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.
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