[PATCH] D112281: WIP: Support: Add Expected<T>::emplaceInto()

[Duncan P. N. Exon Smith via Phabricator via llvm-commits]

dexonsmith abandoned this revision.
dexonsmith added a comment.

In D112281#3079734 <https://reviews.llvm.org/D112281#3079734>, @dblaikie wrote:

> Yeah, similarly mixed feelings.
>
> Might be at some point we would put up with using a small selection of macros (google's done something similar, begrudgingly, to handle absl::Status) to make it possible to actually create a local variable with initialization from an Expected result - I think that requires statement expressions or other shenanigans to make that work, though, which is also annoying, on top of the macros.

A couple of other ideas that are implementable, but I'm not sure are really better than just doing the boilerplate:

  struct ChallengingType {
    ChallengingType(std::string S, bool Flag, int I, ...);
  };
  Expected<std::string> makeString();
  Expected<std::string> makeInt();
  
  void f1(bool Flag) {
    Optional<ChallengingType> CT;
  
    // Filters arguments for `Expected<T>`, joining all errors.
    // Calls `emplace()` with moved values if there are none.
    if (Error E = CT.emplaceWithExpected(makeString(), Flag, makeInt(), ...))
      return E;
  
    // As above, but generalized to types other than Optional.
    // Returns Expected<X> if object returns X.
    if (Error E = emplaceWithExpected(CT, makeString(), Flag, makeInt(), ...))
      return E;
  
    // As above, but instead of filtering for Expected, filters for a temporary
    // lambda returned by `delayExpected()`.
    if (Error E = emplaceWithDelayedExpected(
            CT,
            delayExpected([&]() {return makeString();}),
            Flag,
            delayExpected([&]() {return makeInt();}),
            ...))
      return E;
  
    // As above, but call any function, not just emplace.
    if (Error E = callWithDelayedExpected(
            delayExpected([&]() {return makeString();}),
            Flag,
            delayExpected([&]() {return makeInt();}),
            ...,
            [&CT](std::string S, bool Flag, int I, ...) {
              CT.emplace(std::move(S), Flag, I, ...);
            }))
      return E;
  
    // ... other code ...
  }

Anyway, I'll abandon this.


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