Modular•4mo ago

Any way to work around that capturing closures cannot be materialized as runtime values?

I'm trying to implement converting data from an existing, possibly nested data structure into native Mojo collections like List, Dict etc. in a generic way. Here's an example for lists with one approach i've been trying:

@value
struct Column[T: CollectionElement]:
    var getter: fn (v: Vector, length: Int, offset: Int) capturing -> List[T]

    fn __init__[ChildType: CollectionElement](inout self: Column[List[ChildType]], child: Column[ChildType]):
        """Initialize a list column from a child column."""

        fn list_getter(v: Vector, length: Int, offset: Int) capturing -> List[List[ChildType]]:
            var l = List[List[ChildType]](capacity=length)
            for idx in range(length):
                l.append(child.get(v, length, offset + idx))
            return l
        self.getter = list_getter
    # ...
    fn get(self, v: Vector, length: Int, offset: Int) -> List[T]:
        return self.getter(v, length, offset)

@value
struct Column[T: CollectionElement]:
    var getter: fn (v: Vector, length: Int, offset: Int) capturing -> List[T]

    fn __init__[ChildType: CollectionElement](inout self: Column[List[ChildType]], child: Column[ChildType]):
        """Initialize a list column from a child column."""

        fn list_getter(v: Vector, length: Int, offset: Int) capturing -> List[List[ChildType]]:
            var l = List[List[ChildType]](capacity=length)
            for idx in range(length):
                l.append(child.get(v, length, offset + idx))
            return l
        self.getter = list_getter
    # ...
    fn get(self, v: Vector, length: Int, offset: Int) -> List[T]:
        return self.getter(v, length, offset)

The issue seems that list_getter needs to close over child in order to access it, but when I add capturing to the function, I get the following error:

TODO: capturing closures cannot be materialized as runtime values

TODO: capturing closures cannot be materialized as runtime values

The TODO seems to indicate that this might be supported in the future, but perhaps there's a way to work around the issue today? Any pointers appreciated.

6 Replies

ModularBot•4mo ago

Congrats @Sören, you just advanced to level 2!

Martin Dudek•2mo ago

Did you find a workaround for that? Facing the same limitation here. Thx

SörenOP•2mo ago

@Martin Dudek unfortunately not. I went with a different design approach in the end.

ModularBot•2mo ago

Congrats @Sören Brunk, you just advanced to level 3!

Martin Dudek•2mo ago

thx

msaelices•2mo ago

I also found the same issue. With no global variables, the capturing closure is the only way I've found to be able to modify values in callbacks, for a event dispatcher pattern.

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Any way to work around that capturing closures cannot be materialized as runtime values?