Difficult Trait Conformance

I have been using mojo nightly mojo 2024.8.305 (f99b2e40). I need someone's help on why this doesn't work:

trait IterDataPipable:
    fn __next__[T: CollectionElement](ref[_] self) -> Optional[T]: pass

@value
struct ListDatapipe[T: CollectionElement](IterDataPipable[T]): 
    var src: List[T]
    var index: Int

    fn __init__(inout self, src: List[T]):
        self.src = src
        self.index = 0

    fn __init__(inout self, src: List[T], index: Int):
        self.src = src
        self.index = index

    fn __iter__(self) -> Self: return self

    fn __next__(ref[_] self) -> Optional[T]:
        self.index += 1
        if self.index == len(self.src) + 1:
            return Optional[T](None)

        return Optional[T](self.src[self.index - 1])

    fn __len__(self) -> Int: return len(self.src) - self.index

trait IterDataPipable:
    fn __next__[T: CollectionElement](ref[_] self) -> Optional[T]: pass

@value
struct ListDatapipe[T: CollectionElement](IterDataPipable[T]): 
    var src: List[T]
    var index: Int

    fn __init__(inout self, src: List[T]):
        self.src = src
        self.index = 0

    fn __init__(inout self, src: List[T], index: Int):
        self.src = src
        self.index = index

    fn __iter__(self) -> Self: return self

    fn __next__(ref[_] self) -> Optional[T]:
        self.index += 1
        if self.index == len(self.src) + 1:
            return Optional[T](None)

        return Optional[T](self.src[self.index - 1])

    fn __len__(self) -> Int: return len(self.src) - self.index

When running:

    var numbers = List(1,2,3,4,5)
    var pipe = ListDatapipe(numbers)
    # var pipe2 = AnotherPipe(pipe)
    # additional pipes
    var n = pipe.__next__()

    var numbers = List(1,2,3,4,5)
    var pipe = ListDatapipe(numbers)
    # var pipe2 = AnotherPipe(pipe)
    # additional pipes
    var n = pipe.__next__()

It keeps saying ListDatapipe doesnt conform to IterDataPipable. I've tried a bunch of different ways of trying to make this work, but none of them work well. I'm trying to get a generic IterDataPipable working so I can chain. Tried getting help from https://discordapp.com/channels/1087530497313357884/1269718914208759870 kapa. I'm convinced its an issue with https://github.com/modularml/mojo/issues/3215

3 Replies

Drakontas•2mo ago

Might not be implementable except by moving [T: CollectionElement] parameter from ListDatapipe to every function of ListDatapipe, but you still run into the issue with var src: List[T] definitely something that needs parameterizable traits to make this easy My mojo -v is 24.4, so ref[_] doesn't parse for me yet, feel free to add it back in:

trait IterDataPipable:
    fn __next__[T: CollectionElement](inout self) -> Optional[T]: pass

@value
struct ListDatapipe[T: CollectionElement](IterDataPipable):
    var src: List[T]
    var index: Int

    fn __init__(inout self, src: List[T]):
        self.src = src
        self.index = 0

    fn __init__(inout self, src: List[T], index: Int):
        self.src = src
        self.index = index

    fn __iter__(self) -> Self: return self

    fn __next__[T: CollectionElement](inout self) -> Optional[T]:
        self.index += 1
        if self.index == len(self.src) + 1:
            return Optional[T](None)

        return Optional[T](self.src[self.index - 1])

    fn __len__(self) -> Int: return len(self.src) - self.index

trait IterDataPipable:
    fn __next__[T: CollectionElement](inout self) -> Optional[T]: pass

@value
struct ListDatapipe[T: CollectionElement](IterDataPipable):
    var src: List[T]
    var index: Int

    fn __init__(inout self, src: List[T]):
        self.src = src
        self.index = 0

    fn __init__(inout self, src: List[T], index: Int):
        self.src = src
        self.index = index

    fn __iter__(self) -> Self: return self

    fn __next__[T: CollectionElement](inout self) -> Optional[T]:
        self.index += 1
        if self.index == len(self.src) + 1:
            return Optional[T](None)

        return Optional[T](self.src[self.index - 1])

    fn __len__(self) -> Int: return len(self.src) - self.index

I made __next__ in the trait take an inout self, copied the parameters onto __next__ impl, and then made that one inout self as well since it mutates self.index and then also removed the [T] from the required traits

Josiah•2mo ago

I still get:

error: no matching function in initialization
        return Optional[T](self.src[self.index - 1])
               ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
note: candidate not viable: expected at most 1 positional argument, got 2
from fastrl_mojo.data.datapipes.base import ListDatapipe
^note: candidate not viable: argument #1 cannot be converted from 'T' to 'T'
from fastrl_mojo.data.datapipes.base import ListDatapipe

error: no matching function in initialization
        return Optional[T](self.src[self.index - 1])
               ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
note: candidate not viable: expected at most 1 positional argument, got 2
from fastrl_mojo.data.datapipes.base import ListDatapipe
^note: candidate not viable: argument #1 cannot be converted from 'T' to 'T'
from fastrl_mojo.data.datapipes.base import ListDatapipe

Yeah sounds like parameterizable traits will be needed to make this actually work, but I appreciate the response!

Drakontas•2mo ago

oh i see, because the T in __next__ may not necessarily be the same T as the one from ListDatapipe yeah, this 100% needs parameterized traits, no way around it

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Difficult Trait Conformance