Why a list is not printable?

Mojo
1

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Is this by design?

2

Hi @w32zhong !

For a quick fix, you can use

list = [1, 2, 3]
print(list.__str__())
print(list.__repr__())

For a longer answer:

So unfortunately this is a limitation from the compiler/language around what is called “conditional conformance”.

The function print expects its arguments to all implement the Writeable trait, however List does not implement Writeable. Why? If you look at the List struct, you’ll see that write_to (the required function for Writeable) is implemented - however the problem is this function is only enabled when T (the type that List holds), implements Representable.
We need some way in the language to express:

struct List[T: AnyType](
    Writeable if T implements Representable,
    # ...
)

This however is not possible ATM - but it is something we hope to have in the language soon!

4 Likes
3

Wait. Maybe I am still unfamiliar with the relevant knowledge to grasp the reason here.

I checked the docs: List | Modular

The List.__str__ also requires the list element to be “Representable”:

__str__[U: Representable & Copyable & Movable, //](self: List[U]) -> String

So why

print(list.__str__())

is okay then?

Why not just add Writable trait to the List? Because this won’t pass the compile-time check?