/bottom halves, add them, sum the elements.
    alias half_size = size // 2
    var lhs = slice[ty, half_size, size](x, 0)
    var rhs = slice[ty, half_size, size](x, half_size)
    return reduce_add[ty, half_size](lhs + rhs)

var x = SIMD[DType.index, 4](1, 2, 3, 4)
print(x)
print("Elements sum:", reduce_add(x))

fn slice[ty: DType, new_size: Int, size: Int](
        x: SIMD[ty, size], offset: Int) -> SIMD[ty, new_size]:
    var result = SIMD[ty, new_size]()
    for i in range(new_size):
        result[i] = SIMD[ty, 1](x[i + offset])
    return result

fn reduce_add[ty: DType, size: Int](x: SIMD[ty, size]) -> Int:
    @parameter
    if size == 1:
        return int(x[0])
    elif size == 2:
        return int(x[0]) + int(x[1])

    # Extract the top/bottom halves, add them, sum the elements.
    alias half_size = size // 2
    var lhs = slice[ty, half_size, size](x, 0)
    var rhs = slice[ty, half_size, size](x, half_size)
    return reduce_add[ty, half_size](lhs + rhs)

var x = SIMD[DType.index, 4](1, 2, 3, 4)
print(x)
print("Elements sum:", reduce_add(x))

[1, 2, 3, 4]
Elements sum: 10

[1, 2, 3, 4]
Elements sum: 10

This makes use of the @parameter decorator to create a parametric if condition, which is an if statement that runs at compile-time. It requires that its condition be a valid parameter expression, and ensures that only the live branch of the if statement is compiled into the program. (This is similar to use of the @parameter decorator with a for loop shown earlier.)

Mojo types are just parameter expressions

While we've shown how you can use parameter expressions within types, type annotations can themselves be arbitrary expressions (just like in Python). Types in Mojo have a special metatype type, allowing type-parametric algorithms and functions to be defined.

For example, we can create a simplified Array that supports arbitrary types of elements (via the AnyTrivialRegType parameter):

from memory import UnsafePointer

struct Array[T: AnyTrivialRegType]:
    var data: UnsafePointer[T]
    var size: Int

    fn __init__(out self, size: Int, value: T):
        self.size = size
        self.data = UnsafePointer[T].alloc(self.size)
        for i in range(self.size):
            (self.data + i).init_pointee_copy(value)

    fn __getitem__(self, i: Int) -> T:
        return self.data[i]

    fn __del__(owned self):
        for i in range(self.size):
            (self.data + i).destroy_pointee()
        self.data.free()

var v = Array[Float32](4, 3.14)
print(v[0], v[1], v[2], v[3])

from memory import UnsafePointer

struct Array[T: AnyTrivialRegType]:
    var data: UnsafePointer[T]
    var size: Int

    fn __init__(out self, size: Int, value: T):
        self.size = size
        self.data = UnsafePointer[T].alloc(self.size)
        for i in range(self.size):
            (self.data + i).init_pointee_copy(value)

    fn __getitem__(self, i: Int) -> T:
        return self.data[i]

    fn __del__(owned self):
        for i in range(self.size):
            (self.data + i).destroy_pointee()
        self.data.free()

var v = Array[Float32](4, 3.14)
print(v[0], v[1], v[2], v[3])

Notice that the T parameter is being used as the formal type for the value arguments and the return type of the __getitem__() function. Parameters allow the Array type to provide different APIs based on the different use-cases.

There are many other cases that benefit from more advanced use of parameters. For example, you can execute a closure N times in parallel, feeding in a value from the context, like this:

fn parallelize[func: fn (Int) -> None](num_work_items: Int):
    # Not actually parallel: see the 'algorithm' module for real implementation.
    for i in range(num_work_items):
        func(i)

fn parallelize[func: fn (Int) -> None](num_work_items: Int):
    # Not actually parallel: see the 'algorithm' module for real implementation.
    for i in range(num_work_items):
        func(i)

Another example where this is important is with variadic generics, where an algorithm or data structure may need to be defined over a list of heterogeneous types such as for a tuple. Right now, this is not fully supported in Mojo and requires writing some MLIR by hand. In the future, this will be possible in pure Mojo.

alias: named parameter expressions

It is very common to want to name compile-time values. Whereas var defines a runtime value, we need a way to define a compile-time temporary value. For this, Mojo uses an alias declaration.

For example, the DType struct implements a simple enum using aliases for the enumerators like this (the actual DType implementation details vary a bit):

struct DType:
    var value : UI8
    alias invalid = DType(0)
    alias bool = DType(1)
    alias int8 = DType(2)
    alias uint8 = DType(3)
    alias int16 = DType(4)
    alias int16 = DType(5)
    ...
    alias float32 = DType(15)

struct DType:
    var value : UI8
    alias invalid = DType(0)
    alias bool = DType(1)
    alias int8 = DType(2)
    alias uint8 = DType(3)
    alias int16 = DType(4)
    alias int16 = DType(5)
    ...
    alias float32 = DType(15)

This allows clients to use DType.float32 as a parameter expression (which also works as a runtime value) naturally. Note that this is invoking the runtime constructor for DType at compile-time.

Types are another common use for aliases. Because types are compile-time expressions, it is handy to be able to do things like this:

alias Float16 = SIMD[DType.float16, 1]
alias UInt8 = SIMD[DType.uint8, 1]

var x: Float16 = 0  # Float16 works like a "typedef"

alias Float16 = SIMD[DType.float16, 1]
alias UInt8 = SIMD[DType.uint8, 1]

var x: Float16 = 0  # Float16 works like a "typedef"

Like var variables, aliases obey scope, and you can use local aliases within functions as you'd expect.

Fully-bound, partially-bound, and unbound types

A parametric type with its parameters specified is said to be fully-bound. That is, all of its parameters are bound to values. As mentioned before, you can only instantiate a fully-bound type (sometimes called a concrete type).

However, parametric types can be unbound or partially bound in some contexts. For example, you can alias a partially-bound type to create a new type that requires fewer parameters:

from collections import Dict

alias StringKeyDict = Dict[String, _]
var b = StringKeyDict[UInt8]()
b["answer"] = 42

from collections import Dict

alias StringKeyDict = Dict[String, _]
var b = StringKeyDict[UInt8]()
b["answer"] = 42

Here, StringKeyDict is a type alias for a Dict that takes String keys. The underscore _ in the parameter list indicates that the second parameter, V (the value type), is unbound. You specify the V parameter later, when you use StringKeyDict.

For example, given the following type:

struct MyType[s: String, i: Int, i2: Int, b: Bool = True]:
    pass

struct MyType[s: String, i: Int, i2: Int, b: Bool = True]:
    pass

It can appear in code in the following forms:

• Fully bound, with all of its parameters specified:

  MyType["Hello", 3, 4, True]

  MyType["Hello", 3, 4, True]

• Partially bound, with some but not all of its parameters specified:

  MyType["Hola", _, _, True]

  MyType["Hola", _, _, True]

• Unbound, with no parameters specified:

  MyType[_, _, _, _]

  MyType[_, _, _, _]

You can also use the star-underscore expression *_ to unbind an arbitrary number of positional parameters at the end of a parameter list.

# These two types are equivalent
MyType["Hello", *_]
MyType["Hello", _, _, _]

# These two types are equivalent
MyType["Hello", *_]
MyType["Hello", _, _, _]

The *_ expression specifically matches any parameters that can be specified by position (positional-only or positional-or-keyword). To unbind keyword-only parameters, use the double-star-underscore expression, **_, which matches any parameters that can be specified by keyword (positional-or-keyword or keyword-only).

@value
struct KeyWordStruct[pos_or_kw: Int, *, kw_only: Int = 10]:
    pass

# Unbind both pos_or_kw and kw_only parameters
fn use_kw_struct(k: KeyWordStruct[**_]):
    pass

def main():
    use_kw_struct(KeyWordStruct[10, kw_only=11]())

@value
struct KeyWordStruct[pos_or_kw: Int, *, kw_only: Int = 10]:
    pass

# Unbind both pos_or_kw and kw_only parameters
fn use_kw_struct(k: KeyWordStruct[**_]):
    pass

def main():
    use_kw_struct(KeyWordStruct[10, kw_only=11]())

When a parameter is explicitly unbound with the _, *_, or **_ expressions, you must specify a value for that parameter to use the type. Any default value from the original type declaration is ignored.

Partially-bound and unbound parametric types can be used in some contexts where the missing (unbound) parameters will be supplied later—such as in aliases and automatically parameterized functions.

Omitted parameters

Mojo also supports an alternate format for unbound parameter where the parameter is simply omitted from the expression:

# Partially bound
MyType["Hi there"]
# Unbound
MyType

# Partially bound
MyType["Hi there"]
# Unbound
MyType

This format differs from the explicit unbinding syntax described above in that the default values for omitted parameters are bound immediately. For example, the following expressions are equivalent:

MyType["Hi there"]
# equivalent to
MyType["Hi there", _, _, True] # Uses the default value for `b`

MyType["Hi there"]
# equivalent to
MyType["Hi there", _, _, True] # Uses the default value for `b`

This format is currently supported for backwards compatibility. We intend to deprecate this format in the future in favor of the explicit unbinding syntax.

Automatic parameterization of functions

Mojo supports "automatic" parameterization of functions. If a function argument type is a partially-bound or unbound type, the unbound parameters are automatically added as input parameters on the function. This is easier to understand with an example:

fn print_params(vec: SIMD[*_]):
    print(vec.type)
    print(vec.size)

var v = SIMD[DType.float64, 4](1.0, 2.0, 3.0, 4.0)
print_params(v)

fn print_params(vec: SIMD[*_]):
    print(vec.type)
    print(vec.size)

var v = SIMD[DType.float64, 4](1.0, 2.0, 3.0, 4.0)
print_params(v)

float64
4

float64
4

In the above example, the print_params function is automatically parameterized. The vec argument takes an argument of type SIMD[*_]. This is an unbound parameterized type—that is, it doesn't specify any parameter values for the type. Mojo treats the unbound parameters on vec as infer-only parameters on the function. This is roughly equivalent to the following codes:

fn print_params[t: DType, s: Int, //](vec: SIMD[t, s]):
    print(vec.type)
    print(vec.size)

fn print_params[t: DType, s: Int, //](vec: SIMD[t, s]):
    print(vec.type)
    print(vec.size)

When you call print_params() you must pass it a concrete instance of the SIMD type—that is, one with all of its parameters specified, like SIMD[DType.float64, 4]. The Mojo compiler infers the parameter values from the input argument.

With a manually parameterized function, you can access the input parameters by name (for example, t and s in the previous example). For an automatically parameterized function, you can access the parameters as attributes on the argument (for example, vec.type).

This ability to access a type's input parameters is not specific to automatically parameterized functions, you can use it anywhere. You can access the input parameters of a parameterized type as attributes on the type itself:

fn on_type():
    print(SIMD[DType.float32, 2].size) # prints 2

fn on_type():
    print(SIMD[DType.float32, 2].size) # prints 2

Or as attributes on an instance of the type:

fn on_instance():
    var x = SIMD[DType.int32, 2](4, 8)
    print(x.type) # prints int32

fn on_instance():
    var x = SIMD[DType.int32, 2](4, 8)
    print(x.type) # prints int32

You can even use this syntax in the function's signature to define a function's arguments and return type based on an argument's parameters. For example, if you want your function to take two SIMD vectors with the same type and size, you can write code like this:

fn interleave(v1: SIMD, v2: __type_of(v1)) -> SIMD[v1.type, v1.size*2]:
    var result = SIMD[v1.type, v1.size*2]()
    for i in range(v1.size):
        result[i*2] = SIMD[v1.type, 1](v1[i])
        result[i*2+1] = SIMD[v1.type, 1](v2[i])
    return result

var a = SIMD[DType.int16, 4](1, 2, 3, 4)
var b = SIMD[DType.int16, 4](0, 0, 0, 0)
var c = interleave(a, b)
print(c)

fn interleave(v1: SIMD, v2: __type_of(v1)) -> SIMD[v1.type, v1.size*2]:
    var result = SIMD[v1.type, v1.size*2]()
    for i in range(v1.size):
        result[i*2] = SIMD[v1.type, 1](v1[i])
        result[i*2+1] = SIMD[v1.type, 1](v2[i])
    return result

var a = SIMD[DType.int16, 4](1, 2, 3, 4)
var b = SIMD[DType.int16, 4](0, 0, 0, 0)
var c = interleave(a, b)
print(c)

[1, 0, 2, 0, 3, 0, 4, 0]

[1, 0, 2, 0, 3, 0, 4, 0]

As shown in the example, you can use the magic __type_of(x) call if you just want to match the type of an argument. In this case, it's more convenient and compact that writing the equivalent SIMD[v1.type, v1.size].

Automatic parameterization of parameters

You can also take advantage of automatic parameterization in a function's parameter list. For example:

fn foo[value: SIMD]():
    pass

# Equivalent to:
fn foo[type: DType, size: Int, //, value: SIMD[type, size]]():
    pass

fn foo[value: SIMD]():
    pass

# Equivalent to:
fn foo[type: DType, size: Int, //, value: SIMD[type, size]]():
    pass

Automatic parameterization with partially-bound types

Mojo also supports automatic parameterization: with partially-bound parameterized types (that is, types with some but not all of the parameters specified).

For example, suppose we have a Fudge struct with three parameters:

@value
struct Fudge[sugar: Int, cream: Int, chocolate: Int = 7](Stringable):
    fn __str__(self) -> String:
        return str("Fudge (") + str(sugar) + "," +
            str(cream) + "," + str(chocolate) + ")"

@value
struct Fudge[sugar: Int, cream: Int, chocolate: Int = 7](Stringable):
    fn __str__(self) -> String:
        return str("Fudge (") + str(sugar) + "," +
            str(cream) + "," + str(chocolate) + ")"

We can write a function that takes a Fudge argument with just one bound parameter (it's partially bound):

fn eat(f: Fudge[5, *_]):
    print("Ate " + str(f))

fn eat(f: Fudge[5, *_]):
    print("Ate " + str(f))

The eat() function takes a Fudge struct with the first parameter (sugar) bound to the value 5. The second and third parameters, cream and chocolate are unbound.

The unbound cream and chocolate parameters become implicit input parameters on the eat function. In practice, this is roughly equivalent to writing:

fn eat[cr: Int, ch: Int](f: Fudge[5, cr, ch]):
    print("Ate", str(f))

fn eat[cr: Int, ch: Int](f: Fudge[5, cr, ch]):
    print("Ate", str(f))

In both cases, we can call the function by passing in an instance with the cream and chocolate parameters bound:

eat(Fudge[5, 5, 7]())
eat(Fudge[5, 8, 9]())

eat(Fudge[5, 5, 7]())
eat(Fudge[5, 8, 9]())

Ate Fudge (5,5,7)
Ate Fudge (5,8,9)

Ate Fudge (5,5,7)
Ate Fudge (5,8,9)

If you try to pass in an argument with a sugar value other than 5, compilation fails, because it doesn't match the argument type:

eat(Fudge[12, 5, 7]())
# ERROR: invalid call to 'eat': argument #0 cannot be converted from 'Fudge[12, 5, 7]' to 'Fudge[5, 5, 7]'

eat(Fudge[12, 5, 7]())
# ERROR: invalid call to 'eat': argument #0 cannot be converted from 'Fudge[12, 5, 7]' to 'Fudge[5, 5, 7]'

You can also explicitly unbind individual parameters. This gives you more freedom in specifying unbound parameters.

For example, you might want to let the user specify values for sugar and chocolate, and leave cream constant. To do this, replace each unbound parameter value with a single underscore (_):

fn devour(f: Fudge[_, 6, _]):
    print("Devoured",  str(f))

fn devour(f: Fudge[_, 6, _]):
    print("Devoured",  str(f))

Again, the unbound parameters (sugar and chocolate) are added as implicit input parameters on the function. This version is roughly equivalent to the following code, where these two values are explicitly bound to the input parameters, su and ch:

fn devour[su: Int, ch: Int](f: Fudge[su, 6, ch]):
    print("Devoured", str(f))

fn devour[su: Int, ch: Int](f: Fudge[su, 6, ch]):
    print("Devoured", str(f))

You can also specify parameters by keyword, or mix positional and keyword parameters, so the following function is roughly equivalent to the previous one: the first parameter, sugar is explicitly unbound with the underscore character. The chocolate parameter is unbound using the keyword syntax, chocolate=_. And cream is explicitly bound to the value 6:

fn devour(f: Fudge[_, chocolate=_, cream=6]):
    print("Devoured",  str(f))

fn devour(f: Fudge[_, chocolate=_, cream=6]):
    print("Devoured",  str(f))

All three versions of the devour() function work with the following calls:

devour(Fudge[3, 6, 9]())
devour(Fudge[4, 6, 8]())

devour(Fudge[3, 6, 9]())
devour(Fudge[4, 6, 8]())

Devoured Fudge (3,6,9)
Devoured Fudge (4,6,8)

Devoured Fudge (3,6,9)
Devoured Fudge (4,6,8)

Legacy syntax (omitted parameters)

You can also specify an unbound or partially-bound type by omitting parameters: for example:

fn nibble(f: Fudge[5]):
    print("Ate", str(f))

nibble(Fudge[5, 4, 7]())

fn nibble(f: Fudge[5]):
    print("Ate", str(f))

nibble(Fudge[5, 4, 7]())

Ate Fudge (5,4,7)

Ate Fudge (5,4,7)

Here, Fudge[5] works like Fudge[5, *_] except in the handling of parameters with default values. Instead of discarding the default value of chocolate, Fudge[5] binds the default value immediately, making it equivalent to: Fudge[5, _, 7].

This means that the following code won't compile with the previous definition for the nibble() function, since it doesn't use the default value for chocolate:

nibble(Fudge[5, 5, 9]())
# ERROR: invalid call to 'nibble': argument #0 cannot be converted from 'Fudge[5, 5, 9]' to 'Fudge[5, 5, 7]'

nibble(Fudge[5, 5, 9]())
# ERROR: invalid call to 'nibble': argument #0 cannot be converted from 'Fudge[5, 5, 9]' to 'Fudge[5, 5, 7]'

TODO

Support for omitting unbound parameters will eventually be deprecated in favor of explicitly unbound parameters using _ and *_.

The rebind() builtin

One of the consequences of Mojo not performing function instantiation in the parser like C++ is that Mojo cannot always figure out whether some parametric types are equal and complain about an invalid conversion. This typically occurs in static dispatch patterns. For example, the following code won't compile:

fn take_simd8(x: SIMD[DType.float32, 8]):
    pass

fn generic_simd[nelts: Int](x: SIMD[DType.float32, nelts]):
    @parameter
    if nelts == 8:
        take_simd8(x)

fn take_simd8(x: SIMD[DType.float32, 8]):
    pass

fn generic_simd[nelts: Int](x: SIMD[DType.float32, nelts]):
    @parameter
    if nelts == 8:
        take_simd8(x)

The parser will complain:

error: invalid call to 'take_simd8': argument #0 cannot be converted from
'SIMD[f32, nelts]' to 'SIMD[f32, 8]'
        take_simd8(x)
        ~~~~~~~~~~^~~

error: invalid call to 'take_simd8': argument #0 cannot be converted from
'SIMD[f32, nelts]' to 'SIMD[f32, 8]'
        take_simd8(x)
        ~~~~~~~~~~^~~

This is because the parser fully type-checks the function without instantiation, and the type of x is still SIMD[f32, nelts], and not SIMD[f32, 8], despite the static conditional. The remedy is to manually "rebind" the type of x, using the rebind builtin, which inserts a compile-time assert that the input and result types resolve to the same type after function instantiation:

fn take_simd8(x: SIMD[DType.float32, 8]):
    pass

fn generic_simd[nelts: Int](x: SIMD[DType.float32, nelts]):
    @parameter
    if nelts == 8:
        take_simd8(rebind[SIMD[DType.float32, 8]](x))

fn take_simd8(x: SIMD[DType.float32, 8]):
    pass

fn generic_simd[nelts: Int](x: SIMD[DType.float32, nelts]):
    @parameter
    if nelts == 8:
        take_simd8(rebind[SIMD[DType.float32, 8]](x))

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