Generics and formatting

[Brian Candler]

Just a minor issue, but playing with this example:

https://go2goplay.golang.org/p/IUL1QDrtOwW

- click "Run" and it works

- click "Format" and it fails with the following error:

prog.go:40:24: expected ']', found ',' (and 6 more errors)

[Ian Lance Taylor]

Hmmm, perhaps we failed to update gofmt the last time we updated the
go2go playground. The gofmt on the tip of the dev.go2go branch does
seem to work correctly. Can you open an issue at
https://golang.org/issue? Thanks.

Ian

[Brian Candler]

Raised as https://github.com/golang/go/issues/43470

[Ian Lance Taylor]

On Sat, Jan 2, 2021 at 10:28 AM 'Brian Candler' via golang-nuts
<golan...@googlegroups.com> wrote:
>
> Raised as https://github.com/golang/go/issues/43470

Thanks.

Ian

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[Brian Candler]

I don't know if this is expected, but this (bad) program causes the go2go playground to give "panic: assertion failed"

https://go2goplay.golang.org/p/VH5SLJNxw3X

- I think it's the compiler itself that's panicing, not the runtime (since it doesn't get as far as the next line).

----

Aside: my general feeling so far is that the error messages given from go programs which misapply generic syntax can be very unhelpful.  Here's one example:

https://go2goplay.golang.org/p/m9YUDBTWwpA

Error given: "missing ',' in type argument list"

Actual problem: a constraint was used in a place where it's not allowed

[Brian Candler]

Here's another unhelpful error: https://go2goplay.golang.org/p/GDxiXiIIti7

Error: "prog.go2:7:11: all type parameters must be named"

Actual problem: the type parameters were named, but no constraint was given.

func blah[T1, T2](x T1) T2 {  // wrong
func blah[T1, T2 any](x T1) T2 {  // fixed

[Ian Lance Taylor]

On Tue, Jan 5, 2021 at 8:11 AM Brian Candler <b.ca...@pobox.com> wrote:
>
> I don't know if this is expected, but this (bad) program causes the go2go playground to give "panic: assertion failed"
>
> https://go2goplay.golang.org/p/VH5SLJNxw3X
>
> - I think it's the compiler itself that's panicing, not the runtime (since it doesn't get as far as the next line).

This is a bug in the type checker, which shouldn't panic. Want to
open an issue?

> Aside: my general feeling so far is that the error messages given from go programs which misapply generic syntax can be very unhelpful. Here's one example:
> https://go2goplay.golang.org/p/m9YUDBTWwpA
>
> Error given: "missing ',' in type argument list"
> Actual problem: a constraint was used in a place where it's not allowed

Here too issues will help. Thanks.

Ian

[Brian Candler]

On 06/01/2021 21:52, Ivan Majeru wrote:

If I understand right the panic is because the `m := NewMap[string, int]` is a function not a function call and after that you try to assign to this function `m[int]`. This works https://go2goplay.golang.org/p/NOaPH-dJyWd .

Sure, I know it was wrong, but I was still able to crash the compiler.

Previously I had tried using the (wrong) map-like syntax like this:

    m["foo"] = 123

and this gave a helpful error message (*):

"foo" (untyped string constant) is not a type

which, if you know about generic type parameters makes sense.

So out of interest, I changed it to see what would happen if I *did* put a type there:

    m[int] = 123

... and that caused the panic.

(*) Helpful-ish.  As you say, m is a function, indeed a non-generic one without type parameters. Trying to qualify a such a function with a type doesn't make sense.  Perhaps a better error would be "cannot use type parameters on a non-generic function" or similar.

[Ivan Majeru]

If I understand right the panic is because the `m := NewMap[string, int]` is a function not a function call and after that you try to assign to this function `m[int]`. This works https://go2goplay.golang.org/p/NOaPH-dJyWd .

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