gistfile1.txt

utils/ownable.cairo:11:5: Cannot compare 'starkware.cairo.common.invoke.invoke.Return' and 'felt'.
    assert existing_owner = 0
    ^***********************^

ownable.cairo

%lang starknet

from starkware.cairo.common.invoke import invoke
from starkware.cairo.common.alloc import alloc

func initialize_owner{syscall_ptr : felt*}(owner_reader, owner_writer, initial_owner : felt):
    alloc_locals

    let (owner_reader_args : felt*) = alloc()
    let existing_owner = invoke(owner_reader, 0, owner_reader_args)
    assert existing_owner = 0

    let (owner_writer_args : felt*) = alloc()
    assert [owner_writer_args] = initialize_owner
    invoke(owner_writer, 1, owner_writer_args)
    return ()
end

user.cairo

%lang starknet

from starkware.cairo.common.registers import get_label_location
from starkware.starknet.common.syscalls import get_caller_address

from utils.ownable import initialize_owner

@storage_var
func _owner() -> (owner : felt):
end

@external
func initialize{syscall_ptr : felt*}():
    let (initial_owner) = get_caller_address()
    tempvar syscall_ptr = syscall_ptr
    let (owner_reader) = get_label_location(_owner.read)
    let (owner_writer) = get_label_location(_owner.write)
    initialize_owner(owner_reader, owner_writer, initial_owner)
    tempvar syscall_ptr = syscall_ptr
    return ()
end