DIP 1017--Add Bottom Type--Final Review
luckoverthere
luckoverthere at gmail.cm
Wed Jan 16 23:40:03 UTC 2019
On Wednesday, 16 January 2019 at 23:08:38 UTC, Paul Backus wrote:
> On Wednesday, 16 January 2019 at 11:23:18 UTC, Johan Engelen
> wrote:
>> This is just another example of using the bottom type to
>> signify `noreturn`, which is trivially done with an attribute
>> and doesn't need a new type. I'd like to see _other_ practical
>> uses of the bottom type.
>>
>> -Johan
>
> Here's an example I ran into while working on my `expectations`
> library. [1]
>
> Internally, an Expected!(T, E) is represented as a
> discriminated union containing either an "expected" value of
> type T or an error value of type E. I would like to implement a
> property method, `value`, that either returns the expected
> value if there is one, or throws an exception if there isn't
> (similar to Rust's `unwrap`).
>
> The obvious implementation looks like this:
>
> struct Expected(T, E) {
> private SumType!(T, E) data;
>
> @property T value() {
> return data.match!(
> (T val) => val,
> (E err) { throw new Exception(err.to!string); }
> );
> }
> }
>
> However, this will not compile, because the second lambda is
> inferred to have a return type of void, and void is not
> implicitly convertible to T. Instead, I have to resort to the
> following ugly workaround:
>
> @property T value() {
> try {
> return data.tryMatch!(
> (T val) => val
> );
> } catch (MatchException _) {
> data.tryMatch!(
> (E err) => throw new Exception(err.to!string)
> );
> }
> }
>
> If the second lambda were inferred to have a return type of
> Tbottom, none of this would be necessary. The first version
> would Just Work™.
>
> [1] http://expectations.dub.pm
Also a simpler work around to your problem:
struct Expected(T, E) {
private SumType!(T, E) data;
@property T value() {
return data.match!(
(T val) => val,
(E err) { throw new Exception(err.to!string); return
T.init; }
);
}
}
Or depending on what you are doing even just using an if
statement if you only have 2 types like that.
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