[generics] default type parameter

[Sebastien Rosset]

First apologies for initially bringing up the topic as a github issue rather than golang-nuts. I am copy-pasting my question here with @ianlancetaylor response.

A default type value can be specified in C++ templates. I was asking if a similar construct been considered for go generic type parameters. Potentially this would make it possible to retrofit existing types without breaking existing code.

For example, consider the existing asn1.ObjectIdentifier type which is a []int. One problem with this type is it's not compliant with the ASN.1 specification, which states each sub-oid may be an INTEGER of arbitrary length (e.g. *big.Int). Potentially ObjectIdentifier could be modified to accept a generic parameter, but that would break a lot of existing code. If there was a way to specify int is the default parameter value, maybe that would make it possible to retrofit existing code.

type SignedInteger interface {
	type int, int32, int64, *big.Int
}
type ObjectIdentifier(type T SignedInteger) []T
// type ObjectIdentifier(type T SignedInteger=int) []T  // `int` would be the default instantiation type.

// New code with generic awareness would compile in go2.
var oid1 ObjectIdentifier(int) = ObjectIdentifier(int){1, 2, 3}

// But existing code would fail to compile:
var oid1 ObjectIdentifier = ObjectIdentifier{1, 2, 3}

Just to be clear, the above asn1.ObjectIdentifier is just an example. I'm not saying using generics is the only way or the best way to solve the ASN.1 compliance issue.

> @sebastien-rosset We have not considered default types for generic type parameters. The language does not have default values for function arguments, and it's not obvious why generics would be different. In my opinion, the ability to make existing code compatible with a package that adds generics is not a priority. If a package is rewritten to use generics, it's OK to require existing code to change, or to simply introduce the generic code using new names.

[Sebastien Rosset]

@ianlancetaylor , thank you for the quick reply. The reason I was asking is because potentially this could have been used to fix `type ObjectIdentifier []int` in the `encoding/asn1` package and the `crypto/x509` package. Currently these package are not fully compliant with the ASN.1 specification, which means in practice some certificates cannot be parsed.

I am trying to fix the encoding/asn1 and crypto/x509 package by adding support for OID values that are greater than 2^31. There are multiple ways to fix the issues, and unfortunately it won't be possible to simply change the ObjectIdentifier type because that would break too many applications. If it's not possible to change the type, then most alternatives seem to be somewhat cumbersome. For reference the PR is https://github.com/golang/go/pull/39795.

Sebastien

[Ian Lance Taylor]

On Fri, Jun 26, 2020 at 11:03 AM Sebastien Rosset <sro...@gmail.com> wrote:
>
> @ianlancetaylor , thank you for the quick reply. The reason I was asking is because potentially this could have been used to fix `type ObjectIdentifier []int` in the `encoding/asn1` package and the `crypto/x509` package. Currently these package are not fully compliant with the ASN.1 specification, which means in practice some certificates cannot be parsed.
>
>
> I am trying to fix the encoding/asn1 and crypto/x509 package by adding support for OID values that are greater than 2^31. There are multiple ways to fix the issues, and unfortunately it won't be possible to simply change the ObjectIdentifier type because that would break too many applications. If it's not possible to change the type, then most alternatives seem to be somewhat cumbersome. For reference the PR is https://github.com/golang/go/pull/39795.

Thanks, understood.

Generics don't solve all problems. I agree that there seems to be a
way that we could modify generics to solve this particular problem.
But it means introducing an idea that the rest of the language has
decided to reject: default values for arguments. I don't think it
would be consistent with the language to permit default values for
type arguments when we do not permit default values for non-type
arguments. While we don't have to be strictly consistent here, I
think we need a good reason to break consistency. And in the larger
scheme of things I don't think that making it easier to make a
backward compatible change to one specific package, a package that is
not all that widely used, is a good enough reason.

I'm not claiming to have the final word, but that is my opinion.

Ian

[Sebastien Rosset]

sure, thank you. I will go through the PR review process for asn1 and x509, maybe some good ideas will come up. 

Sebastien 

[Sebastien Rosset]

As an aside, the most common use of the encoding/asn1 package is most likely crypto/x509. x509. Certificate exposes public fields that use the asn1.ObjectIdentifier, so asn1 ends up being exposed in a lot of applications, such as for TLS connection management.

[Ian Lance Taylor]

On Fri, Jun 26, 2020 at 12:54 PM Sebastien Rosset <sro...@gmail.com> wrote:
>
> As an aside, the most common use of the encoding/asn1 package is most likely crypto/x509. x509. Certificate exposes public fields that use the asn1.ObjectIdentifier, so asn1 ends up being exposed in a lot of applications, such as for TLS connection management.

True, but will those packages be affected by a change in the encoding/asn1 API?

When I said that the package was not widely used I should have said
that not many other packages import it, and thus not many other
packages are affected by any changes. I shouldn't have implied that
only a few Go programs import it at all.

Ian

> On Friday, June 26, 2020 at 12:04:09 PM UTC-7, Sebastien Rosset wrote:
>>
>> sure, thank you. I will go through the PR review process for asn1 and x509, maybe some good ideas will come up.
>> Sebastien
>>
>> On Friday, June 26, 2020 at 11:51:05 AM UTC-7, Ian Lance Taylor wrote:
>>>
>>> On Fri, Jun 26, 2020 at 11:03 AM Sebastien Rosset <sro...@gmail.com> wrote:
>>> >
>>> > @ianlancetaylor , thank you for the quick reply. The reason I was asking is because potentially this could have been used to fix `type ObjectIdentifier []int` in the `encoding/asn1` package and the `crypto/x509` package. Currently these package are not fully compliant with the ASN.1 specification, which means in practice some certificates cannot be parsed.
>>> >
>>> >
>>> > I am trying to fix the encoding/asn1 and crypto/x509 package by adding support for OID values that are greater than 2^31. There are multiple ways to fix the issues, and unfortunately it won't be possible to simply change the ObjectIdentifier type because that would break too many applications. If it's not possible to change the type, then most alternatives seem to be somewhat cumbersome. For reference the PR is https://github.com/golang/go/pull/39795.
>>>
>>> Thanks, understood.
>>>
>>> Generics don't solve all problems. I agree that there seems to be a
>>> way that we could modify generics to solve this particular problem.
>>> But it means introducing an idea that the rest of the language has
>>> decided to reject: default values for arguments. I don't think it
>>> would be consistent with the language to permit default values for
>>> type arguments when we do not permit default values for non-type
>>> arguments. While we don't have to be strictly consistent here, I
>>> think we need a good reason to break consistency. And in the larger
>>> scheme of things I don't think that making it easier to make a
>>> backward compatible change to one specific package, a package that is
>>> not all that widely used, is a good enough reason.
>>>
>>> I'm not claiming to have the final word, but that is my opinion.
>>>
>>> Ian
>

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[Sebastien Rosset]

Below are the publicly exposed asn1.ObjectIdentifier fields in the golang/go repo. It has fairly limited exposure,

but I'm sure some people will argue it's too much (maybe ok in go 2.x but not 1.x?).

1. encoding/asn1:
1. My guess is this would be primarily used by SNMP applications such as https://github.com/soniah/gosnmp/search?q=asn1&unscoped_q=asn1
   
2. crypto/x509/pkix package has 4 exported fields using asn1.ObjectIdentifier
3. crypto/x509 package:
1. ECDSA keys
2. Used in the x509.Certificate struct:

// A Certificate represents an X.509 certificate.

type Certificate struct {

...

UnhandledCriticalExtensions []asn1.ObjectIdentifier

UnknownExtKeyUsage []asn1.ObjectIdentifier

PolicyIdentifiers []asn1.ObjectIdentifier

}

I doubt there are many applications that actually inspect these 3 fields.

[Robert Engels]

What about an option to disable the signal on the thread as it crosses the CGo boundary? This seems better than disabling the async preemption entirely?

On Jun 26, 2020, at 4:45 PM, Sebastien Rosset <sro...@gmail.com> wrote:



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[Sebastien Rosset]

Did you reply to the wrong thread?

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[Robert Engels]

Yes I did. Sorry all. 

On Jun 26, 2020, at 7:42 PM, Sebastien Rosset <sro...@gmail.com> wrote:



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