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<p class="MsoNormal"><span lang="EN-US">David Alayachew<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Wed Jun 14 13:00:06 UTC 2023<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">> Hello Rémi,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">> <o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">> To start off, Nothing is only going to take you a short distance. Nothing works decently enough for the type T. But for the Exception, Nothing will not work as a type parameter because the type parameter it is supposed
to be matching is E, and E extends Exception. Nothing does not extend Exception (presumably), and even if it did, this would be a constantly growing problem that we do not want to put into the language. What if you need R to extend List\<String>, for example?<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">This is a flawed understanding of the question that was asked. A 'Bottom type' ('Nothing' here) is a type that extends every other type and is not instantiatable.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> <o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Brian Goetz<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Wed Jun 14 17:50:53 UTC 2023<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">> [...]<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">> What you are probably reacting to here is "but why does Fail have to say T, it doesn't use it." And the answer is: "get over that, and then you're done."
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">> You are trying to invent a new generics feature to avoid putting a `T` you don't use in your Error declaration. But that T (and maybe E) are needed to unify the two under Result<T,E> -- just like in the second Haskell
example above. <o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">A valid reason in my opinion to want a `nothing` type is that it would solve the issue of unchecked casts your solution will introduce.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> <o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">```<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">static Result<String> getContentOfFile(Path path) {<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> // simple mock implementation<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> if (!Files.exist(path)) {<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> return new Fail<>(IOException("..."));<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> } else {<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> return new Succ<>("Data");<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> }<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">}<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">static Result<List<String>> getLinesOfFile1(Path path) {<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> return switch(getContentOfFile(path)) {<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> case Succ<String>(var contents) -> new Succ<>(contents.lines().toList());<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> case Fail<String> error -> (Result<List<String>>) error; // requires unchecked cast<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> }<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">}<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">static Result<List<String>> getLinesOfFile2(Path path) {<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> return switch(getContentOfFile(path)) {<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> case Succ<String>(var contents) -> new Succ<>(contents.lines().toList());<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> case Fail<String>(var error) -> new Fail<>(error); // requires redundant allocation<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> }<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">}<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">```<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">One maybe slightly better solution is to introduce an `into` method on `Fail` to hide the cast<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">```<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">record Fail<T>(Throwable t) extends Result<T> {<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> public <R> Fail<R> into() {<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> return (Fail<R>)this;<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> }<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">}<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">static Result<List<String>> getLinesOfFile3(Path path) {<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> return switch(getContentOfFile(path)) {<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> case Succ<String>(var contents) -> new Succ<>(contents.lines().toList());<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> case Fail<String> error -> error.into();<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> }<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">```<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">In the end, once we get value types, `getLinesOfFile2` wont really be an issue, (maybe someone even makes an extra optimization for sealed types where all subclasses are value types). Additionally, like Dan Smith said,
a bottom type would only be useful here if have declaration-site variance, which we don't have (*yet*).<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Kind regards<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Robbe Pincket<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
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