Modular•12mo ago

Confusion with owned integers

I tried the replacing String from the example in the docs and got an error when running build. Docs example

fn set_fire(owned text: String) -> String:
    text += "🔥"
    return text

fn mojo():
    let a: String = "mojo"
    let b = set_fire(a)
    print(a)
    print(b)

mojo()

fn set_fire(owned text: String) -> String:
    text += "🔥"
    return text

fn mojo():
    let a: String = "mojo"
    let b = set_fire(a)
    print(a)
    print(b)

mojo()

My modified example

fn take_ownership_int(owned x: Int) -> Int:
    x += 5
    return x


fn main():
    let x = 10
    let y = take_ownership_int(x)
    print("x = " + String(x) + " and y = " + String(y))

fn take_ownership_int(owned x: Int) -> Int:
    x += 5
    return x


fn main():
    let x = 10
    let y = take_ownership_int(x)
    print("x = " + String(x) + " and y = " + String(y))

Error

❯ mojo build hello-world/take_ownership.mojo
/path/to/hello-world/take_ownership.mojo:9:26: error: use of uninitialized value 'x'
    print("x = " + String(x) + " and y = " + String(y))
                         ^
/path/to/hello-world/take_ownership.mojo:7:5: note: 'x' declared here
    let x = 10
    ^
mojo: error: failed to run the pass manager

❯ mojo build hello-world/take_ownership.mojo
/path/to/hello-world/take_ownership.mojo:9:26: error: use of uninitialized value 'x'
    print("x = " + String(x) + " and y = " + String(y))
                         ^
/path/to/hello-world/take_ownership.mojo:7:5: note: 'x' declared here
    let x = 10
    ^
mojo: error: failed to run the pass manager

--- If I replace Int with String it builds ok.

fn take_ownership_str(owned x: String) -> String:
    x += " mojo"
    return x


fn main():
    let hello = "hello"
    let hello_mojo = take_ownership_str(hello) 
    print("hello = " + hello + " and hello_mojo = " + hello_mojo)

fn take_ownership_str(owned x: String) -> String:
    x += " mojo"
    return x


fn main():
    let hello = "hello"
    let hello_mojo = take_ownership_str(hello) 
    print("hello = " + hello + " and hello_mojo = " + hello_mojo)

22 Replies

b4rdarian•12mo ago

Btw the error is similar to what I get if I call

take_ownership_str(hello^)
print(hello)

take_ownership_str(hello^)
print(hello)

᲼᲼•12mo ago

You are passing the ownership from one scope to another and thus this variable gets "consubed" by a function I think it's similar to Rust, but I'm not perfect not in Rust nor in Mojo, these are just my thoughts

ModularBot•12mo ago

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Stole•12mo ago

Yeah, this is correct You can't use the original one declared because that was consumed by your function call, albeit it did pass ownership back when you assigned it to y in your Int example Even though you do have that object under y, x is now invalid I think the compiler's behavior on this isn't fully fleshed out, or blocked on traits, and that's why it worked with String when it ideally shouldn't

rdickert•12mo ago

@b4rdarian I tried the above code but changed let x to var x:

def check_int():
    var x = 10
    let y = take_ownership_int(x)
    print("x = " + String(x))
    print("y = " + String(y))

check_int()

def check_int():
    var x = 10
    let y = take_ownership_int(x)
    print("x = " + String(x))
    print("y = " + String(y))

check_int()

This works and prints x=10 and y=15, but there is a compiler warning: 'x' was declared as a 'var' but never mutated, consider switching to a 'let' So making x mutable unblocks the code and clearly copies x as expected, but the compiler tells us to switch to let, which gives a compiler error. The docs note: "Currently, Mojo always makes a copy when a function returns a value." So I'm not quite sure where this leaves us.

b4rdarian•12mo ago

I think the compiler's behavior on this isn't fully fleshed out, or blocked on traits, and that's why it worked with String when it ideally shouldn't

The example with the String was taken from the docs. Either the docs are wrong or I am missing something about how owned should behave. Further down in the example it talks about how using ^ actually gives complete ownership, making the passed variable out of scope.

gabrieldemarmiesse•12mo ago

I think that without ^, if the argument is owned, it should just make a copy (which seems pretty logical to me). Maybe the behavior is changing? I wouldn't be surprised if it changed more in the future, they are working on it indeed.

b4rdarian•12mo ago

I suspect it relates to how Ints are actually stored under the hood with comparison to String objects. I played a bit with another example from the docs (https://docs.modular.com/mojo/programming-manual.html#transfer-arguments-owned-and) yesterday,

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b4rdarian•12mo ago

Added a __copyinit__ to UniquePointer to allow copying.

struct UniquePointer:
    var ptr: Int

    fn __init__(inout self, ptr: Int):
        self.ptr = ptr

    # Allows us to move ownership, eg owned_argument(UniquePointer(123)^)
    fn __moveinit__(inout self, owned existing: Self):
        self.ptr = existing.ptr

    # Allows us to pass by copy, eg owned_argument(UniquePointer(123))
    fn __copyinit__(inout self, existing: Self):
        self.ptr = existing.ptr

    fn __del__(owned self):
        self.ptr = 0

struct UniquePointer:
    var ptr: Int

    fn __init__(inout self, ptr: Int):
        self.ptr = ptr

    # Allows us to move ownership, eg owned_argument(UniquePointer(123)^)
    fn __moveinit__(inout self, owned existing: Self):
        self.ptr = existing.ptr

    # Allows us to pass by copy, eg owned_argument(UniquePointer(123))
    fn __copyinit__(inout self, existing: Self):
        self.ptr = existing.ptr

    fn __del__(owned self):
        self.ptr = 0

From the example

fn take_ptr(owned p: UniquePointer):
    print_no_newline("take_ptr: ")
    print(p.ptr)


fn use_ptr(borrowed p: UniquePointer):
    print_no_newline("use_ptr: ")
    print(p.ptr)

fn take_ptr(owned p: UniquePointer):
    print_no_newline("take_ptr: ")
    print(p.ptr)


fn use_ptr(borrowed p: UniquePointer):
    print_no_newline("use_ptr: ")
    print(p.ptr)

Last call will fail as expected if uncommented.

fn owned_and_borrowed_example():
    let p = UniquePointer(1234)
    use_ptr(p)
    take_ptr(p)
    use_ptr(p)
    take_ptr(p ^)
    # This will cause a compile error as `p` is no longer in scope
    # The `^` operator ends the lifetime of the value being binded to `p`.
    # use_ptr(p)

fn owned_and_borrowed_example():
    let p = UniquePointer(1234)
    use_ptr(p)
    take_ptr(p)
    use_ptr(p)
    take_ptr(p ^)
    # This will cause a compile error as `p` is no longer in scope
    # The `^` operator ends the lifetime of the value being binded to `p`.
    # use_ptr(p)

These will fail compilation if the the commented lines are uncommented

fn owned_and_borrowed_int():
    let x: Int = 1234
    use_int(x)
    take_int(x)
    # use_int(x)


fn owned_and_borrowed_bool():
    let x: Bool = False
    use_bool(x)
    take_bool(x)
    # use_bool(x)


fn owned_and_borrowed_float():
    let x: Float32 = 1234.0
    use_float(x)
    take_float(x)
    # use_float(x)

fn owned_and_borrowed_int():
    let x: Int = 1234
    use_int(x)
    take_int(x)
    # use_int(x)


fn owned_and_borrowed_bool():
    let x: Bool = False
    use_bool(x)
    take_bool(x)
    # use_bool(x)


fn owned_and_borrowed_float():
    let x: Float32 = 1234.0
    use_float(x)
    take_float(x)
    # use_float(x)

ModularBot•12mo ago

Congrats @b4rdarian, you just advanced to level 1!

b4rdarian•12mo ago

I think the docs need updating / clarification. I believe it relates to stack vs heap. So copying only works for objects stored in the heap.

Weichen-16小時的自由人•12mo ago

I'm encountering the same problem. I'm using modular playground. The version of mojo kernel should be v0.4.0 at the time.
According to Argument mutability and ownership I modified the function from the example in the docs.
My modified example:

fn add_owned(owned x: Int, owned y: Int) -> Int:
    x += 10
    y += 20
    return x + y

fn add_owned(owned x: Int, owned y: Int) -> Int:
    x += 10
    y += 20
    return x + y

Situation 1: a and b are declared as var

fn main_1():
    var a: Int = 100
    var b: Int = 200
    print("a: ", a, "b: ", b)
    let c = add_owned(a, b)
    print("c: ", c)
    print("a: ", a, "b: ", b)
   
main_1()

fn main_1():
    var a: Int = 100
    var b: Int = 200
    print("a: ", a, "b: ", b)
    let c = add_owned(a, b)
    print("c: ", c)
    print("a: ", a, "b: ", b)
   
main_1()

The result:

a:  100 b:  200
c:  330
a:  100 b:  200

a:  100 b:  200
c:  330
a:  100 b:  200

It works.
Situation 2 a and b are declared as let

fn main_2():
    let a: Int = 100
    let b: Int = 200
    print("a: ", a, "b: ", b)
    let c = add_owned(a, b)
    print("c: ", c)
    print("a: ", a, "b: ", b)
   
main_2()

fn main_2():
    let a: Int = 100
    let b: Int = 200
    print("a: ", a, "b: ", b)
    let c = add_owned(a, b)
    print("c: ", c)
    print("a: ", a, "b: ", b)
   
main_2()

The result:

expression failed to parse (no further compiler diagnostics)

expression failed to parse (no further compiler diagnostics)

It looks like Mojo didn't make copies of a and b separately.

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Weichen-16小時的自由人•12mo ago

So, did you mean it is not a bug?

b4rdarian•12mo ago

I can't really say. It might be, all I am saying is that it doesn't work as it's described in the docs for primitive types.

b4rdarian•12mo ago

Btw I am also basing my understanding on rust documentation on ownership https://doc.rust-lang.org/book/ch04-01-what-is-ownership.html#ownership-and-functions

What is Ownership? - The Rust Programming Language

b4rdarian•12mo ago

In that integers are copied rather than owned. You might be right and it's a bug. I am on an M2 MacBook.

❯ mojo --version
mojo 0.4.0 (9e33b013)

❯ mojo --version
mojo 0.4.0 (9e33b013)

VSCode's mojo extension is not picking up on the error. Only when trying to compile.

rdickert•12mo ago

@b4rdarian great research. I think this must be on the right track. This is probably worth filing an issue on the repo

b4rdarian•12mo ago

Good call. Will do.

b4rdarian•12mo ago

Apparently I am not the first to notice. There is already an issue open https://github.com/modularml/mojo/issues/747. I can see @Weichen-威辰 has added his experience to it.

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b4rdarian•12mo ago

I will do the same.

Weichen-16小時的自由人•12mo ago

Thanks for the link. It helps me to get a better understanding of ownership.

ModularBot•12mo ago

Congrats @Weichen-威辰, you just advanced to level 1!

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Confusion with owned integers