Re: [llvm-dev] RFC: Modernizing our use of auto

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Re: [llvm-dev] RFC: Modernizing our use of auto

Richard Smith via cfe-dev
On Dec 16, 2018, at 11:44 AM, Stephen Kelly via llvm-dev <[hidden email]> wrote:

>
> On 25/11/2018 14:43, Stephen Kelly via llvm-dev wrote:
>> However this is a proposal for more modern thinking regarding the permissiveness of auto in LLVM codebases.
>> Currently the rule on the use of auto is here:
>
> Hi,
>
> Thanks for the input on this topic, which has appeared here on the mailing list, on the phab review, and on IRC.
>
> Almost all respondants generally expressed the claim "The type must be obvious if auto is used", though as I wrote before the guide uses auto in context that the type is not obvious:
>
> for (const auto &Val : Container) { observe(Val); }
>
> It seems that the respondants wish for 'almost never auto'. Fair enough, if the existing practice supports that.
>
> There is one problem though, or rather ~56,000 problems. That's how many uses of auto there are in the entire codebase currently, according to someone on IRC.

I find this to be a helpful line of argument.  We should, as a community, decide what the right thing is regardless of the existing art.  As you say, the current patches going in have been arbitrary, so making an argument based on what is in the code base isn’t particularly informative.

> Do those uses conform to the guide? If they don't, then should the guide be updated? Are the types there 'obvious’?

If/when we revise the policy, then it would make sense for non-conforming uses of auto to be changed.  However, I don’t think that actually making a widespread change would be high priority...

> How did all of those uses get into the codebase? Does it indicate that the guide is not followed, or does it indicate that the guide is too subjective, or that maybe the 'the type must be obvios' guide does not reflect the 'reality at the coalface' anymore? Should those uses of auto be changed?

My understanding is that there has been no widely understood or accepted policy, so different coders and reviewers are doing different things.

> How is a new contributor to react to any of that? What are the real criteria that we can use to determine where auto will cause a patch to be rejected? Does it only depend on who you get as a reviewer?

Right now, it is quite arbitrary.  This is a bug, not a feature.

> Here is a quote from this thread from Chris and supported by Chandler and Quentin at least:
>
> > Perhaps the rule came be rewritten more generally, as
> > in “auto should generally only be used when the inferred
> > type is obvious from context, e.g. as a result of a cast
> > like dyn_cast or as a result of constructing a value with
> > a constructor containing a type name.”?
>
> Is it reasonable to have that as a rule if there are ~12000 uses which break that rule?

Yes.  If you’d like to make progress on this, I think you should start by building consensus.  It seems like there is widespread enough acknowledgement that the current state of things is broken, but there is no concrete proposal for a coding standards change.  Please prepare a patch so we can discuss it.

-Chris


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Re: [llvm-dev] RFC: Modernizing our use of auto

Richard Smith via cfe-dev


On 31/12/2018 04:54, Chris Lattner wrote:
On Dec 16, 2018, at 11:44 AM, Stephen Kelly via llvm-dev [hidden email] wrote:
On 25/11/2018 14:43, Stephen Kelly via llvm-dev wrote:
However this is a proposal for more modern thinking regarding the permissiveness of auto in LLVM codebases.
Currently the rule on the use of auto is here:
Hi,

Thanks for the input on this topic, which has appeared here on the mailing list, on the phab review, and on IRC.

Almost all respondants generally expressed the claim "The type must be obvious if auto is used", though as I wrote before the guide uses auto in context that the type is not obvious:

for (const auto &Val : Container) { observe(Val); }

It seems that the respondants wish for 'almost never auto'. Fair enough, if the existing practice supports that.

There is one problem though, or rather ~56,000 problems. That's how many uses of auto there are in the entire codebase currently, according to someone on IRC.
I find this to be a helpful line of argument.


Given what you wrote below, maybe you are missing a negation somewhere in this sentence?


We should, as a community, decide what the right thing is regardless of the existing art.


The existing art is part of 'the community deciding what to do'.


And yes, I think it makes sense to 'standardize existing practice' where possible.


How did all of those uses get into the codebase? Does it indicate that the guide is not followed, or does it indicate that the guide is too subjective, or that maybe the 'the type must be obvios' guide does not reflect the 'reality at the coalface' anymore? Should those uses of auto be changed?
My understanding is that there has been no widely understood or accepted policy, so different coders and reviewers are doing different things.


And different contexts within LLVM are fine with auto, but seem to get campaigning from other parts who have a different interpretation of the guideline:


https://reviews.llvm.org/D33672#inline-475812


How is a new contributor to react to any of that? What are the real criteria that we can use to determine where auto will cause a patch to be rejected? Does it only depend on who you get as a reviewer?
Right now, it is quite arbitrary.  This is a bug, not a feature.


Do we have some idea of who is interested in fixing the bug? It can't be just one person fixing it - this is a community issue. You've suggested that the guideline needs an update, and I've already suggested an update. Is it only the two of us? How can we proceed?


Here is a quote from this thread from Chris and supported by Chandler and Quentin at least:

Perhaps the rule came be rewritten more generally, as
in “auto should generally only be used when the inferred
type is obvious from context, e.g. as a result of a cast
like dyn_cast or as a result of constructing a value with
a constructor containing a type name.”?
Is it reasonable to have that as a rule if there are ~12000 uses which break that rule?

    
If you’d like to make progress on this, I think you should start by building consensus.


Well, I'm trying to find out what the positions people have are, but even though there are so many existing usages of auto, this thread is not getting responses from the people who put them there. So, the code says one thing, and the guideline says arguably the same thing, but people have alternative interpretations of the guideline. But at least the responses here are not really representative.

For me that means I'm not able to get my clang-query features (https://steveire.wordpress.com/2018/11/11/future-developments-in-clang-query/) upstream because I'm getting reviews which say "remove auto, here and everywhere in this file." in


 https://reviews.llvm.org/D54408?id=173770#inline-480255


That's a bit of a difficult review comment, given the ways it is already used throughout the code.


It seems like there is widespread enough acknowledgement that the current state of things is broken, but there is no concrete proposal for a coding standards change.  Please prepare a patch so we can discuss it.


I made a proposal in my initial mail in this thread. See the end of the email: https://lists.llvm.org/pipermail/llvm-dev/2018-November/127953.html

Phab is not well-suited to discussion like this, so we should probably keep it on the mailing list for now.


But, here's some updated thinking:


New guidelines should

* Be easy to follow
* Have some consistency
* Be modern
* Be welcoming (or at least non-hostile) to newcomers
* Standardize existing practice in LLVM
* Achieve a consensus of support about the spirit of the guideline (consensus is not unanimity) and be acceptable to people who dislike auto

Can we agree on that much to start?


On the Phab review, some people expressed that they liked the examples of when auto is acceptable. Here is an updated attempt at guideline text for that section:



```
Some are advocating a policy of "almost always ``auto``" in C++11, however LLVM
uses a more moderate stance. Don't "almost always" use ``auto``, but it may be used where either the Concept or the type is obvious from the context. Here are
some cases where use of ``auto`` would make sense:

* Where the type appears elsewhere in the line (eg a dyn_cast)
* Where the variable name or initializing expression provides enough information (`auto SR = getSourceRange()`)
* Where the context makes the *Concept* obvious, even if the type is not obvious (eg, where the instance is only used as an iterator, or in an algorithm as a container-like concept, or only with a validity check, or an AST Matcher).

Exceptions may arise, but they should only arise in exceptional cases. If the case is not exceptional, apply the guidelines in review discussion.
```


The most important thing here is that it does not accept your proposal that 'the type must be obvious'. Instead, it recognizes that `auto` is really "an unspecified concept" - unspecified only because we can't specify the concept in C++ yet.

However, the point/concern seems to be that readers of code should know the instance may be used in its local scope.

That's why these guidelines would allow `auto Ctors` in


llvm::Optional<std::pair<std::string, MatcherCtor>>
 getNodeConstructorType(ASTNodeKind targetType) {
   const auto &Ctors = RegistryData->nodeConstructors();
   auto It = llvm::find_if(
       Ctors, [targetType](const NodeConstructorMap::value_type &Ctor) {
         return Ctor.first.isSame(targetType);
       });
   if (It == Ctors.end())
     return llvm::None;
   return It->second;
 }
 

because `Ctors` is obviously the Container *concept*, and knowing exactly what type it is is not necessary in the local context.

However, in the below code it would probably not be ok because we're calling methods on the instance which opens up more possibilities (is it a base interface? etc):


void SomeClass::foo(int input)
{
    auto Ctors = getCtors(input);

    m_widgets = Ctors->calculate();
}

Here, `Ctors` is definitely not a Container concept, we don't know what kind of concept it is, so we should know the type by seeing it typed in the code.

Another example from earlier in the thread:


 template <typename BaseT, typename DerivedT>
 void registerIfNodeMatcher(...) {
   auto NodeKind = ASTNodeKind::getFromNodeKind<DerivedT>();
   if (!NodeKind.isNone())
     NodeCtors[NodeKind] = std::make_pair(MatcherName, Descriptor);
 }

Here, `NodeKind` is used as an Optional (or Maybe) *concept*. All we do is a validity check. So, `auto` should be allowed here.

This 'the concept must be obvious' guideline is also what allows the use of `auto` for iterators.


What do people think of "Either the Concept or the type should be obvious from the context" as a baseline guideline?


Thanks,

Stephen.



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Re: [llvm-dev] RFC: Modernizing our use of auto

Richard Smith via cfe-dev


On Sat, Jan 5, 2019 at 11:37 AM Stephen Kelly via cfe-dev <[hidden email]> wrote:


On 31/12/2018 04:54, Chris Lattner wrote:
On Dec 16, 2018, at 11:44 AM, Stephen Kelly via llvm-dev [hidden email] wrote:
On 25/11/2018 14:43, Stephen Kelly via llvm-dev wrote:
However this is a proposal for more modern thinking regarding the permissiveness of auto in LLVM codebases.
Currently the rule on the use of auto is here:
Hi,

Thanks for the input on this topic, which has appeared here on the mailing list, on the phab review, and on IRC.

Almost all respondants generally expressed the claim "The type must be obvious if auto is used", though as I wrote before the guide uses auto in context that the type is not obvious:

for (const auto &Val : Container) { observe(Val); }

It seems that the respondants wish for 'almost never auto'. Fair enough, if the existing practice supports that.

There is one problem though, or rather ~56,000 problems. That's how many uses of auto there are in the entire codebase currently, according to someone on IRC.
I find this to be a helpful line of argument.


Given what you wrote below, maybe you are missing a negation somewhere in this sentence?


We should, as a community, decide what the right thing is regardless of the existing art.


The existing art is part of 'the community deciding what to do'.


And yes, I think it makes sense to 'standardize existing practice' where possible.


How did all of those uses get into the codebase? Does it indicate that the guide is not followed, or does it indicate that the guide is too subjective, or that maybe the 'the type must be obvios' guide does not reflect the 'reality at the coalface' anymore? Should those uses of auto be changed?
My understanding is that there has been no widely understood or accepted policy, so different coders and reviewers are doing different things.


And different contexts within LLVM are fine with auto, but seem to get campaigning from other parts who have a different interpretation of the guideline:


https://reviews.llvm.org/D33672#inline-475812


How is a new contributor to react to any of that? What are the real criteria that we can use to determine where auto will cause a patch to be rejected? Does it only depend on who you get as a reviewer?
Right now, it is quite arbitrary.  This is a bug, not a feature.


Do we have some idea of who is interested in fixing the bug? It can't be just one person fixing it - this is a community issue. You've suggested that the guideline needs an update, and I've already suggested an update. Is it only the two of us? How can we proceed?


Here is a quote from this thread from Chris and supported by Chandler and Quentin at least:

Perhaps the rule came be rewritten more generally, as
in “auto should generally only be used when the inferred
type is obvious from context, e.g. as a result of a cast
like dyn_cast or as a result of constructing a value with
a constructor containing a type name.”?
Is it reasonable to have that as a rule if there are ~12000 uses which break that rule?

    
If you’d like to make progress on this, I think you should start by building consensus.


Well, I'm trying to find out what the positions people have are, but even though there are so many existing usages of auto, this thread is not getting responses from the people who put them there. So, the code says one thing, and the guideline says arguably the same thing, but people have alternative interpretations of the guideline. But at least the responses here are not really representative.


You're discounting some aspects:

- Code is written downstream without much consideration for the guideline and then upstreamed
- Review process isn't perfect: we let things slip, we don't always want to nitpick on things like `auto`, etc.
- Some developers just don't know the guidelines and because of the point above, some cases slip.
- People move back and forth between projects (i.e. LLVM is not their main project) and thus aren't focused on every details of the code guidelines (For instance I would review a patch today and not necessarily catch on every style aspect).

For the particular case of auto, there is an even more complex effect: since the intention is that "types should be obvious when reading the code", someone very familiar with some libraries will always unconsciously infer the returned type from API calls (from conventions, past experience, etc.), but someone who is less familiar with this area of the codebase would be quickly confused.
Now add to this the fact that the reviewers are (in general) familiar with the area they are reviewing, they aren't necessarily in a good position to catch immediately all the uses of auto that makes the types less obvious to other people.

The view you mentioned above is a good example of this: one reviewer points very accurately that "if you have to explain that in the variable's name, justify it in review comments" then "This really shouldn't be auto".
To me this is the important part of the guideline: make it easy for anyone to read the code and infer the types (ideally without extra steps like clicking in an IDE to debunk it).

On the other hand, another reviewer mentions that there is a specific pattern here and that "people get used to it very quickly when they start actively working on the codebase."
This is also a valid point, and while I'm very much in favor of the current guideline in general, there are pattern that are so much repetitive that it can be worthwhile to endorse them as the kind of things you need to know for this area.

The question in this case is if:

const auto ValueToCast = ....getAs<DefinedOrUnknownSVal>();

has an "obvious" type of being an Optional<DefinedOrUnknownSVal>.

I claim that this example is less about `auto` itself but rather a question about can you consider that `getAs` is such a "core" pattern of this area of the code base that we can accept as "common knowledge" that it always wrap the returned type in an optional.
And if we do, then the current guideline is actually fulfilled: "the type is already obvious from the context" (getAs being part of the context at this point).

There are other such example, for example I believe we can assume that all the standard STL algorithm are known and so in this case: 
 llvm::all_of(Container, [] (const auto &Value) { ...})
 
The use of auto in the lambda should be OK (assuming c++14 and the type of Container is obvious).


For me that means I'm not able to get my clang-query features (https://steveire.wordpress.com/2018/11/11/future-developments-in-clang-query/) upstream because I'm getting reviews which say "remove auto, here and everywhere in this file." in


 https://reviews.llvm.org/D54408?id=173770#inline-480255


That's a bit of a difficult review comment, given the ways it is already used throughout the code.


It seems like there is widespread enough acknowledgement that the current state of things is broken, but there is no concrete proposal for a coding standards change.  Please prepare a patch so we can discuss it.


I made a proposal in my initial mail in this thread. See the end of the email: https://lists.llvm.org/pipermail/llvm-dev/2018-November/127953.html

Phab is not well-suited to discussion like this, so we should probably keep it on the mailing list for now.


But, here's some updated thinking:


New guidelines should

* Be easy to follow
* Have some consistency
* Be modern
* Be welcoming (or at least non-hostile) to newcomers
* Standardize existing practice in LLVM
* Achieve a consensus of support about the spirit of the guideline (consensus is not unanimity) and be acceptable to people who dislike auto

Can we agree on that much to start?


It depends what you mean by "standardize existing practice in LLVM", this seems like a "guideline" to define the new guideline, more than a rule. I.e. if some area are "abusing" auto for example, this is not automatically a reason to standardize, this may just be an indication that some cleanup is needed there.

 


On the Phab review, some people expressed that they liked the examples of when auto is acceptable. Here is an updated attempt at guideline text for that section:



```
Some are advocating a policy of "almost always ``auto``" in C++11, however LLVM
uses a more moderate stance. Don't "almost always" use ``auto``, but it may be used where either the Concept or the type is obvious from the context. Here are
some cases where use of ``auto`` would make sense:

* Where the type appears elsewhere in the line (eg a dyn_cast)
* Where the variable name or initializing expression provides enough information (`auto SR = getSourceRange()`)
* Where the context makes the *Concept* obvious, even if the type is not obvious (eg, where the instance is only used as an iterator, or in an algorithm as a container-like concept, or only with a validity check, or an AST Matcher).


The later point isn't clear to me: even if you're only using the instance as an iterator, I may want to know what types the iterator is actually iterating on.
I'm not saying that the idea you have here is not desirable, just that the language used does not help me visualize what is / isn't OK: it does not fit your first criteria "Be easy to follow".
 


Exceptions may arise, but they should only arise in exceptional cases. If the case is not exceptional, apply the guidelines in review discussion.


This last sentence seems general to the full document rather than this section?
 

```


The most important thing here is that it does not accept your proposal that 'the type must be obvious'. Instead, it recognizes that `auto` is really "an unspecified concept" - unspecified only because we can't specify the concept in C++ yet.

However, the point/concern seems to be that readers of code should know the instance may be used in its local scope.

That's why these guidelines would allow `auto Ctors` in


llvm::Optional<std::pair<std::string, MatcherCtor>>
 getNodeConstructorType(ASTNodeKind targetType) {
   const auto &Ctors = RegistryData->nodeConstructors();
   auto It = llvm::find_if(
       Ctors, [targetType](const NodeConstructorMap::value_type &Ctor) {
         return Ctor.first.isSame(targetType);
       });
   if (It == Ctors.end())
     return llvm::None;
   return It->second;
 }
 

because `Ctors` is obviously the Container *concept*

It is only obvious after your read the following, but more importantly: why use auto here? It wouldn't hurt to write:    const NodeConstructorMap &Ctors = RegistryData->nodeConstructors(); 
I.e. auto does not make the code more readable to me in this case.
On the other hand, if the type of Ctors is explicit, the lambda argument type could be auto to me (it becomes obvious from the local context and the use of llvm::find_if).
 

, and knowing exactly what type it is is not necessary in the local context. 

However, in the below code it would probably not be ok because we're calling methods on the instance which opens up more possibilities (is it a base interface? etc):


void SomeClass::foo(int input)
{
    auto Ctors = getCtors(input);

    m_widgets = Ctors->calculate();
}

Here, `Ctors` is definitely not a Container concept, we don't know what kind of concept it is, so we should know the type by seeing it typed in the code.

Another example from earlier in the thread:


 template <typename BaseT, typename DerivedT>
 void registerIfNodeMatcher(...) {
   auto NodeKind = ASTNodeKind::getFromNodeKind<DerivedT>();
   if (!NodeKind.isNone())
     NodeCtors[NodeKind] = std::make_pair(MatcherName, Descriptor);
 }

Here, `NodeKind` is used as an Optional (or Maybe) *concept*. All we do is a validity check. So, `auto` should be allowed here.

This 'the concept must be obvious' guideline is also what allows the use of `auto` for iterators.


What do people think of "Either the Concept or the type should be obvious from the context" as a baseline guideline?

You forgot to add "and knowing exactly what type it is is not necessary in the local context" after "Concept", it seems that this is necessary for your definition.

I'm still fairly unconvinced, because the concept of "concept" seems too fuzzy to be applicable in such a guideline. What is a "Concept" other than a class that honor an API? How is your previous example " m_widgets = Ctors->calculate();" not just obvious that Ctors is an instance of a Concept that "can calculate a widget"?

Best,

-- 
Mehdi


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Re: [llvm-dev] RFC: Modernizing our use of auto

Richard Smith via cfe-dev


On 05/01/2019 21:56, Mehdi AMINI wrote:
You're discounting some aspects:


Not surprising! :)

Thanks for your contribution. Lots of things I hadn't considered here.



- Code is written downstream without much consideration for the guideline and then upstreamed


I hadn't considered this. Can you either quantify it, or point to some examples? I guess you're referring to some large commits which might not have been reviewed for particulars of style. Can you point me to some?


- Review process isn't perfect: we let things slip, we don't always want to nitpick on things like `auto`, etc.


Yes, even in that case, the submitter thinks the submitted code makes sense and the reviewer doesn't think it doesn't make sense. At least not so much that they'd block a commit because of it :).


- Some developers just don't know the guidelines and because of the point above, some cases slip.
- People move back and forth between projects (i.e. LLVM is not their main project) and thus aren't focused on every details of the code guidelines (For instance I would review a patch today and not necessarily catch on every style aspect).

For the particular case of auto, there is an even more complex effect: since the intention is that "types should be obvious when reading the code", someone very familiar with some libraries will always unconsciously infer the returned type from API calls (from conventions, past experience, etc.), but someone who is less familiar with this area of the codebase would be quickly confused.


Intermediate AST Matchers typically look like

 auto ConstrExpr = cxxConstructExpr(hasType(recordDecl(hasName(ClassName))));

`auto` is used exclusively when writing AST Matchers variables. No one complains about not reading the exact type of `ConstrExpr` in the code. And the people who work on those parts of the code tend to be the people most against all use of `auto`.


(You could write `StatementMatcher` instead of `auto` above if you wished, and `DeclarationMatcher`, `TypeMatcher` etc for other matchers. Unless you were familiar with all the implementation details of AST Matchers those type names wouldn't mean anything to you. In fact, they're not type names - they're just typedefs. Typedefs are what we used to use before we were able to use auto, when the actual type doesn't matter. You can look up the actual expansion of the typedef if you wish, but it won't make the above code clearer to you, because if you know what AST Matchers are, the exact type of `ConstrExpr` *does not matter*. Only the concept matters. It's an AST Matcher and it can be nested inside other AST Matchers and passed to Finder->addMatcher).


So, you don't see the exact type, and that doesn't affect your confusion about the above line. It could be `auto`, `StatementMatcher` or `internal::Matcher<Stmt>` without affecting your confusion. If you felt confusion about that line of code, it wasn't about the type.


I claim that this example is less about `auto` itself but rather a question about can you consider that `getAs` is such a "core" pattern of this area of the code base that we can accept as "common knowledge" that it always wrap the returned type in an optional.
And if we do, then the current guideline is actually fulfilled: "the type is already obvious from the context" (getAs being part of the context at this point).


We probably don't need to dive into the details here. You will notice though that the conclusion from the same people who discussed that revisited it in https://reviews.llvm.org/D54877#inline-484380 and concluded retrospectively that `auto` was fine.

 


There are other such example, for example I believe we can assume that all the standard STL algorithm are known and so in this case: 
 llvm::all_of(Container, [] (const auto &Value) { ...})
 
The use of auto in the lambda should be OK (assuming c++14 and the type of Container is obvious).


If you require that something about the type (either the type itself or the concept - ie being a container) or the type passed to the lambda be obvious, you can choose either one. My proposal for the LLVM guideline is not to make both the container and the lambda arg `auto`. But you could choose one or the other. I think you're agreeing with that.




For me that means I'm not able to get my clang-query features (https://steveire.wordpress.com/2018/11/11/future-developments-in-clang-query/) upstream because I'm getting reviews which say "remove auto, here and everywhere in this file." in


 https://reviews.llvm.org/D54408?id=173770#inline-480255


That's a bit of a difficult review comment, given the ways it is already used throughout the code.


It seems like there is widespread enough acknowledgement that the current state of things is broken, but there is no concrete proposal for a coding standards change.  Please prepare a patch so we can discuss it.


I made a proposal in my initial mail in this thread. See the end of the email: https://lists.llvm.org/pipermail/llvm-dev/2018-November/127953.html

Phab is not well-suited to discussion like this, so we should probably keep it on the mailing list for now.


But, here's some updated thinking:


New guidelines should

* Be easy to follow
* Have some consistency
* Be modern
* Be welcoming (or at least non-hostile) to newcomers
* Standardize existing practice in LLVM
* Achieve a consensus of support about the spirit of the guideline (consensus is not unanimity) and be acceptable to people who dislike auto

Can we agree on that much to start?


It depends what you mean by "standardize existing practice in LLVM", this seems like a "guideline" to define the new guideline, more than a rule. I.e. if some area are "abusing" auto for example, this is not automatically a reason to standardize, this may just be an indication that some cleanup is needed there.


Certainly! I tried to quantify the existing use of `auto` a bit in a previous email. Can you do a more-useful quantification?


 


On the Phab review, some people expressed that they liked the examples of when auto is acceptable. Here is an updated attempt at guideline text for that section:



```
Some are advocating a policy of "almost always ``auto``" in C++11, however LLVM
uses a more moderate stance. Don't "almost always" use ``auto``, but it may be used where either the Concept or the type is obvious from the context. Here are
some cases where use of ``auto`` would make sense:

* Where the type appears elsewhere in the line (eg a dyn_cast)
* Where the variable name or initializing expression provides enough information (`auto SR = getSourceRange()`)
* Where the context makes the *Concept* obvious, even if the type is not obvious (eg, where the instance is only used as an iterator, or in an algorithm as a container-like concept, or only with a validity check, or an AST Matcher).


The later point isn't clear to me: even if you're only using the instance as an iterator, I may want to know what types the iterator is actually iterating on.


Hmm, the existing guideline says that these iterators are ok as `auto`. I think you're saying that if there's a reason to have an exception for a particular iterator then you can have an exception for it. I agree with that! Are you thinking of some exceptional case?


I'm not saying that the idea you have here is not desirable, just that the language used does not help me visualize what is / isn't OK: it does not fit your first criteria "Be easy to follow".
 


Exceptions may arise, but they should only arise in exceptional cases. If the case is not exceptional, apply the guidelines in review discussion.


This last sentence seems general to the full document rather than this section?


Yes, I'm just trying to satisfy people who might be worried :).


 

```


The most important thing here is that it does not accept your proposal that 'the type must be obvious'. Instead, it recognizes that `auto` is really "an unspecified concept" - unspecified only because we can't specify the concept in C++ yet.

However, the point/concern seems to be that readers of code should know the instance may be used in its local scope.

That's why these guidelines would allow `auto Ctors` in


llvm::Optional<std::pair<std::string, MatcherCtor>>
 getNodeConstructorType(ASTNodeKind targetType) {
   const auto &Ctors = RegistryData->nodeConstructors();
   auto It = llvm::find_if(
       Ctors, [targetType](const NodeConstructorMap::value_type &Ctor) {
         return Ctor.first.isSame(targetType);
       });
   if (It == Ctors.end())
     return llvm::None;
   return It->second;
 }
 

because `Ctors` is obviously the Container *concept*

It is only obvious after your read the following, but more importantly: why use auto here? It wouldn't hurt to write:    const NodeConstructorMap &Ctors = RegistryData->nodeConstructors();


Why write `NodeConstructorMap`? That's not a type. If the reason for not using `auto` is that "the type must be obvious", then surely you're obliged to expand typedefs? `NodeConstructorMap` could be a std::vector for all you know...


The only thing you care about is that it's a container. It's a short function that uses the variable with `llvm::find_if`. Both the shortness and the use with `find_if` are relevant. This thing is a container, and I can't think of a container where the precise type matters in a small algorithmic function like this.


But the *Container-ness* of it does matter. And it is obvious. That's the concept that is obvious, so it should be allowed by the guidelines and should pass review.


I.e. auto does not make the code more readable to me in this case.


`NodeConstructorMap` doesn't "make the type obvious" either.


On the other hand, if the type of Ctors is explicit, the lambda argument type could be auto to me (it becomes obvious from the local context and the use of llvm::find_if).


Fair enough - I agree that this is a consequence of the guideline I propose, as I wrote above.


 

, and knowing exactly what type it is is not necessary in the local context. 

However, in the below code it would probably not be ok because we're calling methods on the instance which opens up more possibilities (is it a base interface? etc):


void SomeClass::foo(int input)
{
    auto Ctors = getCtors(input);

    m_widgets = Ctors->calculate();
}

Here, `Ctors` is definitely not a Container concept, we don't know what kind of concept it is, so we should know the type by seeing it typed in the code.

Another example from earlier in the thread:


 template <typename BaseT, typename DerivedT>
 void registerIfNodeMatcher(...) {
   auto NodeKind = ASTNodeKind::getFromNodeKind<DerivedT>();
   if (!NodeKind.isNone())
     NodeCtors[NodeKind] = std::make_pair(MatcherName, Descriptor);
 }

Here, `NodeKind` is used as an Optional (or Maybe) *concept*. All we do is a validity check. So, `auto` should be allowed here.

This 'the concept must be obvious' guideline is also what allows the use of `auto` for iterators.


What do people think of "Either the Concept or the type should be obvious from the context" as a baseline guideline?

You forgot to add "and knowing exactly what type it is is not necessary in the local context" after "Concept", it seems that this is necessary for your definition.


Correct! Thanks for the clarification. I mentioned earlier in the thread various things about use of `auto` being ok where the type is irrelevant, but indeed I didn't put that in the proposed guideline and your clarification is useful.



I'm still fairly unconvinced, because the concept of "concept" seems too fuzzy to be applicable in such a guideline. What is a "Concept" other than a class that honor an API? How is your previous example " m_widgets = Ctors->calculate();" not just obvious that Ctors is an instance of a Concept that "can calculate a widget"?


Indeed, but if you go down that path, you arrive at AAA, and I don't want to propose that. I said that in the first line of this thread, but expressing the reason for that in terms of Concepts clarifies the reason for that in my mind. So, thanks for the discussion!


There are more general concepts, such as Iterator (ie if you see this being used in a raw for loop or to compare with the result of an algorithm it is not surprising), Container/Range (ie, if this is passed directly to a range-capable algorithm such as find_if, it is not surprising), Optional/Maybe (if all we do in the local context of the `auto var` is check for validity and then pass the variable to something else - a function, a container etc - that is not surprising).

Isn't there something more 'core' about those than 'has a foo() method returning int'? Don't they occur very often?

Thanks,

Stephen.



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