Store Reference in struct

Is there an example on how to create a struct with a Reference field?

17 Replies

I saw a couple examples from Chris here if it's helpful https://github.com/modularml/mojo/discussions/1568#discussioncomment-8251023

GitHub

Inferred Mutability: Reachability-based reference lifetime/mutabili...

Summary of idea: The borrow checker shall track the reachability from one reference to another reference within the function (whenever feasible). Inter-procedurally, the type signature may associat...

TilliFe•9mo ago

Is someone able to provide a real example of a struct with a reference as a field? This would be greatly appreciated. 🙂 (The examples by Chris does not specify what thisLifetime actually is in an instantiation)

Michael K•9mo ago

@value
struct RefString[L: MutLifetime]:
    var s: Reference[String, __mlir_attr.`1: i1`, L]


fn main():
    var a = String("hello")
    let b = String("world")

    let r = Reference(a)
    let rs = RefString(r)
    rs.s[] += b

    print(a)

@value
struct RefString[L: MutLifetime]:
    var s: Reference[String, __mlir_attr.`1: i1`, L]


fn main():
    var a = String("hello")
    let b = String("world")

    let r = Reference(a)
    let rs = RefString(r)
    rs.s[] += b

    print(a)

gabrieldemarmiesse•9mo ago

In this example, if the struct rs is returned, I wonder how it works, since the string it's pointing to is on the stack and will be destroyed when the function returns

TilliFe•9mo ago

Sure this works. But what if we want to use other structs than Strings? E.g. a Tensor or something? Then what is L?

gabrieldemarmiesse•9mo ago

I think it's the same

TilliFe•9mo ago

I t is not: @value struct RefTensor[L: MutLifetime]: var tensor_ref: Reference[Tensor, __mlir_attr.1: i1, L] fn main(): var a = TensorDType.float32 var r = Reference(a) var rs = RefTensor(r) # Error: cannot construct 'RefTensor[?]' from 'Reference[Tensor[f32], 1, *"a`0"]' value in 'var' initializer

benny•9mo ago

i’ve had trouble getting References working well, idk that they are ready for production code yet

Michael K•9mo ago

struct RefTensor[L: MutLifetime]:
    var tensor_ref: Reference[Tensor, __mlir_attr.1: i1, L]

struct RefTensor[L: MutLifetime]:
    var tensor_ref: Reference[Tensor, __mlir_attr.1: i1, L]

should be

struct RefTensor[L: MutLifetime]:
    var tensor_ref: Reference[Tensor[DType.float32], __mlir_attr.1: i1, L]

struct RefTensor[L: MutLifetime]:
    var tensor_ref: Reference[Tensor[DType.float32], __mlir_attr.1: i1, L]

Tensor was missing its type parameter.

TilliFe•9mo ago

crazy times

ModularBot•9mo ago

Congrats @TilliFe, you just advanced to level 3!

TilliFe•9mo ago

thank you

Michael K•9mo ago

I think this is what Reference is designed to fix. If I understand your question correctly, this example demonstrates how that works .

@value
struct RefString[L: MutLifetime]:
    var s: Reference[String, __mlir_attr.`1: i1`, L]

fn build_string_reference[
    L: MutLifetime
](s: Reference[String, __mlir_attr.`1: i1`, L]) -> RefString[L]:
    return RefString(s)


fn main():
    var a = String("hello")
    let b = String("world")
    let rs = build_string_reference(Reference(a))

    rs.s[] += b
    print(rs.s[])

@value
struct RefString[L: MutLifetime]:
    var s: Reference[String, __mlir_attr.`1: i1`, L]

fn build_string_reference[
    L: MutLifetime
](s: Reference[String, __mlir_attr.`1: i1`, L]) -> RefString[L]:
    return RefString(s)


fn main():
    var a = String("hello")
    let b = String("world")
    let rs = build_string_reference(Reference(a))

    rs.s[] += b
    print(rs.s[])

I think the problem you are describing is what References and Lifetimes are designed to manage. The lifetime of a gets shared with its references and prevents the immediate destruction of a after its last use. I altered the last line so a is not used and you can see the result still prints correctly because the string has not been destroyed.

gabrieldemarmiesse•9mo ago

I had in my mind something like this, wondering how it would work:

@value
struct RefString[L: MutLifetime]:
    var s: Reference[String, __mlir_attr.`1: i1`, L]

fn some_factory_func() -> RefString:
    var a = String("hello")
    let b = String("world")

    let r = Reference(a)
    let rs = RefString(r)
    rs.s[] += b

    return rs


fn main():
    print(some_factory_func().s[])

@value
struct RefString[L: MutLifetime]:
    var s: Reference[String, __mlir_attr.`1: i1`, L]

fn some_factory_func() -> RefString:
    var a = String("hello")
    let b = String("world")

    let r = Reference(a)
    let rs = RefString(r)
    rs.s[] += b

    return rs


fn main():
    print(some_factory_func().s[])

But that can't work because when calling some_factory_func, it's not possible to specify a lifetime yet. (i get

/projects/open_source/mojo-stdlib-extensions/trying_stuff.mojo:5:27: error: 'RefString' expects 1 input parameter, but 0 were specified, fn some_factory_func() -> RefString:

) Furthermore, the variable a is on the stack, thus it will be destroyed when some_factory_func returns.

ModularBot•9mo ago

Congrats @gabrieldemarmiesse, you just advanced to level 8!

gabrieldemarmiesse•9mo ago

I guess that maybe it would work if a was on the heap and not on the stack? But I'm unsure on how to do this with Mojo

nickfury3398•9mo ago

Is there some way to explicitly pass in L? I would like to have a RefString in the field of another struct

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Store Reference in struct