Hello fellow gophers,
Ian announced the intention to update how type switches work in the
generics proposal in a golang-nuts thread from this August:
https://groups.google.com/g/golang-nuts/c/iAD0NBz3DYw
It included this from Ian (which is a snippet of his item #4 from his first
post there):
----------------------------------------------------------------------------------
In a type switch on a type parameter with a type list, every case
listed must be a type that appears in the type list (“default” is also
permitted, of course). A case will be chosen if it is the type matched
by the type argument, although as discussed above it may not be the
exact
type argument: it may be the underlying type of the type argument. To
make
that rule very clear, type switches will not be permitted for type
parameters that do not have type lists.
----------------------------------------------------------------------------------
There was a follow-on discussion in that thread on the desired type switch
behavior, including gophers like Rog Peppe and Jimmy Frasche commenting on
what they were hoping this meant.
I think the conclusion of that conversation might have been that adjustment
in type switch behavior suggested by Ian was not adopted as part of the
current proposal... but I'm not 100% sure, and wanted to check if that is
indeed the case (vs. maybe it was adopted, and perhaps I am just missing
how it works in the current go2go prototype, or maybe it is still planned
to be adopted but maybe the proposal and/or prototype haven't been updated
yet).
The most definitive comment from Ian as part of that conversation might
have been:
----------------------------------------------------------------------------------
https://groups.google.com/g/golang-nuts/c/iAD0NBz3DYw/m/5KceLsiLBAAJ
----------------------------------------------------------------------------------
Given a type parameter, there are two interesting pieces of
information. One is what the actual type argument is; we can already
determine that by writing code like "var x T; switch
(interface{})(x).(type) { ... }". The other is which type in the type
list was matched by the type argument. The latter is the purpose of
the type switch suggested here. Without something like that type
switch, there is no straightforward way for a generic function to
determine which type in a type list is matched by a type argument.
See also
https://go.googlesource.com/proposal/+/refs/heads/master/design/go2draft-type-parameters.md#identifying-the-matched-predeclared-type
I'm not at all wedded to this. We can continue to omit it. It just
seems like adding a capability that does not otherwise exist.
----------------------------------------------------------------------------------
The current draft seems to hold out the following as a future option:
----------------------------------------------------------------------------------
https://go.googlesource.com/proposal/+/refs/heads/master/design/go2draft-type-parameters.md#identifying-the-matched-predeclared-type
----------------------------------------------------------------------------------
One way to handle this would be to permit type switches on the type T,
with the
proviso that the type T would always match a type defined in the
constraint. This
kind of type switch would only be permitted if the constraint lists
explicit types,
and only types listed in the constraint would be permitted as cases.
----------------------------------------------------------------------------------
In any event, I am curious about the latest thinking and/or latest status.
If this has been discussed at length elsewhere since that August thread, I
would be happy for a pointer if anyone has one.
Thanks, and sorry if the answer is I am just confused ;-)
Best regards,
thepudds
--
You received this message because you are subscribed to the Google Groups
"golang-nuts" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to golang-nuts+unsubscr...@googlegroups.com.
To view this discussion on the web visit
https://groups.google.com/d/msgid/golang-nuts/dbdc0e64-b41d-4c69-9218-5972dc3b21a3n%40googlegroups.com.
