What do keyword `as`?

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I am reading docs https://docs.harelang.org/bufio#read_line
Signature of function is next:
fn read_line(file: io::handle) ([]u8 | io::EOF | io::error);

But follow code is reqired using `as` keyword to get it's compiled:

use fmt;
use io;
use bufio;
use memio;
use strings;

export fn main() void = {

   let mystr = strings::toutf8("hello\nworld");
   let buf = memio::fixed(mystr);
   const line = bufio::read_line(&buf) as []u8;


It seems that it is forced to return the first tagged union, but I am
not sure about it.
Also how can I handle errors with `match` and `[]u8`?
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<CADVGtmv9_pvVwS2hkmzQWtGxW1DDkFQzcG-APvQ5YcPYwDwo4g@mail.gmail.com> (view parent)
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"as" is a "type assertion", it asserts that a tagged union has a
particular type and pulls out the value of that type. If the value has a
different type, a runtime assertion is raised. In this sample code, we
can know in advance that it will not fail because we construct a memio
reader which always has data available for the first read.

You can handle errors like so:

const line = match (bufio::read_line(&buf)) {
case let line: []u8 =>
	yield line;
case io::EOF =>
	// handle EOF
case let err: io::error =>
	// do something with err
// do something with line
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