[go-nuts] Re: invalid recursive type alias

['Bryan Mills' via golang-nuts]

The compiler is behaving as designed in both cases.

See https://golang.org/issue/25187 (and the 
associated https://golang.org/issue/25141).


On Saturday, December 8, 2018 at 12:33:48 PM UTC-5, Jan Mercl wrote:
>
> This code compiles fine
>
> package main
> 
> type node struct {
> next *node
> }
> 
> func main() {}
>
> (https://play.golang.org/p/ZYg0EciQnOQ)
>
> This code does not
>
> package main
> 
> type node = struct {
> next *node
> }
> 
> func main() {}
>
> (https://play.golang.org/p/gWWX8ngPsS6)
>
> The error is
>
> prog.go:3:6: invalid recursive type alias node
> prog.go:3:6: node uses 
> prog.go:3:13:  uses node
>
> The specification remains silent about what is considered "invalid 
> recursive type alias" so I'm not sure, but it seems to me the first and 
> second programs should both compile fine. In both cases, so to say "the 
> pointer breaks the cycle" while type checking. But maybe/probably I'm 
> mistaken.
>
> Can anybody enlighten me please and explain if the compiler is right or if 
> it's a bug?
>
> Thanks in advance.
>
> -- 
>
> -j
>

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[go-nuts] Re: invalid recursive type alias

[Space A.]

Explained in proposal 
:

Type
 
> cycles
>
> In a type alias declaration, in contrast to a type declaration, T2 must 
> never refer, directly or indirectly, to T1. For example type T = *T and type 
> T = struct { next *T } are not valid type alias declarations. In 
> contrast, if the equals signs were dropped, those would become valid 
> ordinary type declarations. The distinction is that ordinary type 
> declarations introduce formal names that provide a way to describe the 
> recursion. In contrast, aliases must be possible to “expand out”, and there 
> is no way to expand out an alias like type T = *T.
>


суббота, 8 декабря 2018 г., 20:33:48 UTC+3 пользователь Jan Mercl написал:
>
> This code compiles fine
>
> package main
> 
> type node struct {
> next *node
> }
> 
> func main() {}
>
> (https://play.golang.org/p/ZYg0EciQnOQ)
>
> This code does not
>
> package main
> 
> type node = struct {
> next *node
> }
> 
> func main() {}
>
> (https://play.golang.org/p/gWWX8ngPsS6)
>
> The error is
>
> prog.go:3:6: invalid recursive type alias node
> prog.go:3:6: node uses 
> prog.go:3:13:  uses node
>
> The specification remains silent about what is considered "invalid 
> recursive type alias" so I'm not sure, but it seems to me the first and 
> second programs should both compile fine. In both cases, so to say "the 
> pointer breaks the cycle" while type checking. But maybe/probably I'm 
> mistaken.
>
> Can anybody enlighten me please and explain if the compiler is right or if 
> it's a bug?
>
> Thanks in advance.
>
> -- 
>
> -j
>

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