how to update APIs for generics

[rsc]

@ianlancetaylor, @griesemer, and I are wondering about different possible plans for updating APIs that would have used generics but currently use interface{}, and we wanted to cast a wider net for ideas.

There exist types and functions that clearly would use generics if we wrote them today. For example:

• type sync.Pool would naturally be sync.Pool[T]
• type sync.Map would naturally be sync.Map[K, V]
• type atomic.Value would naturally be atomic.Value[T]
• type list.List would naturally be list.List[T]
• func math.Abs would naturally be math.Abs[T]
• func math.Min would naturally be math.Min[T]
• func math.Max would naturally be math.Max[T]

There are certainly more of these, both in Go repos and everyone else's code.
The question is what should be the established convention for updating them.

One suggestion is to adopt an "Of" suffix, as in PoolOf[T], MinOf[T], and so on.
For types, which require the parameter list, that kind of works.
For functions, which will often infer it, the result is strange:
there is no obvious difference between math.MinOf(1, 2) and math.Min(1, 2).
Any new, from-scratch, post-generics API would presumably not include the Of - Tree[K,V], not TreeOf[K,V] -
so the "Of" in these names would be a persistent awkward reminder of our pre-generics past.

Another possibility is to adopt some kind of notation for default type parameters.
For example, suppose you could write

type List[E any (= interface{})] struct { ... }

func Min[T comparable (= float64)](x, y T) T { ... }

The (= ...) sets the default for a type parameter.
The rule for types could be that the bracketed type parameter list may be omitted when all parameters have defaults,
so saying List would be equivalent to List[interface{}], which, if we are careful, would be identical to the current code,
making the introduction of a generic List as list.List not a backwards-incompatible change.

The rule for functions could be that when parameters have defaults,
those defaults are applied just before the application of default constant types in the type inference algorithm.
That way, Min(1, 2) stays a float64, while Min(i, j) for i, j of type int, infers int instead.

The downside of this is a little bit more complexity in the language spec,
while the upside is potentially smoother migration for users.
Like the "Of" suffix, an API with type defaults would be a persistent awkward reminder of our pre-generics past,
but it would remind mainly the author of the code rather than all the users.
And users would not need to remember which APIs need an "Of" suffix added.

Are there other options we haven't considered?
Are there arguments for or against these options that haven't been mentioned?

Thanks very much.

[randall77]

var i int
Min(i, 5)   // is Min[int]
Min(3, 5) // is Min[float64]

Would definitely be confusing. Especially if you converted from the former to the latter while debugging something.

[mibk]

And what about using !go1.18 and go1.18 build tags and actually change the definition, so modules using go 1.18 would need to be adjusted to the new syntax?

[bcmills]

@mibk, !go1.18 and go1.18 build tags would change the definition of the package, not its contents as viewed by the importer.

[rsc]

Min is probably just a bad example.
I agree with Keith that Min(i, j) being int but Min(1, 2) not being int is probably too surprising.
(And Min(1, 2) must stay a float64 for x := Min(1,2).)
So let's take Min off the table.

[nirui]

But what about

func min() int {
    return math.Min(3, 5)
}

or

func getPool() int {
    return pool.Get()
}

etc? Will they still be inferred as math.Min[float64]&sync.Pool[interface{}]{}, or they will be inferred to math.Min[int]&sync.Pool[int]{} to match the return type?

IMO a reasonable default is fine. math was accept&/returning float64 only, dudes just silently sit there casting int32 to float64 before feeding them to math.Abs (maybe crying internally at the same time). Now you can have math.Abs take and return float64, int32, int, uint, PackOf6 etc. So at very least the func Min[T comparable (= float64)](x, y T) T { ... } (or whatever syntax it will become) approach is not a lost.

[ianlancetaylor]

I want to note that this example:

var i int
Min(i, 5)   // is Min[int]
Min(3, 5) // is Min[float64]

is an issue in general, even without default type arguments. Suppose we have a hypothetical

alg.Min[T constraints.Ordered](a, b T) T

with no default type argument. Now we get

var f float64
Min(f, 5) // is Min[float64]
Min(3, 5) // is Min[int]

So I don't think introducing default type arguments makes this any worse.

[Merovius]

Personally, I don't like the asymmetry this would create between type-parameters and regular parameters. Default values for arguments is something that has often been asked about, but we never did it. To me, the case for default type arguments isn't really stronger than for default regular arguments though.

I also don't like introducing a new language feature just for a one-time migration. If the motivator for this feature is just the migration of pre-generics APIs to post-generics APIs, it will become de-facto obsolete in a year or so. I don't like that idea.

[rogpeppe]

Default type arguments, however, are motivated by a one-time migration of pre-generics APIs to post-generics APIs. At some point, there won't be any pre-generics APIs left (i.e. every new API created after go 1.18 is a post-generics API) and they become obsolete.

I'm not sure that's true. With a small tweak to the rules, default type arguments could be useful any time a type parameter is added to an existing API, which I suspect will be more than we currently think, particularly in larger programs.

The rule that I'd suggest tweaking would be this one:

The rule for types could be that the bracketed type parameter list may be omitted when all parameters have defaults

in favour of:

The rule for types is that they may be omitted if that parameter and all following parameters have defaults and all the following types are omitted as well.

That change could also be made at a later date without problem, I think.

I don't really buy the parallel with default function arguments - functions are first-class values in Go but generic types and functions are not, so the impact of adding default arguments to functions is considerably deeper and has more runtime implications (think of the issues that Python has with default arguments, for example) than adding them for type arguments.

[Merovius]

I'm not sure that's true. With a small tweak to the rules, default type arguments could be useful any time a type parameter is added to an existing API, which I suspect will be more than we currently think, particularly in larger programs.

I think wanting to add parameters to functions is even more common. And yet we never made that easier with default arguments. There are many changes to APIs which are fairly commonly useful. We don't support most of them.

If I want to, say, add a context.Context argument to a function, I have to come up with a new name for the migration. I don't see why adding type parameters should get any preferential treatment here.

[rogpeppe]

I think wanting to add parameters to functions is even more common. And yet we never made that easier with default arguments.

It might be more common, but adding function default parameters to the language is hard. I don't think anyone knows how to do it, and it would have big implications for existing code (I'm not sure it would be possible to add them without breaking compatibility). By contrast, adding default type arguments is easy.

[AndrewHarrisSPU]

I hope I'm not saying this in too abruptly - maybe I'm missing something - but would the rule rewrite effectively mean this:

The rule for types is that they may be omitted if that parameter and all following parameters have defaults and all the following types are omitted as well. Also, that the default won't be changed at a later point in time.

[apparentlymart]

I must admit that I'm not really sure how I feel about default types for type parameters or default values for function parameters, but one difference I can see between those two situations is that the patterns often employed to create later-extensible function signatures (e.g. a single argument of a struct type representing all the args, or the de-facto functional options pattern) are not applicable to type signatures.

In other words, if I have the foresight to predict that a particular function might need additional arguments later then there are API design patterns I can follow to make it possible to do so without a breaking change. It isn't clear yet what I would do if I anticipate additional type parameters being added in future. Perhaps optional type parameters with defaults is an answer to that (with that also being useful to retrofit type parameters where there were originally none), or perhaps there's some other answer (which may or may not address the problem of retroactively introducing type parameters to a pre-generics codebase).

[Merovius]

One more data-point: This strategy wouldn't help with sort.Slice, for example. At least as far as I can tell. Even though it looks like we might lean towards making that slices.Sort, I feel like this speaks against this as a general strategy. Many generic APIs can't just replace an interface{} with a type-parameter.

[rsc]

sort.Slice was always an odd-ball, because of the comparison function taking indexes and not values. It was never going to transition smoothly to generics.

[Merovius]

Yes. That's pretty much my argument :) I don't see a general strategy for smooth transition to generics working.

Personally, I'd rather figure out if we can do a v2 of stdlib packages or something like that and lean into cleaning up APIs for generics.

[rsc]

Most types are nothing like sort.Slice.

[earthboundkid]

containers/heap has the same problem as sort.

[zephyrtronium]

Using new major versions for stdlib packages, e.g. sort/v2, seems like a clean solution to me. I've seen a few people mention this, generally with a disclaimer like "there seems to be opposition to the idea," but I haven't seen the counterarguments. As a bonus, the precedent could also facilitate redesigns independent of generics, like #26263 and #22697.

[smyrman]

I don't see any inherent difficulty with net/http/v2 and encoding/json/v2 and archive/zip/v2. I don't think it would make sense to have net/v2/http`. Maybe I'm missing something.

@ianlancetaylor, usually sub-packages import the parent package, so I wold think there are potentially acceptable reasons you might want the versioning to be after the first element. E.g. if introducing a breaking change to net.Listener in net/v2 (arbitrary example), then surely you would need a v2 pacakge of net/http as well, and then net/v2/http would be a sensible way of communicating that dependency.

Given package versioning in the standard library is done infrequently and with great care, then the added duplication and potential cleanup that would result of introducing versioning after the first path element, might be an acceptable trade-off.

[ianlancetaylor]

I don't think the interesting argument is whether the subpackage imports the parent package. I think the interesting argument is whether we would normally want to have a v2 version of all the subpackages at the same time. For example, it seems to me that writing a v2 of encoding/json or encoding/xml in no way implies that we need or want a v2 of encoding/binary or encoding/base64 or encoding/hex, or, for that matter, of encoding itself.

For the case of archive/zip, note that archive/zip does not import archive. In fact, there is no archive package.

[smyrman]

I think the interesting argument is whether we would normally want to have a v2 version of all the sub-packages at the same time.

I completely agree to that. I am not to worried either way, as I think the answer to this question would become more clear with proto-types. The more v2 proto-types (related to generics or not) the better. If the proto-type doesn't have to be in code, but could be in the form of a Gist or a blog article, then anyone can write one.

I won't be writing any proto-types in code, but I though I would do a few articles to visualize how a v2 of different packages might look like for generics in particular. I have started out with an article for sync/v2, showing a potential sync/v2/atomic as well. I will share it when it's in a more ready state. I also started looking at math/v2 which has a higher number of sub-packages. If anyone feeels inspired to write a piece on this, and beats me to it, then I don't mind.

[lpar]

None of this is meant to imply that versioning standard library packages is a good idea, or a bad idea. It does solve the problem at hand, but it's a little weird to have to remember to write math/v2.

Well, that’s a problem with Go’s package system in general, not specific to generics. I can’t be the only person who has gotten half way through using a third party library for something and only then discovered that I want the v2 version. (Or in some cases the v4 version, I’m looking at you UUID.)

A solution, of course, would be for the default to be the most recent if the version is not explicitly specified. Perhaps doing generics using regular package naming conventions could apply some pressure to solve that problem for packages in general?

There is no math2.h or java.lang.Math2.

On the other hand, Java’s library is now a decent date and time library plus an unholy mess of obsolete non-thread-safe functions that programmers have to remember to avoid. And there’s java.io2, they just decided to call it nio for New IO instead.

[smyrman]

I though I would do a few articles to visualize how a v2 of different packages might look like for generics in particular. I have started out with an article for sync/v2, showing a potential sync/v2/atomic as well. I will share it when it's in a more ready state.

My progress on this has been very slow as I have been busy with other stuff, but here is an early draft that might already be useful to read:

• intro: https://github.com/smyrman/blog/blob/master/2021-09-v2-for-generics/1-v2-for-generics.md
• sync/v2 + sync/v2/atomics: https://github.com/smyrman/blog/blob/master/2021-09-v2-for-generics/2-sync-v2.md

There is a very nice overview of suggestions to changes in #48287 (comment), but I have tried to go a little-bit more in depth. E.g. could we extend the assembly syntax to allow writing a generic version of the various functions in atomic? If so, we could reduce the number of functions from 29 to 5.

I haven't gotten down to write anything about the math package yet, not to mention math/big. I probably won't get the time to do so anytime soon, but I would actually accept PRs, and be sure to mention anyone contributing to the article if/when I decide to release them for a wider audience.

[ianlancetaylor]

Another approach is to observe that the language supports type aliases to support transitions. So let's use type aliases.

type Pool[T any] ...
type Pool = Pool[interface{}]

The rules here would be:

1. The type alias must be at the same scope as the type (which currently means only at package scope).
2. The type alias identifier would be the same as the alias target (otherwise it is an ordinary alias).
3. The type alias identifier must not have any type parameters.
4. The type alias target must specify arguments for all type parameters.

This alias rule does not work for functions. We can either say there is no transition for functions, or we can introduce another rule.

A function may be defined both with and without type parameters. A reference to the function (not a call) with no type arguments is permitted, and gets the version without type parameters. A call of the function with no type arguments gets the version with type parameters and does type inference as usual. If type inference fails, the call is instead made to the version without type parameters (and may fail if the arguments type are not assignable).

That gives us

func Min[T constraints.Ordered](a, b T) T { ... }
func Min(a, b float64) float64 { return Min[float64](a, b) }

[mvdan]

On the minus side, it's a bit odd to permit two different objects with the same name in a scope; in order to not break all kinds of invariants both in the implementation and in the go/types API, such a pair of type declarations would probably need to be considered a single object, but that is also strange: What is the object named "Pool"? Where is is declared?

Don't we have to worry about this problem anyway, due to #46477?

[gbarr]

An additional benefit for having 2 declarations as opposed to default types in the declaration is that they can have separate documentation. The new generic documentation can be kept clear without having to mention the compatibility defaults and the second declaration also allows the developer to mark it as deprecated in the documentation.

[akavel]

Seeing that this proposal (as many others too) is adding a language feature, I'd like to ask a question: would this language feature be defensible alone if the need for the transition from non-generics to generics didn't occur? I.e., if generics were there from the start, would this feature be deemed valuable enough to warrant its inclusion in the language? (I don't know the answer, I'd just like to make sure the Core Team themselves are consciously aware of the answer and take it into account when making a decision.)

[ianlancetaylor]

@akavel No, I don't think this language feature would be defensible if generics has been there from the start. It's a good question to ask. Still we have to do something (though we don't necessarily have to add a language feature, we can use different packages or different type/function names).

[GGCristo]

As an alternative to the "you can define a function more than once", i.e.

func Min[T constraints.Ordered](a, b T) T { ... }
func Min(a, b float64) float64 { return Min[float64](a, b) }

We could potentially simplify slightly more and say that you can define exactly one function with the same name without a body and the types therein will be used as a fallback if type unification fails. So, the above would be

func Min[T constraints.Ordered](a, b T) T { ... }
func Min(a, b float64) float64

I would like to add to @kylelemons's idea that the default function should be close to the fallback function, so code is more readable.
Also, this could be an alternative way of doing this in case using the "func" keyword feels like a hack

[T = float64]
func Min[T constraints.Ordered](a, b T) T { ... }

[ianlancetaylor]

It's clearly essential to always have a crazy idea that everybody can reject, so here is one. default is already a keyword. So let's permit, only at package scope,

default Pool Pool[interface{}]
default Min Min[float64]

[griesemer]

This is essentially the same as the alias idea above (at least for types), except that a keyword is used.

[changkun]

Not necessarily need the default keyword, this might be more consistent with the subject:

type Pool[T any] ...
type Pool = Pool[interface{}] // type alias

func Min[T constraints.Ordered](a, b T) T { ... }
func Min = Min[float64]       // func alias

(° ο°)...

[jpap]

I like the idea of keeping the declaration as one: with a separate alias/default declaration, one of the two parts might end up far from the other, reducing source code readability.

What about using the default keyword with the original proposal:

type List[E any default interface{}] struct { ... }

func Min[T comparable default float64](x, y T) T { ... }

func Print2[T1 any default string, T2 any default interface{}](s1 []T1, v2 []T2) { ... }

[bcmills]

One suggestion is to adopt an "Of" suffix, as in PoolOf[T], MinOf[T], and so on.
For types, which require the parameter list, that kind of works.
For functions, which will often infer it, the result is strange:
there is no obvious difference between math.MinOf(1, 2) and math.Min(1, 2).
Any new, from-scratch, post-generics API would presumably not include the Of - Tree[K,V], not TreeOf[K,V] -
so the "Of" in these names would be a persistent awkward reminder of our pre-generics past.

I'm going to go out on a limb and suggest that we use the Of suffix for generic container types, regardless of whether they are new. Then the Of in the names is no longer a persistent awkward reminder, but a consistent pattern.

Constructor functions for generic containers get the Of suffix because the type has it, and because they can't generally be inferred locally anyway: list.NewListOf[T]().

math gets a mulligan for using the good names for the 64-bit-only variants: v2 it. import "math/v2" and the only call sites that change are the ones that can't infer arguments (e.g. Nan()) or pass only untyped constants. Then maybe we can revisit the awkward inclusions for that package at the same time. (Do people really use math.Yn on a regular basis‽)

I think atomic.Value needs more thought anyway (#47657 (comment)). So maybe v2 that package too, or use names that aren't just ValueOf..?

If we do the above, what are the remaining problematic names?

[rsc]

Didn't that ship sail with map?

[bcmills]

Maybe, but as a built-in it's already funny-looking. (And its second type parameter spills outside the brackets! 😅)

[kylelemons]

The idea clearly exists already, as it was mentioned in this thread but I think it's worth its own suggestion as I think it aligns pretty nicely with what we're asking package maintainers to do in general:

Don't introduce generics in most of these libraries. /v2 them:

• "sync" -> "sync/v2" gets generic Value, Pool, Map
• "math" -> "math/v2" gets generic Abs, Min, etc
• "list", "heap", "ring", etc get "list/v2", etc for their generic versions.

The normal packages should be implementable in terms of the generic version, so if it doesn't compromise performance we could do the go fix trick to rewrite them all to /v2.

[twmb]

If choosing a v2 approach, I think it'd be worth it for gopls / goimports / etc. to adopt a convention of importing v2 by default if go.mod uses go.1.18+ (which it should, if a library / binary is using generics).

[lu4p]

Alternatively /v2/math, /v2/sync, etc, to fit the usual module-version style. That means we'd have a single v2 directory with all the new API in. As you say, all the current packages should be implementable in terms of the new API.

I think this is not the right approach because stdlib packages might want to version independently from another in the future. (e.g. math/v3 while sync is still at v2)

[Merovius]

@lu4p That's not something precluded by putting the version in front. Effectively, the v3/math might exists, while v2/sync doesn't, until we decide to revamp it again. The std as a module is already special (in that you don't need to put the module path into the import), there's no reason to force it to upgrade all packages at once.

[nickkeets]

What if we take this one step further? Go 1.18 becomes Go 2 and importing v2/math etc happens by default, with import math. But, we also keep the old versions too as v1/math etc. We do break compatibility this way, but in a way that can be ported automatically with go fix.

[ncruces]

@nickkeets, if the Go community really stands by SIV, there's no reason to exclude the standard library from the associated ugliness.

[rogpeppe]

I support this suggestion, particularly as I see it as having the potential to be generally useful into the future given a minor tweak to the rules.

I'd question the need for the parentheses. I understand that they're there to suggest the "maybe" aspect of the default but I don't think they pull their weight, and there's considerable precedent from other languages (e.g. Rust's default type parameters, Python's default function parameters) that they could be omitted without much confusion.

That is, I think it would be fine if the examples were written thus:

type List[E any = interface{}] struct { ... }

func Min[T comparable = float64](x, y T) T { ... }

[AlaxLee]

Drop the parentheses is good idea. "=" usually used for "equal" or "assignment", if we can use another word instead of "="?
For example:
type List[E any default interface{}] struct { ... }

[AndrewHarrisSPU]

I agree the sense in which E would be assigned interface{} as a default value by = feels a bit out of place here. default seems a little more obvious. Really, I think the type assertion syntax is the strongest analogy: E.(interface{}) in the production of type List[E.(interface{}) any] struct { ... }. (Maybe it's clearer to say it's a type judgement, as 'assertion' can imply something that can fail?)

The spec states the following for the type assertion from an interface x.(T):

... even though the dynamic type of x is known only at run time, the type of x.(T) is known to be T in a correct program.

Here we would be analogously stating for a type judgement E.(interface{}):

... even though the concrete type of E is only known at instantiation time, the type of E.(interface{}) is known to be interface{} in a correct program.

This judgement holds even if no other instantiation of E can be reached ... so, there is a subtle difference between E any and E.(any) any.

For Min[T.(float64) comparable](x, y T) T { ... }, when something like Min(1, 2) returns float64, maybe it's sensible to say it's a judgement?

[AlaxLee]

IMHO, There may be a confusing place here.
When using x.(T), it often means:

if x's type is T, right; if not T, wrong.

When using E.(interface{}) in type List[E.(interface{}) any] struct { ... }, it means:

if E's type is interface{}, right; if not interface{}, right too.

[DeedleFake]

Is this something that could be handled by the go directive in go.mod? Something like go 1.18 and up changes them to generic versions, with a go fix directive to add an explicit sync.Map[interface{}, interface{}] and the like? This would obviously only work for the standard library, but external modules can use a major version update to introduce generics instead.

It seems like overkill for something like this, and if a clean alternative can be found than that's probably better, but I hadn't seen it discussed anywhere and introducing incompatibilities was one of the primary motivations for adding the directive, if I remember correctly.

[bcmills]

That option came up a bit in #48287 (comment). (I think the tricky part is in defining the syntax for the declaration site.)

[suared]

Why would that be required? Wouldnt it be a 1 time fix?

[akavel]

A proposal/idea related to various more or less vague "go fix" approaches already mentioned around here and there, but specific enough and subtly different enough to warrant separate mention, given this is clearly a brainstorming discussion:

• Treat go version directive in go.mod as transition point, such that:
  • for code having version < go1.18, new packages are available via import "go2/math" etc., old packages via regular import "math"
  • for code having version >= go1.18, new packages are now available via import "math" etc., old packages via import "go1compat/math"
  • a go fix upgrade (or similar) rewrites the imports & bumps the go version directive in go.mod

(obviously, the "go2/" and "go1compat/" prefixes being subject to bikeshedding, and go1.18 subject to change to newer)

edit: Notably, AFAIU this is also basically how Rust editions are working, so Rust community leaders could be consulted about any nonobvious pros or cons of this approach. FWIW, from a distance it seems to be working for them well enough that they repeated this "phase transition" a few times already. Also, if adopted, this could possibly also help with other bigger "Go2" changes if needed at some point.

edit 2: This also seems to me fairly similar to the /v2, /v3, etc. versioning in 3rd-party packages, as introduced with the modules system. The difference being, Go stdlib under this proposal would come with a "deflation" mechanism via go fix, basically purely for ergonomy/ease-of-writing's sake. Though the disadvantage of this "deflation system" would be that when reading Go code, one could then not be immediately sure which version of the stdlib the code is using - only being able to discern this after looking into go.mod.

[ianlancetaylor]

I think this could work but I think it would be difficult for people to understand. It's always problematic when the exact same code means different things based on some other file. It makes it much harder to understand the code when reading it. In this case the problem is somewhat mitigated because it is a one time transition cost. But still.

[oxplot]

We could teach go.mod replace to work with stdlib packages.

As an example:

module xyz

go 1.18

replace sort => typed/sort

require (
  ...
)

Benefits:

• Backward compatibility: all parameterized types are under typed/ (or something similar)
• Explicit: no magical compiler behavior choosing one version over another based on some heuristic.
• New code can trivially use the go.mod replace (and go mod init can add the replace directive automatically for Go versions that support generics).
• Existing code can be migrated by explicitly naming the import path import sort "typed/sort".
• Go 2 will rename all typed/* to * and provide code upgrade tool.

[Merovius]

This seems unhelpful to me. Library code is going to want to use the new versions of packages and can't rely on replace. So they will have to type out the entire path. At that point, it seems simpler and less confusing to just have everybody type out the full import path.

[renthraysk]

Is the plan to keep the generics behaving the same as the original?

As implemented as a generic bits.RotateLeft() the inliner is assigning a much higher cost (almost x2) to the function. Whilst doesn't prevent it from inlining, it prevents the caller, QVarint(), from inlining (non-generic cost is < 50, generic cost is > 120), even though the assembly generated from the generic code is practically identical to the assembly generated from the non-generic code.

type Unsigned interface {
	~uint8 | ~uint16 | ~uint32 | ~uint64 | ~uint
}

func BitWidth[T Unsigned]() uint {
	return uint(8 + (T(8<<8) >> 8) + (T(16<<16) >> 16) + (T(32<<32) >> 32))
}

func RotateLeft[T Unsigned](x T, k int) T {
	n := BitWidth[T]()
	s := uint(k) & (n - 1)
	return x<<s | x>>(n-s)
}

func QVarint[T Unsigned](x T) (uint64, int) {
    y := uint64(RotateLeft(x, 2))
    n := uint(8) << (y & 0b11)
    if w := BitWidth[T](); w == 64 || n <= w {
        s := w + 2 - n
        return y >> (s % 64), int(n / 8)
    }
    // truncated 
    return 0, -1
}

func main() {
	x := uint64(1 << 56)
	y, n := QVarint(x)
	_, _ = y, n
}

[rsc]

It is definitely the case that if we write generic APIs that we expect to be inlined, we will need to pay attention to the cost assumed by the inliner in higher-level frames. Thanks for raising this issue.

[bmhatfield]

A possible awful idea for rejection: implementation-specific suffixes akin to _unix, _windows, etc, but for generics/not: _generic, _typed. This would allow us to compile a typed version in contexts where generics are not available, and a generic version where they are. Also possible/less collide-y would be version build suffixes instead (_1.17, _1.18`).

This builds on the observation:

For functions, which will often infer [the type], the result is strange:
there is no obvious difference between math.MinOf(1, 2) and math.Min(1, 2).

[clarkmcc]

If these suffixes behave anything like existing suffixes, then you could only flip generics on or off for the entire project at compile time. In my opinion, a solution that allows incremental migration of existing projects rather than an all or nothing approach is pretty valuable.

[jxsl13]

I wonder how much compilation time it would take if the function signatures would also be distinguished by their parameter lists. Preferring a generic implementation over the interface one.
Just a thought..

[matt0xFF]

For generic math, type/function aliases would be better than default parameters.

Edit: As long as you can add parameters in an alias to a different name (type mgl32.Vector3 = mgl.Vector3[float32]), I think all of these cases would work with a combination that and default parameters - if default parameters are included, they don't require aliases to the same name.

For example the mathgl packages include 3D math types used by OpenGL bindings / games etc. There are 32 and 64 bit versions of each type, mgl32.Vec3/mgl64.Vec3, mgl32.Mat2x3/mgl64.Mat2x3, etc. Type aliases would allow each package to point to (and be compatible with) common generic versions of each type, but a default parameter wouldn't work since each package needs a different one (same thing for the functions, although wrapper functions would also be fine if they can be optimized away).

If Go ever permits array size type parameters (#44253), the type alias mechanism could also be used to aid migration in the same way (Vector3 could become an alias for Vector[3]).

Another example of math code (mathf.go) in the wild uses the f suffix (sinf, cosf) for 32 bit versions of math functions - type aliases would work there while default parameters wouldn't.

For those suggesting library authors break API compatibility, then all packages using those libraries re-writing to use the new API - I think we need to be realistic that this will not rapidly or completely happen. Many tasks currently work fine without generics (for example, 64-bit math code), so many applications and libraries would need to change a large number of files for zero or marginal benefits (changes that may include forking dependencies that are no longer actively maintained).

Even new code might continue to use "math" instead of "math/v2", if you are using 64-bit math, the former is shorter, and what's the difference?

I think the likely outcome is that users would need to be familiar with multiple ways to do the same thing (across many libraries) for the foreseeable future. Creating such a situation in the name of simplicity seems like a false economy.

[wolverian]

(Off-topic)

Because v1 would be implemented in terms of v2, so anything not reflected as an API change (like performance improvements and the like) you'd benefit from indefinitely, even if you use v1.

You're probably talking about the standard library here specifically, but I'm wondering how many projects in the Go ecosystem generally do this. Definitely my own vague understanding has been that people usually abandon old versions when they make breaking changes instead of re-implementing them in terms of the newer version. Some statistics about this would be super interesting!

[Merovius]

You're probably talking about the standard library here specifically

FWIW, I'm not. The explicit intention of this discussion is to create general guidance for how to do this. And the mechanisms we've developed over the last years are generally applicable. And I believe they are what should be our general guidance.

Whether that's actually followed is a fair question. But as far as I'm concerned, that's a question for the maintainers of the relevant modules. As so often in open source, it's up to them to what degree they want to support their users and it's up to you to decide if you want to trust a particular maintainer.

[matt0xFF]

@Merovius

But if it's worth introducing a new language feature, it definitely is worth releasing a new package. Language features are far more costly than releasing new packages.

In order to understand your position better, can you elaborate on this (with respect to this particular feature)? I have some idea of the costs of breaking compatibility, and in addition, I think users having to remember to add v2 to certain commonly-used packages like math, but not others, will forever add a (small) amount of cognitive overhead to learning and using Go.

However, I don't have as clear an idea of the costs of adding this particular language feature. For example, if there is some particular way users are likely to abuse it, or if there is reason to believe it will be an ongoing source of compiler bugs.

[lu4p]

I have some idea of the costs of breaking compatibility, and in addition, I think users having to remember to add v2 to certain commonly-used packages like math, but not others, will forever add a (small) amount of cognitive overhead to learning and using Go.

It's perfectly valid to use the old version of the package, because v1 will be implemented using the v2 package. I expect most users to use gopls (via their editor), which should automatically add imports with the latest version.

[wolverian]

The explicit intention of this discussion is to create general guidance for how to do this. And the mechanisms we've developed over the last years are generally applicable. And I believe they are what should be our general guidance.

Thank you, guidance on this topic is a good idea.

[uluyol]

How about the following rule (inspired by Java) for types: if a type instantiation doesn't contain type parameters, then the type parameters default to the constraint types. For example:

type [T interface{}] List struct {...}

var p List[bytes.Buffer] // type-safe usage
var p List               // == List[interface{}], same as today

Pros:

• Works well for container types
• Most generic things in Go today seem to be containers, since writing generic functions is tricky
• No additional syntax
• Default rule is simple

Cons:

• Doesn't work for interfaces that can be used as constraints but not regular values (e.g. comparable)
• Doesn't work for making concrete functions generic
• Prevents future type inference on generic type literals (if we have S[T], S{...} is defined to mean something with this rule)

[Merovius]

Doesn't work for interfaces that can be used as constraints but not regular values (e.g. comparable)

That excludes 4 of the 7 examples in the top-post.

[AndrewHarrisSPU]

Your cons list, and a good amount of discussion here and elsewhere, highlight that a constraint does not express everything a programmer might wish about the shape of the eventually unified type. They are sufficient for a lot of useful, common cases but I think it'd be worth examining the kinds of affordances that may be possible for less common cases.

[earthboundkid]

Metapoint: I don't think this issue worked very well with the GitHub discussions format. Discussions seem to work better when there is one big proposal with many small details to work out (e.g. there is going to be a slices package, what should go in it?), and then the discussions each burrow in on a detail (e.g., let's figure out a name for slices.Compact). At this point, this issue is still too up in the air (change generic type aliases? add v2 packages? something else?) to drill down on specifics, so each subdiscussion tends to repeat one or two of the main themes with minor variations in the specifics, which makes it very hard to follow.

[deanveloper]

This kind of discussion also doesn’t really fit Github issues either, that’d also be extremely messy.

I think threaded discussion with recursing sub-threads (a la Reddit) would be best for this kind of discussion.

[suared]

I may be “under-thinking” the problem, it feels like too much work on the language to come up with what feels like a workaround. The use cases I can think of feel like they are handled with existing constructs. This is my use case summary, what am I missing?:
• User code upgrade to 1.18 from previous – their code is updated using go fix so that it is seamless (in the example of a current different return type the source code would be re-written at that point to cast to the pre 1.18 type so the rest of the program would function as originally intended).
• User wants to import a new package that has a go.mod of => 1.18 – the existing modules approach would error and tell them they need to upgrade to 1.18 to use the library. When they do, bullet 1 would take effect
What am I missing?

[earthboundkid]

A lot of people need to support multiple versions of Go simultaneously. See the Go2 transition doc for specific criticisms of "the Python 3 problem" we want to avoid,

because there is no backward compatibility, it is impossible to mix Python 2 and Python 3 code in the same program. This means that for a typical program that uses a range of libraries, each of those libraries must be converted to Python 3 before the program can be converted. Since programs are in various states of conversion, libraries must support Python 2 and 3 simultaneously.

Because simultaneous support for 2 & 3 wasn't possible at the start, the Python 3 transition dragged on for a decade…

[suared]

Thanks for the reference, it was a great read! re: Python 3 issue and potential solution to this thread seems to be already in the article referenced. It may just be a matter of agreeing how to characterize some of the changes to follow the articles advice. In my mind, the 3rd use case that I now see is:

• User chooses to update their own code but relies on packages with earlier version dependencies - In the article the suggestion is to use the version to enable the compiler to compile it correctly. This is possible because of modules if it can be assumed to be reasonable to expect all "legacy" packages to now minimally support modules. The <1.18 in go.mod of pulled in packages would enable the compiler to use the "old" behavior.

Are there any other missing use cases? As a language user, my bias would be to keep things simple so the language has clear expectations. Having that hidden or forced into action by requiring something like a go fix to be run potentially even on pulled in packages would be preferable in my view. What I think makes Go unique vs a java or other in its ability to do this is that other than plugin there is no real "binary" interface that I am aware as in java's jar, etc. The source is available to be recompiled and must be even for plugin so I would favor whatever is simplest as a user so the don't need to navigate multiple versions of core library seemingly equivalent documentation. Just my .02. Very interesting problem, thanks for the insights!

[Tang-RoseChild]

how about suffix 'X'?

[HALtheWise]

At the risk of adding one more proposal to this already lengthy discussion:

1. Create "math/v2" with the new APIs under obvious names (func Min[T numeric](...)) and (where necessary) the old APIs under ungainly names but marked as deprecated.
2. Create "math/v1" that consists of aliases and thin wrappers around "math/v2" types and functions, and exposes a consistent API with the current "math" package.
3. In Go tooling, resolve import "math" to either "math/v1" or "math/v2" depending on the go language version in go.mod. Explicitly specifying is always allowed.
4. go fix (or a similar command) should bump the go version in your go.mod and modify the import paths and/or usages in your source code so there is no behavior change. At it's simplest, that means replacing all "math" imports with "math/v1", but a smarter migration tool could instead replace references with their new names Min -> MinFloat64, with generics-aware specializations Min -> Min[float64], or even use type information to notice when inference rules would do the right thing anyway Min -> Min

This approach is intended to keep the standard library as similar as possible to what public packages need to do, and step 3 is the only new piece required for this to work.

The warts of this approach are also warts any open-source library author will face, and the (optional) additional changes below will help with community migrations as well. Each of these would need to go through its own proposal process.

• Language change: Allow function aliases similar to type aliases. Let "math/v1" declare func Min = math.Min[float64] instead of needing to make a wrapper function. This also avoids performance concerns with the wrapper, and improves consistency between func, type, and const, simplifying the language.
• Make godoc propagate documentation from aliased-to types and funcs, either automatically or with a special directive in the comment. This prevents needing to maintain both the old and the new doc comments in many cases.
• Make go fix automatically upgrade any dependency to a higher version whenever all referenced symbols in the old version are simply aliases to symbols in a higher version.
• Release a script that helps package authors make v1 "alias packages"

[hherman1]

Feels a bit magical for “math” to be special. Do we really want devs to have to learn this or think about it?

[ncruces]

While I agree that functions aliases have been unnecessary so far (function identity is not particularly relevant, one line wrappers work), @ianlancetaylor's proposal is different.

This proposal is not about redirecting users to new packages. For this, the current type aliases plus function wrappers would be sufficient.

This proposal, AFAICS is about overloading names within the same package, which has as been noted, is novel in Go. And if we are to overload names, I think aliasing is definitely clearer than wrapping.

[HALtheWise]

I was using math as an example, the intent would be that all standard library packages would use this process, where new major versions can become the default in a new go release.

[ChrisHines]

As far as I know, forwarding functions shouldn't create a performance penalty. Since Go 1.12 functions that do nothing but call another function are inlined.

[Merovius]

@ncruces This thread is not about @ianlancetaylor's suggestion. It's about @HALtheWise's suggestion. Which would be a case of redirecting to new packages.

[jsshapiro]

I suspect it is far too late to introduce this comment, but it may be useful to record it for later when we look back at this.

Long and short: the reason we are talking about default types here is that we aren't doing type inference. In languages where type inference is adopted, none of the issues being raised here exist because the type parameters are auto-populated by inference.

This can definitely be done in a go-like language, as we showed in BitC. I definitely wouldn't adopt everything we tried - if only because the result wouldn't be go. But the heart of the problem here is that type variables are getting instantiated explicitly.

Circling back to an early example in this thread:

var f float64
Min(f, 5) // is Min[float64]
Min(3, 5) // is Min[int]

It appears to me that this is a type error, because the first invocation of Min has incompatible argument types. I'm new to go, so I may not be understanding the arithmetic promotions correctly, but I think there is no implicit promotion in this case, so the example would need to be written as:

var f float64
Min(f, float64(5))

If the promotion is required to be explicit, there is neither ambiguity (to the compiler) nor surprise (to the user).

We went to significant lengths in BitC to use type classes to arrange for this type of implicit promotion on binary operators. At the time, I was pushing the type classes idea to see what kinds of things could be expressed with them. We ended up with something similar to go's untyped constants, but we got there using type classes. The problem, ultimately, is that type class constraints accumulate in much the way that explicitly declared exception types accumulate: incomprehensibly.

Jonathan Shapiro (the BitC architect).

[zephyrtronium]

Long and short: the reason we are talking about default types here is that we aren't doing type inference. In languages where type inference is adopted, none of the issues being raised here exist because the type parameters are auto-populated by inference.

I think we are talking about default types because we are doing type inference and the result of that inference isn't always what we need for backward compatibility.

Circling back to an early example in this thread:

var f float64
Min(f, 5) // is Min[float64]
Min(3, 5) // is Min[int]

It appears to me that this is a type error, because the first invocation of Min has incompatible argument types. I'm new to go, so I may not be understanding the arithmetic promotions correctly, but I think there is no implicit promotion in this case,

There is. (Although I don't think "promotion" is really the correct term.) 5 is a float64 with Min(f, 5) because the constant is representable as float64 and that is the only type that can be assigned to the second parameter given unification with the type of the first, but it is an int with Min(3, 5) because the context doesn't require it to be any particular type and int is its default as an integer constant.

so the example would need to be written as:

var f float64
Min(f, float64(5))

If the promotion is required to be explicit, there is neither ambiguity (to the compiler) nor surprise (to the user).

If the promotion is required to be explicit, then we need a different definition, because that would be a breaking change.

[jsshapiro]

@zephyrtronium

I'm cautious about driving this discussion off topic, but...

I mis-read the language specification about untyped constants in Go. As a result, I misunderstood the contexts in which they can appear. So my statement about explicit promotion is wrong.

At the risk of being pedantic, in both BitC and Go, "5" is a constant having a type 'T that can be looked at in one of two ways:

• 'T inhabits a kind UntypedInt that might be described as "one of the types in the set of things that can represent 5" (this appears to me to be the Go view), or
• 'T satisfies the type class UntypedInt 'T

The difference between the two isn't so obvious for single variable type classes, except to note that instances of type classes can be explicitly defined (I'll come back to this). In either framing, ambiguity arises if nothing forces 'T to specialize to a concrete type. Some form of resolution rule is then required, at which point you no longer have complete inference. Having said that, "sound" is mandatory while "complete" is merely very nice to have, and in this instance it's probably fine to have a tie breaker. Both BitC and Go have such a resolution rule.

Regarding backwards compatibility, I'm surprised that inference isn't generating backwards compatible outcomes. In particular, the Min(3,5) example resolving to float64 strikes me as resulting from a flaw in Go's original definition of the UntypedInt kind: it shouldn't have included floating point types. BitC's version of UntypedInt does not include any float types, so we didn't run into the corresponding incompatibilities.

A cautionary note for the future:

In later evolutions of BitC, we introduced literal kinds. One of the things we explored was using them for fixed precision integer width promotion. At that point we would have typed the constant 3 as:

3: SignedInt 'T 3 => 'T

meaning "A signed fixed precision integer type having at least three bits." Because that's the least size required to represent "3".

So If we saw Min(3, 7), we would end up with an initial (partial) specialization:

Min: Orderable 'T, SignedInt 'T 3, SignedInt 'T 4 => 'T -> 'T -> 'T

which simplifies to:

Min: SignedInt 'T 4 => 'T -> 'T -> 'T

But the concrete type of 'T would remain ambiguous pending further resolution by unification or constraint. If 'T remained ambiguous at the end of type resolution, it was resolved using the ad hoc resolution rule, which (in BitC) can informally be stated as "the smallest natural signed int type that satisfies the surviving constraints." Type classes allowed BitC to support implicit integer and floating point size promotion. It also gave us a way to deal with binary operators that promote mixed arguments implicitly:

Add: Promotable 'T1  'T2  'TResult => 'T1 -> 'T2 -> 'TResult

and we defined Promotable at all of the usual combinations of types that are familiar from C.

In retrospect, [ab]using type classes for integer bit size was not a good idea, because you end up with things like:

SignedInt 'T1 's1, SignedInt 'T2 's2, SignedInt 'TResult  'sResult,
  's1 <= 'sResult, 's2 <= 'sResult
instance Promotable 'T1 'T2 'TResult 

which is an enormous number of instances! This exponentially increased the time required for type class resolution in binary operators while adding zero actual value for real programmers. We had introduced "literal kind" for array length typing (where it did provide value) and got carried away seeing what it could do in other places. Literal kind has some value in typed assembly language and in typing certain kinds of safe pointer arithmetic, but it's a blade with some exponentially sharp edges. Actually, that could probably be said for multivariable type classes in general. I suppose the lesson is "not everything should be automated by magic."

The "last gasp" resolution strategy in BitC can be viewed as a "weak" type class constraint - one that is applied only if it is required for disambiguation. The contemplated "specified default" idea for Go can similarly be viewed as a weak constraint. I think you will discover that it is problematic, because the defaults end up becoming part of the type so that they can be satisfied at the value's point of origin. What's going to happen is that two type variables T1 and T2 with different defaults, specified in packages separated by twenty cross-package calls will get unified, whereupon you will have contradicting default resolutions. Merely reporting the error in an actionable way isn't an easy thing to do (we put a lot of effort into reporting unification-driven errors actionably). Looking back, my feeling now is that it is cleaner (and easier to explain) to adopt a single "fallback int" and a single "fallback float" type and avoid explicit default resolutions. If that means renaming the functions, then so be it.

As @rsc and @robpike note, there is limited experience with generics in Go so far. Apologies for wandering a bit here; perhaps it will be a useful cautionary note at some point in the future.

The Go community hates to break existing code, which is a very good thing. Would it be possible to modify the compiler temporarily under the "doesn't include floats" interpretation of UntypedInt and learn how many GitHub projects would actually be broken? It strikes me that a completely automated source rewriter could be built...

[Merovius]

In particular, the Min(3,5) example resolving to float64 strikes me as resulting from a flaw in Go's original definition of the UntypedInt kind: it shouldn't have included floating point types.

Perhaps (personally, I disagree). But that's neither here nor there for this discussion, as it's a choice that has been made and we can't break it now.

[Merovius]

FWIW:

Would it be possible to modify the compiler temporarily under the "doesn't include floats" interpretation of UntypedInt and learn how many GitHub projects would actually be broken?

That's likely possible, yes. The Go corpus is intended for these kinds of analysis. It hasn't been updated in years, but the rules for constants have always been in the language and I can't imagine why anything about their usage rates would have changed.

It strikes me that a completely automated source rewriter could be built...

It definitely can. But we can't rely on people using it. Especially as you are suggesting to break the language, not just the tooling. It also seems very hard to me to decide (after the change) if a broken build is due to a) a programming error, or b) the automatic source rewriter not having been applied.

[jsshapiro]

Thinking further about constraint accumulation, it will be interesting to see how type constraints evolve in Go. At some point I suspect the language will need to consider intersection and union types to deal with them comprehensively.

[fzipp]

The type set concept for constraint interfaces models both intersection and union:

https://tip.golang.org/ref/spec#Structure_of_interfaces

• The set of specific types of a non-empty interface is the intersection of the specific types of its interface elements.

• The type set of a union of terms t1|t2|…|tn is the union of the type sets of the terms.

[jabolopes]

If we had a Go equivalent of the absl::StatusOr, let's call it ErrorOr (other names are possible), we could write ParseNumber like the following instead:

func ParseNumber[T constraints.Number](s string, base int) ErrorOr[T]

The typical (value, error) approach has 4 possible states:

1. (an actual value, nil)
2. (not an actual value, error)
3. (an actual value, error)
4. (not an actual value, nil)

But only states 1 and 2 are sensible in most cases. The ErrorOr can only have these sensible states.

Another advantage of ErrorOr is that it combines well with higher order functions. For example, we could write:

func WithFunc[T any](myfunc func()T) T

WithFunc(func()int { ... })

WithFunc(func()ErrorOr[int] { ... })

instead of having 2 versions of WithFunc, one for errors and one for no errors:

func WithFunc[T any](myfunc func()T) T

func WithFuncErr[T any](myfunc func()(T, error)) (T, error)

This would make it easier to write generic libraries and APIs.

[Merovius]

@jabolopes

Haskell can panic (or throw) if the caller tries to take the value when the Either doesn't contain a value, for example:

Fair enough.

For multiple return values, a struct can be used or a tuple type (if we wanted to define one).

ISTM what you are really suggesting, then, is to de-facto replace Go's current multiple-return semantics with tuples. No reason to single out errors.

That would still leave us with the problem of arguments.

I don't understand what this means. Is taking arguments passing arguments via pointer?

No, I just mean "a function which takes an argument". Your WithFunc can't compose those.

I think this is yet another example of objecting to the proposal because it doesn't solve problems that it is not trying to solve.

Exactly. I'm objecting, because it provides too little value for too much harm. Saying "I don't intend to provide much value" doesn't make enough out of too little.

The currying bit I don't think plays a part here and it's rather orthogonal to the discussion.

The currying bit is what makes your WithFunc being unable to compose functions with arguments. It's the dual to composing multiple return types. So it's really the same problem.

I think calling it functional composition is somewhat misleading because it shifts the main argument of the proposal into a conversation about whether Go is or is not a functional language, which is really besides the point. I think the main point y'all here have been completely avoiding is the fact that (T, error) can't instantiate a single-return generic function.

That is saying "function composition doesn't work for functions which return an error". And one of my points is, that singling out "functions which return an error" for this is very limiting and prevents this from providing enough value to be worth its cost.

FWIW I'd be far more interested in a language change which makes multiple-return values behave like tuples (though I'm not sure we could do that). Your suggestion is just a special case of that.

I think in abseil, the main value ErrorOr provides is that C++ doesn't have multiple return values. I'm not sure that if C++ had multiple return values, ErrorOr would exist.

[jabolopes]

@kylelemons

The definition of Do that I had in my mind was slightly different from those, namely, it would be something like:

func Do[T any](g *Group, key string, f func() T) (v T, shared bool)

where f could be instantiated with an error returning function (e.g., func()ErrorOr[T]) or not (e.g., func()T). So it would be the instantiation of f that would decide whether Do would return an error or not. This would also simplify the internal implementation of SingleFlight, which would only need to store a single return value per call (i.e., ErrorT[T]) rather than 2 values per call (i.e., (T, error)).

@Merovius

ISTM what you are really suggesting, then, is to de-facto replace Go's current multiple-return semantics with tuples. No reason to single out errors.

Not exactly. I'm suggesting introducing a new type ErrorOr to do error handling for new APIs instead of using (T, error).

I think the option to use tuples instead of multiple-return semantics is an alternative to ErrorOr and a different way to go about the problem.

I think both options are possible and have different pros and cons. I think the ErrorOr approach is easier to do because it's just defining a new generic type, whereas the tuples approach would require changing parts of the semantic of Go, which is a much bigger proposal.

In any case, I'd be happy to discuss both options.

FWIW I'd be far more interested in a language change which makes multiple-return values behave like tuples (though I'm not sure we could do that).

Your suggestions regarding treating a (x, y, z) as a tuple of arguments instead of separate arguments, the suggestion of currying, the suggestion of improving composition, the suggestion of making multiple-return values behave like tuples, are all still valid and possible. The ErrorOr does not get in the way of that and if anything it makes certain things easier because functions become more composable.

So I don't understand why you'd object to ErrorOr when it doesn't get in the way of what you're trying to achieve and it even makes it easier for you to continue investing in the language features you care about. So it seems to me that by objecting to this proposal you're effectively making it more difficult for you to go further beyond this proposal and introducing those additional language features that you mentioned.

I think in abseil, the main value ErrorOr provides is that C++ doesn't have multiple return values. I'm not sure that if C++ had multiple return values, ErrorOr would exist.

In C++, it's possible: you can do something similar with the following code. So I don't think the argument is applicable:

std::tuple<T, int> MyFunc() ...
auto [x, err] = MyFunc();
if (!err) ...

One more thing that we haven't discussed is that we could extend Go with additional syntax so that ErrorOr could behave monadically so that we could solve the problem with exceptions without actually requiring exceptions. I think this is out of scope of this discussion, so I will just give a quick example.

Let's say we have a new operator (I will use <- but other syntactic choices are possible), then we could do the following:

func MyFunc(...) ErrorOr[T] ...

func OtherFunc(...) ErrorOr[T] {
  x := <- MyFunc(...)
  ...
}

func OtherFunc2(...) ErrorOr[T] {
  x := MyFunc(...)
  if x.Err() != nil {
    return x
  }
  ...
}

In this example, OtherFunc and OtherFunc2 are equivalent but in OtherFunc we use this new operator to avoid having to explicitly write the code that propagates the error up the chain call. This operator has the same meaning as the explicit error handling in OtherFunc2.

[Merovius]

So I don't understand why you'd object to ErrorOr when it doesn't get in the way of what you're trying to achieve

Because every change has a cost and must justify it. The less it does, the more difficult it is to justify that cost. I bring up these other things to demonstrate how small the benefits of your suggestion really are - it's a special case of a special case. What's left is

1. Preventing people from accidentally returning both a value and an error. Which seems a non-existent problem to me.
2. Preventing people from intentionally returning both a value and an error. Which is arguably counterproductive.
3. Preventing people from returning neither, which both seems a non-existent problem and it doesn't even do that particularly well.
4. Allow people to compose error-returning functions, which doesn't need this, as functions could already return struct{ T; error }.

You are correct, that none of this is really talking about the cost of what you suggest. It just make statements about the benefits. For the cost, I'd mostly say

1. The significant work required to move the ecosystem from one convention to the other
2. Syntactical noise (which is subjective, though)

[ncruces]

I think in abseil, the main value ErrorOr provides is that C++ doesn't have multiple return values. I'm not sure that if C++ had multiple return values, ErrorOr would exist.

I'll go further than that: I'm not sure if C++ had exceptions from the start, we'd have absl::StatusOr.
https://google.github.io/styleguide/cppguide.html#Exceptions

[leaxoy]

The "multiple invalid states" need to be guaranteed at the construction of ErrorOr.

That's the easy part. For the hard part, you suggest:

func (e ErrorOr[T]) Val() T ...



func (e ErrorOr[T]) Err() error ...

How is that different from just returning (T, error)? i.e. What would prevent the user from calling Val() before checking Err()?

That's why I said the only really viable design is my Match method. It's the only way to guarantee that only the correct branch is executed. If you don't guarantee that, you might as well stay with (T, error), it's idiomatic and provides the same level of safety.

The (T，error) is product type, while ErrorOr[T] or Result[T] is sum type.

For each element has two state, the first case will produce 4 states, and then second only produce 2 states.

When handling result, the first case will check all 4 states, but the second one only 2 states.

The proper way is introduce tagged union or sum type, and add Result[T] like rust.

[robpike]

@jabolopes Your part of this discussion has drifted far from the original intent of this issue and is now taking up about half the space. Can you please move the discussion to a separate issue and do so in the form of a proposal?

This proposal is originally about how to introduce generic versions of things like Min and Map. You are asking for a new way to think about errors. That deserves to be a separate conversation.

[jabolopes]

Hi Rob, thank you so much for the suggestion to create a new proposal. Created #51931.

Apologies for taking up so much space. When I wrote the first comment I didn't expect the thread to grow so much.

Thanks,
Jose

[smasher164]

I was recently discussing this on twitter, but instead of suggesting to library designers to provide two versions of each function, one with a predefined constraint, and one with a func value as a parameter -- why not provide functions for those operators in the constraints package?

That way, instead of choosing between Sort(v) and Sort(v, func() { ... }), they can just do

Sort(v, constraints.LessThan[T])

[willfaught]

@rsc:

• func math.Abs would naturally be math.Abs[T]
• func math.Min would naturally be math.Min[T]
• func math.Max would naturally be math.Max[T]

I don't think we'd want to make math.Abs, Min, or Max generic anyway, since they take into account NaN, infinities, and signed zeroes. Are there any other examples of wanting to abstract a non-interface type to a type variable in the stdlib?

I think we should follow Java's approach, with raw types. The Go equivalent would be:

• You can change any non-generic type into a generic type, but the constraints must be basic interfaces if the type will be used "raw."
• Uses of the type that don't instantiate it are implicitly instantiated with its constraint types (typically interface{}/any).

So var x list.List means var x list.List[any], assuming the constraint is any.

We would get:

• Old code compiled with a pre-generics compiler continues to work.
• Old code compiled with a post-generics compiler continues to work.
• Old code with added generics continues to work with old pre-generics code.
• Old code with added generics works with post-generics code that uses it generically.

Due to the basic interface constraint limitation, we wouldn't be able to convert, say,

func Max(x, y int64) int64 {
    if x > y {
        return x
    }
    return y
}

var max func(int64, int64) int64 = Max

to

func Max[T constraints.Ordered](x, y T) T {
    if x > y {
        return x
    }
    return y
}

var max func(?, ?) ? = Max

because there's nothing we can put for ?, because constraints.Ordered can't be used as a type for a value.

For the cases where we'd need to add a second, generic, variant of a function, I suggest appending the constraint to the name, rather than "Of". E.g. FooAny[T any], FooReader[T io.Reader], etc. for Foo. So, in the case of Max, we'd have Max and MaxOrdered.

[perj]

I've been holding off commenting here, because from the discussion it doesn't sound like this is a very popular choice, but since nobody mentioned it this way yet and this issue has resurfaced, I figured I would.

One obvious thing for me to consider when it's about a single or a few symbols that needs a new version is to version those symbols. That's how I've been solving the Setup function in one of my packages, without generics being involved. I renamed it to SetupV2, changing the signature, then made a new Setup with the old signature calling the new version, also indicating that's what it does in the documentation.

I think that's concise and delivers the message, while still being backwards compatible.

It's possible to extend it slightly based on some comments in this thread. For example, one could make sure to also create a SetupV1 when there need for SetupV2 arise. SetupV1 and the old Setup would be identical, both calling SetupV2. One could then start thinking about go.mod maneuvering to change the signature of Setup to that of SetupV2 instead of SetupV1 based on the version number in there, but it would be a later problem. Or perhaps to drop the versionless one altogether.