proposal: spec: direct reference to embedded fields in struct literals

[adg]

Consider

type E struct {
    A int
}

type T struct {
    E
}

This works:

T{E: E{A: 1}}

This does not:

T{A: 1}

Makes some struct literals more verbose than they need be, and makes them asymmetrical to their usage (where you can access the embedded struct's fields directly).

Can we allow it?

(cc @bradfitz)

[adg]

One possible argument against is that it allows you to effectively specify the same field twice, for example:

T{E: E{A: 1}, A: 2}

But this case already exists

S{B: 2, B: 3}

and is disallowed, so maybe that isn't worth worrying about.

[bradfitz]

FWIW, @adg and I both tripped over this independently. We both assumed T{A: 1} would work.

[griesemer]

I've wished to be to able to do this many times (go/ast is a prime example).

I don't know that we ever thought through all the consequences, but it's perhaps worthwhile considering.

[dominikh]

Is there any known reason it wasn't considered back in #164?

[griesemer]

@dominikh issue #164 was about a "programmer error" or misunderstanding of the spec as written.

Usually we don't look at each such error and consider a spec change. However this has come up before (it has restricted what I wanted to do in APIs significantly) so maybe it's time to at least investigate the consequences of such a language change more thoroughly.

That said, language changes are really very low priority. We'll get to it when we get to it.

[josharian]

r's comment on this on golang-nuts was:

It might one day, but as it stands the requirement to provide more information is more robust against changes in the data types.

[ghasemloo]

I have also written code that assumed this would work.

Considering the reading and assignment of embedded fields work it is quite unintuitive that this doesn't.

https://go.dev/play/p/wad3pxSez5I

[zombiezen]

@neild and I were discussing this and there are two things that would have to be addressed (not showstoppers, just considerations to make):

1. What happens if both the anonymous field containing the embedded field and the embedded field itself are specified?

T{E: E{}, A: 42}

This is probably a compile error, much like specifying duplicate fields in struct literals today.

2. What happens if the anonymous field is a pointer type? Should setting an embedded field cause the anonymous field to be implicitly allocated?

I also suspect that this could be done before Go 2, since it makes previously invalid Go programs valid, and shouldn't alter the meaning of existing Go programs.

[ghasemloo]

How much should we be consistent with assignments? If 1 is a compile error should the following also be a compile error?

https://go.dev/play/p/ZVzskYi0ZlV

For 2, in assignments you would get a runtime error. I feel the idea of allocation on the fly can obscure memory allocation and if you have multiple nested embedding a simple int assignment would do a lot more than a user would expect and surprise them. So I feel it might be better to just have a runtime error as in the case of assignment and no hidden memory allocation:

https://go.dev/play/p/beA1MwFh9Y4

Also that is the behavior without embedding:

https://go.dev/play/p/5VyVof6WnC9

[neild]

The assignment rules already cover the case where the same value (or part of a value) appears multiple times in an assignment.

The assignment proceeds in two phases. First, the operands of index expressions and pointer indirections (including implicit pointer indirections in selectors) on the left and the expressions on the right are all evaluated in the usual order. Second, the assignments are carried out in left-to-right order.

[ianlancetaylor]

What should happen for the case that @zombiezen mentions above, in which a field is embedded via a pointer? Here are some possibilities:

1. silently allocate the pointer
2. panic at run time with a nil dereference error
3. give a compilation error

It seems like choice 3 is the best one. The compiler can always tell how a field is embedded, so it can tell that a field is embedded via a pointer.

Is it too confusing for struct literals to permit direct embedded fields but not fields embedded via pointers?

[jimmyfrasche]

1 or 3 are both valid options. 2 doesn't make sense given that 3 is a valid option.

3 would incentivize non-pointer embedding to get the simpler behavior even when a pointer embedding might make more sense.

1 seems more uniform and hence easier to use. Composite literals aren't necessary. They exist to make things easier.

[jimmyfrasche]

Hmm, that is a strong argument for option 3

[griesemer]

Given a struct type T with arbitrary embeddings (directly, or indirectly), if the following code is valid:

var x T
x.f1 = v1
x.f2 = v2
...

for (possibly embedded and possibly exported) struct fields f1, f2, ..., and corresponding values v1, v2, ..., it seems that it should be valid to write:

T{f1: v1, f2: v2, ...}

and vice versa.

This rule alone would take care of visibility across packages and potential field name ambiguities. It would also explain what happens with pointer indirections (it will panic).

This is perhaps the simplest and most intuitive rule and would permit code transformation from one style to the other without change of semantics (which is currently the case for the restricted form of struct literals).

If the compiler is permitted to report an error when a struct literal is known to panic (which is easy to decide) then using a struct literal would be the "safer" choice. But one would lose the ability to transform code between the two variants without semantic change. I don't know if that's a good trade-off.

[jimmyfrasche]

You could also only implicitly create the pointer if you could explicitly create it. In @jba's example you couldn't write pkg.S{A: 1} because you couldn't write pkg.S{u: &pkg.u{A: 1}}

[networkimprov]

Possible alternatives:

T{E{A: 1}}

T{E.A: 1}

[darkfeline]

If we implicitly allocating embedded pointers, if you have a long chain of embedded pointers, then it's non-obvious that:

x := Foo{Value: v}

is implicitly expanding to:

x := Foo{Bar: &Bar{Baz: &Baz{Spam: &Spam{Egg: &Egg{Value: v}}}}}

I don't think I like having non-obvious allocations like that.

[jba]

@griesemer, your rule seems obvious (in hindsight). Could you go further and say that the sequence of assignments is actually the meaning of the struct literal? Interestingly, the spec never really gives the exact meaning of a struct literal. It strongly suggests it by saying keys are field names and values must be assignable to their respective fields. It even says omitted fields get the zero value. But it never actually says that the given values are assigned to the fields.

[karlbateman]

How much should we be consistent with assignments? If 1 is a compile error should the following also be a compile error?
https://goplay.googleplex.com/p/9CL_5Owvzi

For 2, in assignments you would get a runtime error. I feel the idea of allocation on the fly can obscure memory allocation and if you have multiple nested embedding a simple int assignment would do a lot more than a user would expect and surprise them. So I feel it might be better to just have a runtime error as in the case of assignment and no hidden memory allocation:
https://goplay.googleplex.com/p/HWSFemHcFf
Also that is the behavior without embedding:
https://goplay.googleplex.com/p/ixVAW3xa-Q

@ghasemloo The links you've provided are not public 🛡️ not sure if that was intentional?

[brackendawson]

Consider

type E struct {
    A int
}

type T struct {
    E
}

This works:

T{E: E{A: 1}}

This does not:

T{A: 1}

If E is generic using the current generics proposal, then there is nowhere to specify the type on the above proposed literal of T.

https://go.googlesource.com/proposal/+/refs/heads/master/design/go2draft-type-parameters.md#embedded-instantiated-type

[griesemer]

If E is generic the example might look like this:

type E[T any] struct {
	A T
}

type T1 struct {
	E[int]
}

type T2[T any] struct {
	E[T]
}

that is, either E is instantiated with a fixed type such as int in T1, or is is instantiated with another type parameter T as in T2.

In these cases the equivalent struct literals would be:

T1{E[int]{A: 1}}
T2[int]{E[int]{A: 1}}

and, per this proposal, it could be:

T1{A: 1}      // we statically know the type of A from the declaration of E[int]
T2[int]{A: 1} // we statically know the type of A from the T2 type argument

So this should be doable even in the presence of generic embedded fields.

Unrelated, please note that

T1{E: E[int]{A: 1}}
T2[int]{E: E[int]{A: 1}}

doesn't work at the moment in the generics prototype, even though it should. See also #44345 .

[brackendawson]

Amusingly your experience today was the reverse of mine. I separately discovered #44345 in the experimental playground and then came here wondering how it impacts this issue.

[Inuart]

I can't seem to find this proposal in the proposals project. Is it missing from there or is it following another review process?

[ianlancetaylor]

Language changes like this one mostly follow a separate process, tracked in #33892.

This issue is on that list but it hasn't gotten much attention recently.

[ValarDragon]

Wanted to echo support of this! Currently there are places I have to avoid using an embedded struct to de-duplicate logic, due to dev UX overhead for downstream users w/ nested struct construction.

[takanuva15]

Wanted to echo support of this!

👍 Agreed, this would make promoted fields much more intuitive. I thought this was already possible but was surprised to learn that embedded fields are directly gettable but not directly settable on instantiation.

[vearutop]

Struct literals (as opposed to var field assignments), are sometimes bringing better readability due to go fmt alignment and less "unnecessary" references to parent var.

type My struct {
	A      string
	Foo    int
	BarBaz bool

	AnotherGroup1    int
	AnotherGroupQuux string
}

type myEmb struct {
	A      string
	Foo    int
	BarBaz bool
}

type Mye struct {
	myEmb

	AnotherGroup1    int
	AnotherGroupQuux string
}

// Better readability due to vertical alignment.
m1 := My{
	A:      "abc",
	Foo:    123,
	BarBaz: true,

	// Another group for indentation.
	AnotherGroup1:    3,
	AnotherGroupQuux: "c",
}

// Worse readability due to lack of alignment and also `m2` all over the place.
m2 := Mye{}

m2.A = "abc"
m2.Foo = 123
m2.BarBaz = true

m2.AnotherGroup1 = 3
m2.AnotherGroupQuux = "c"

I'd be happy if this proposal is developed into a language change.