How do I do a typedef in mojo?

Question

I want to implement a class with a property whose type I can change globally at compile time. For example,

let ID_TYPE = Int

@value
struct MyStruct:
    var a: ID_TYPE

However, let seems not to be applicable to define types, as I receive the error invalid call to '__del__': argument #0 cannot be converted from 'Int' to 'Int'mojo

What can I do to avoid having to manually change all places where the ID_TYPE occurs in my code when I change e.g. from Int to Uint8?

Answer 1

Compile-time variables can be set via the alias keyword. Then, the example reads as follows:

alias ID_TYPE = Int

@value
struct MyStruct:
    var a: ID_TYPE

Answer 2

The best way to change it during compile time is to use compile-time defines, for example:

from sys.param_env import is_defined
from tensor import Tensor, TensorSpec

alias float_type: DType = DType.float32 if is_defined["FLOAT32"]() else DType.float64

let spec = TensorSpec(float_type, 256, 256)
var image = Tensor[float_type](spec)

then during compilation you can do:

mojo -D FLOAT_32 main.mojo

you can read more here: https://docs.modular.com/mojo/stdlib/sys/param_env.html