Hi all,
A few weeks ago we added mozilla::Result<V, E> to MFBT [0][1]. I was asked
to inform dev-platform about this, so here's a quick overview.
mozilla::Result<V, E> is based on Rust's Result type [2]. It contains
either a success value of type V or an error value of type E. For example,
a function Foo that returns an `int` on success or some `Error` (enum,
pointer, etc) on failure, would have this signature:
Result<int, Error> Foo();
mozilla::Ok is an empty struct that can be used when a function doesn't
return anything on success:
Result<Ok, Error> Bar() { ... return Ok(); }
The MOZ_TRY(expr) macro is similar to Rust's try! macro: if `expr` is an
error it propagates it to the caller, else it continues:
Result<V, E> Baz() { MOZ_TRY(Bar()); ... }
There's also a MOZ_TRY_VAR macro that can be used when you want to store
the return value on success. Result has isOk(), isErr(), unwrapOk(),
unwrapErr() methods that do what you'd expect. It also has the
MOZ_MUST_USE_TYPE annotation, so the static analysis builds will complain
if you ignore the return value of a function that returns Result.
Internally, Result uses mozilla::Variant, but there are some cases that can
be stored more efficiently. For instance, Result<Ok, Error&> just stores an
Error* pointer and Ok is represented as nullptr. This is more efficient and
will also make it easier to call functions that return Result from JIT
code. The documentation [3] has more info.
The long-term plan is to use Result in SpiderMonkey, to simplify our
error/OOM handling [4][5].
Many thanks to Jason Orendorff (jorendorff) for doing most of the work by
writing the initial Result patches, and to Jeff Walden (Waldo) for his
thorough code reviews.
Jan
[0] https://bugzilla.mozilla.org/show_bug.cgi?id=1283562
[1] https://searchfox.org/mozilla-central/source/mfbt/Result.h
[2] https://doc.rust-lang.org/std/result/
[3]
https://searchfox.org/mozilla-central/rev/cc2a84852bd4e6f6d8d4d5b17b8382bb5d005749/mfbt/Result.h#148-158
[4] https://bugzilla.mozilla.org/show_bug.cgi?id=1277368
[5] https://searchfox.org/mozilla-central/source/js/public/Result.h
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