Modular•9mo ago

Was there any specific reason for the current template syntax?

Mojo currently uses the [ and ] characters to specify templates instead of the < and > characters that we see in most other languages. Initially one would think it is to avoid the known ambiguity the latter characters can cause, however the current characters also make the grammar ambiguous.

fn function[t: Int]() -> Int:
    return t


fn main():
    let x = function[0]()  # function call

fn function[t: Int]() -> Int:
    return t


fn main():
    let x = function[0]()  # function call

@value
struct object[t: Int]:
    pass


fn main():
    let y = object[0]()  # struct instantiation

@value
struct object[t: Int]:
    pass


fn main():
    let y = object[0]()  # struct instantiation

from collections.vector import DynamicVector


@value
struct callable(CollectionElement):
    fn __call__(self) -> Int:
        return 1


fn main():
    let vector = DynamicVector[callable]()
    let z = vector[0]()  # member access + call

from collections.vector import DynamicVector


@value
struct callable(CollectionElement):
    fn __call__(self) -> Int:
        return 1


fn main():
    let vector = DynamicVector[callable]()
    let z = vector[0]()  # member access + call

In all of these cases the syntax is identifier[literal](), but every case parses into a different AST. So I wonder if there was any specific reason for choosing the current syntax or it was only for using different tokens than < and >.

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