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Bug 1550108 - Add mapErr method to Result r=froydnj

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Adds an explicit error mapping function to Result, to simplify
using MOZ_TRY... macros.

Differential Revision: https://phabricator.services.mozilla.com/D47466

--HG--
extra : moz-landing-system : lando
This commit is contained in:
Doug Thayer 2019-09-27 22:15:35 +00:00
parent 076597dbc2
commit d27f7199e1
1 changed files with 32 additions and 0 deletions
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32
mfbt/Result.h
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@ -9,6 +9,7 @@
#ifndef mozilla_Result_h
#define mozilla_Result_h
#include <type_traits>
#include "mozilla/Alignment.h"
#include "mozilla/Assertions.h"
#include "mozilla/Attributes.h"
@ -433,6 +434,37 @@ class MOZ_MUST_USE_TYPE Result final {
return isOk() ? RetResult(f(unwrap())) : RetResult(unwrapErr());
}
/**
* Map a function V -> W over this result's error variant. If this result is
* a success, do not invoke the function and move the success over.
*
* Mapping over error values invokes the function to produce a new error
* value:
*
* // Map Result<V, int> to another Result<V, int>
* Result<V, int> res(5);
* Result<V, int> res2 = res.mapErr([](int x) { return x * x; });
* MOZ_ASSERT(res2.unwrapErr() == 25);
*
* // Map Result<V, const char*> to Result<V, size_t>
* Result<V, const char*> res("hello, map!");
* Result<size_t, E> res2 = res.mapErr(strlen);
* MOZ_ASSERT(res2.unwrapErr() == 11);
*
* Mapping over a success does not invoke the function and copies the error:
*
* Result<int, V> res(5);
* MOZ_ASSERT(res.isOk());
* Result<int, W> res2 = res.mapErr([](V v) { ... });
* MOZ_ASSERT(res2.isOk());
* MOZ_ASSERT(res2.unwrap() == 5);
*/
template <typename F>
auto mapErr(F f) -> Result<V, std::result_of_t<F(E)>> {
using RetResult = Result<V, std::result_of_t<F(E)>>;
return isOk() ? RetResult(unwrap()) : RetResult(f(unwrapErr()));
}
/**
* Given a function V -> Result<W, E>, apply it to this result's success value
* and return its result. If this result is an error value, then return a