[dart-misc] Re: [analyzer-discuss] Functions including return statements with and without value

['Brian Wilkerson' via Dart Misc]

Moved discussion to '***@dartlang.org'.

On Wed, Mar 7, 2018 at 11:24 PM, Anatoly Pulyaevskiy <

myFunc() {
if (isSomething) return;
return "some value";
}
Analyzer warning: Functions can't include return statements both with and
without values.
As we know in JS this is not an issue and sometimes is even used as a
feature. Which is the case with Firebase Database JS client.
(example: https://firebase.google.com/docs/reference/js/
firebase.database.Reference#transaction)
As I'm working on a Dart package which wraps JS client I needed to come up
with a way to get this working.
Function _createTransactionHandler<T>(TransactionHandler<T> handler) {
// This is a workaround for analyzer which complains if a function
contains
// return statements with and without value.
void undefined() {}
// Firebase requires us to return JS `undefined` to indicate we want
// to abort the transaction.
return (currentData) {
final data = dartify(currentData);
final result = handler(data);
if (result._abort) return undefined(); // workaround continues...
return jsify(result.data);
};
}
Which works (with latest Dart2 SDK, 2.0.0-dev.32.0). But I was wondering
if there is any plans in Dart2 to statically restrict this kind of usage?
My gut feeling says that since `void` in Dart2 is more like a value of
special type (we can use it in generics like Future<void>) then my
workaround should be statically allowed, but I just wanted to confirm it
with the team.
Thanks!
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[Matan Lurey]

Funny you mentioned this: https://github.com/dart-lang/sdk/issues/32443

On Thu, Mar 8, 2018 at 7:36 AM 'Brian Wilkerson' via Dart Misc <

Post by 'Brian Wilkerson' via Dart Misc
On Wed, Mar 7, 2018 at 11:24 PM, Anatoly Pulyaevskiy <

myFunc() {
if (isSomething) return;
return "some value";
}
Analyzer warning: Functions can't include return statements both with and
without values.
As we know in JS this is not an issue and sometimes is even used as a
feature. Which is the case with Firebase Database JS client.
https://firebase.google.com/docs/reference/js/firebase.database.Reference#transaction
)
As I'm working on a Dart package which wraps JS client I needed to come
up with a way to get this working.
Function _createTransactionHandler<T>(TransactionHandler<T> handler) {
// This is a workaround for analyzer which complains if a function
contains
// return statements with and without value.
void undefined() {}
// Firebase requires us to return JS `undefined` to indicate we want
// to abort the transaction.
return (currentData) {
final data = dartify(currentData);
final result = handler(data);
if (result._abort) return undefined(); // workaround continues...
return jsify(result.data);
};
}
Which works (with latest Dart2 SDK, 2.0.0-dev.32.0). But I was wondering
if there is any plans in Dart2 to statically restrict this kind of usage?
My gut feeling says that since `void` in Dart2 is more like a value of
special type (we can use it in generics like Future<void>) then my
workaround should be statically allowed, but I just wanted to confirm it
with the team.
Thanks!
--
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[tatumizer-v0.2]

Can dart
return (void)0
?

[Anatoly Pulyaevskiy]

Thanks, subscribed. There is one more linked to it
(https://github.com/dart-lang/sdk/issues/32177) where ) left a comment with
this use case.

I also found an alternative way to achieve what I needed. Didn't think I
can create JS binding for undefined, but it works actually in both ddc and
dart2js:

@JS()
external get undefined;

Function _createTransactionHandler<T>(TransactionHandler<T> handler) {
return (currentData) {
final data = dartify(currentData);
final result = handler(data);
if (result._abort) return undefined;
return jsify(result.data);
};
}


I'll probably switch to this version as it seems more reliable.

Post by Matan Lurey
Funny you mentioned this: https://github.com/dart-lang/sdk/issues/32443
On Thu, Mar 8, 2018 at 7:36 AM 'Brian Wilkerson' via Dart Misc <

Post by 'Brian Wilkerson' via Dart Misc
On Wed, Mar 7, 2018 at 11:24 PM, Anatoly Pulyaevskiy <

myFunc() {
if (isSomething) return;
return "some value";
}
Analyzer warning: Functions can't include return statements both with
and without values.
As we know in JS this is not an issue and sometimes is even used as a
feature. Which is the case with Firebase Database JS client.
https://firebase.google.com/docs/reference/js/firebase.database.Reference#transaction
)
As I'm working on a Dart package which wraps JS client I needed to come
up with a way to get this working.
Function _createTransactionHandler<T>(TransactionHandler<T> handler) {
// This is a workaround for analyzer which complains if a function
contains
// return statements with and without value.
void undefined() {}
// Firebase requires us to return JS `undefined` to indicate we want
// to abort the transaction.
return (currentData) {
final data = dartify(currentData);
final result = handler(data);
if (result._abort) return undefined(); // workaround continues...
return jsify(result.data);
};
}
Which works (with latest Dart2 SDK, 2.0.0-dev.32.0). But I was wondering
if there is any plans in Dart2 to statically restrict this kind of usage?
My gut feeling says that since `void` in Dart2 is more like a value of
special type (we can use it in generics like Future<void>) then my
workaround should be statically allowed, but I just wanted to confirm it
with the team.
Thanks!
--
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To file a bug report or feature request, go to http://www.dartbug.com/new
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[Anatoly Pulyaevskiy]

Sorry about formatting issues, here is plain-text version of the same code
snippet:

@JS()
external get undefined;

Function _createTransactionHandler<T>(TransactionHandler<T> handler) {
return (currentData) {
final data = dartify(currentData);
final result = handler(data);
if (result._abort) return undefined;
return jsify(result.data);
};
}

Thanks, subscribed. There is one more linked to it (
https://github.com/dart-lang/sdk/issues/32177) where ) left a comment
with this use case.
I also found an alternative way to achieve what I needed. Didn't think I
can create JS binding for undefined, but it works actually in both ddc and
@JS()
external get undefined;
Function _createTransactionHandler<T>(TransactionHandler<T> handler) {
return (currentData) {
final data = dartify(currentData);
final result = handler(data);
if (result._abort) return undefined;
return jsify(result.data);
};
}
I'll probably switch to this version as it seems more reliable.

Post by Matan Lurey
Funny you mentioned this: https://github.com/dart-lang/sdk/issues/32443
On Thu, Mar 8, 2018 at 7:36 AM 'Brian Wilkerson' via Dart Misc <

Post by 'Brian Wilkerson' via Dart Misc
On Wed, Mar 7, 2018 at 11:24 PM, Anatoly Pulyaevskiy <

myFunc() {
if (isSomething) return;
return "some value";
}
Analyzer warning: Functions can't include return statements both with
and without values.
As we know in JS this is not an issue and sometimes is even used as a
feature. Which is the case with Firebase Database JS client.
https://firebase.google.com/docs/reference/js/firebase.database.Reference#transaction
)
As I'm working on a Dart package which wraps JS client I needed to come
up with a way to get this working.
Function _createTransactionHandler<T>(TransactionHandler<T> handler) {
// This is a workaround for analyzer which complains if a function
contains
// return statements with and without value.
void undefined() {}
// Firebase requires us to return JS `undefined` to indicate we want
// to abort the transaction.
return (currentData) {
final data = dartify(currentData);
final result = handler(data);
if (result._abort) return undefined(); // workaround continues...
return jsify(result.data);
};
}
Which works (with latest Dart2 SDK, 2.0.0-dev.32.0). But I was
wondering if there is any plans in Dart2 to statically restrict this kind
of usage?
My gut feeling says that since `void` in Dart2 is more like a value of
special type (we can use it in generics like Future<void>) then my
workaround should be statically allowed, but I just wanted to confirm it
with the team.
Thanks!
--
You received this message because you are subscribed to the Google
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For HOWTO questions, visit http://stackoverflow.com/tags/dart
To file a bug report or feature request, go to
http://www.dartbug.com/new
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['Lasse R.H. Nielsen' via Dart Misc]

On Wed, May 9, 2018 at 6:56 PM Anatoly Pulyaevskiy <

Oh, just looked at my own example again. The problem is not in the
_createTransactionHandler itself (which actually always returns Function)
but in the closure created by this method.
That closure must return an object representing the value, and null is a
valid value in this case. To abort the transaction it must return undefined
(in Dart semantics it must not return anything or in other words return
void).

That's not actually Dart semantics. Dart's `void` return type does not mean
"don't return anything". It means "returns something you shouldn't use",
which is most likely *null*.
In JS compiled Dart code, both JS *null* and JS *undefined* are considered
the Dart *null* value.

If you actually *need* to return the JS *undefined* values, then explicitly
returning the result of the external undefined getter is the correct thing
to do. Do not depend on there being a difference between `return;` and
`return null;`, that's not something the language promises, and it's not
unlikely that the front-end will end up making the two indistinguishable
(because they are at the Dart language level).

/L

On Wed, May 9, 2018 at 9:14 AM Anatoly Pulyaevskiy <

Lasse,
The way I understand it is that this issue is more JS-specific rather
than Dart-specific.
In JS null and undefined are two different things, which means null ==
undefined would return false.
I can return null from my function but the JS code that invokes it
doesn’t care about null, it only checks for undefined, so I needed a way to
do exactly what JS code expects.
Basically I need a function which only returns Function or undefined, and
never null.
In Dart there is no equivalent for undefined, as far as I know, void is
probably close but not exactly. So that’s why I had to declare binding for
undefined.
I should probably change return type to dynamic to not confuse type
checking system.
Hope this makes sense.
Best,
Anatoly
On Wed, May 9, 2018 at 1:29 AM 'Lasse R.H. Nielsen' via Dart Misc <

On Thu, Mar 8, 2018 at 7:04 PM Anatoly Pulyaevskiy <

Post by Anatoly Pulyaevskiy
Sorry about formatting issues, here is plain-text version of the same
@JS()
external get undefined;
Function _createTransactionHandler<T>(TransactionHandler<T> handler) {
return (currentData) {
final data = dartify(currentData);
final result = handler(data);
if (result._abort) return undefined;
return jsify(result.data);
};
}

I'm not sure why this is better than just using: return null;
What you have here is a method declared as returning a *Function*. You
can't return a *void* expression inside that, and you can't use a
return statement with no expression.
You can only return *void* expressions from funtions with return type
*void* (or async functions with return type *Future<void>*), and you
can only use return with no expression where you can return void
expressions. We might not have this nailed down on all implementations yet,
but that's the design.
Every return in this function must return something that can be assigned
to *Function*, which is any function value ... or *null*. If your
*undefined* value isn't one of those, this should give you a runtime
error, so luckily it is, because it is actually *null*.
In more detail, in the return undefined; statement, "*undefined*" has
type *dynamic*, so you get a runtime down-cast from *dynamic* to *Function
*which should check that the actual value (which you have made the JS *undefined
*value) is assignable to *Function*. It is, because JS compiled Dart
treats *undefined *as the Dart *null *value, so you might as well just
return *null *directly.
/L

Post by Anatoly Pulyaevskiy

Thanks, subscribed. There is one more linked to it (
https://github.com/dart-lang/sdk/issues/32177) where ) left a comment
with this use case.
I also found an alternative way to achieve what I needed. Didn't think
I can create JS binding for undefined, but it works actually in both ddc
@JS()
external get undefined;
Function _createTransactionHandler<T>(TransactionHandler<T> handler) {
return (currentData) {
final data = dartify(currentData);
final result = handler(data);
if (result._abort) return undefined;
return jsify(result.data);
};
}
I'll probably switch to this version as it seems more reliable.

Post by Matan Lurey
https://github.com/dart-lang/sdk/issues/32443
On Thu, Mar 8, 2018 at 7:36 AM 'Brian Wilkerson' via Dart Misc <

Post by 'Brian Wilkerson' via Dart Misc
On Wed, Mar 7, 2018 at 11:24 PM, Anatoly Pulyaevskiy <

myFunc() {
if (isSomething) return;
return "some value";
}
Analyzer warning: Functions can't include return statements both
with and without values.
As we know in JS this is not an issue and sometimes is even used as
a feature. Which is the case with Firebase Database JS client.
https://firebase.google.com/docs/reference/js/firebase.database.Reference#transaction
)
As I'm working on a Dart package which wraps JS client I needed to
come up with a way to get this working.
Function _createTransactionHandler<T>(TransactionHandler<T> handler) {
// This is a workaround for analyzer which complains if a
function contains
// return statements with and without value.
void undefined() {}
// Firebase requires us to return JS `undefined` to indicate we want
// to abort the transaction.
return (currentData) {
final data = dartify(currentData);
final result = handler(data);
if (result._abort) return undefined(); // workaround continues...
return jsify(result.data);
};
}
Which works (with latest Dart2 SDK, 2.0.0-dev.32.0). But I was
wondering if there is any plans in Dart2 to statically restrict this kind
of usage?
My gut feeling says that since `void` in Dart2 is more like a value
of special type (we can use it in generics like Future<void>) then my
workaround should be statically allowed, but I just wanted to confirm it
with the team.
Thanks!
--
You received this message because you are subscribed to the Google
Groups "Dart Analyzer Discussion" group.
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For HOWTO questions, visit http://stackoverflow.com/tags/dart
To file a bug report or feature request, go to
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