Conditional typing of fields.

Suppose I have the following struct: (in pseudo Mojo)

struct IntWrapper[N: Int]:
    @parameter
    if N <= 255:
        var inner: UInt8
    else:
        var inner: Int

struct IntWrapper[N: Int]:
    @parameter
    if N <= 255:
        var inner: UInt8
    else:
        var inner: Int

For usecases where I know that it is definite that the field inner cannot hold values larger than N, then is it possible to type the fields conditionally? You can do this easily in C++, impossible to do in Rust. What about Mojo?

1 Reply

benny•7mo ago

I don’t think this is currently supported by a do really like the idea, the closest is can think of is something like this

trait IntWrapper:
    …

struct UIntWrapper[N: Int](IntWrapper):
    var inner UInt8

struct SIntWrapper[N: Int](IntWrapper):
    var inner: Int

fn createWrapper[N: Int]() -> IntWrapper:
    @parameter
    if N > 255:
        return SIntWrapper[N]()
    return UIntWrapper[N]()

trait IntWrapper:
    …

struct UIntWrapper[N: Int](IntWrapper):
    var inner UInt8

struct SIntWrapper[N: Int](IntWrapper):
    var inner: Int

fn createWrapper[N: Int]() -> IntWrapper:
    @parameter
    if N > 255:
        return SIntWrapper[N]()
    return UIntWrapper[N]()

Alternatively you could have the struct take in a type as a parameter, along with N, and figure that out as a separate function The code will be a little messier but you don’t have to do type gymnastics with traits and can just return the passed in type

struct IntWrapper[N: Int, T: DType]():
    var inner: T

fn createWrapper[N: Int]() -> IntWrapper:
    @parameter
    if N > 255:
       return IntWrapper[N, Int]()
    return IntWrapper[N, UInt8]()

struct IntWrapper[N: Int, T: DType]():
    var inner: T

fn createWrapper[N: Int]() -> IntWrapper:
    @parameter
    if N > 255:
       return IntWrapper[N, Int]()
    return IntWrapper[N, UInt8]()

you have to define an init function but those are the two easiest ways I can think of implementing what your asking

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Conditional typing of fields.