Generics and constraints question

[Brian Candler]

Question: how to make a generic function which takes a type T where the constraint is "*T satisfies a given interface"?

func (m *MyMap[KT, VT]) init() {

        m.data = make(map[KT]VT)

}

type Initializer interface {

init()

}

func mynew[T Initializer]() *T {

t := new(T)

t.init()

return t

}

/* or:
func mynew[T Initializer]() *T {

var t T

t.init()

return &t

}

*/

func main() {

m := mynew[MyMap[string, int]]()

}

gives: 

type checking failed for main prog.go2:34:13: MyMap[string, int] does not satisfy Initializer: wrong method signature

        got func (*MyMap[KT, VT]).init()

        want func (Initializer).init()

Full code: https://go2goplay.golang.org/p/INcenfcInSl

[roger peppe]

You can do this with type-list interfaces: https://go2goplay.golang.org/p/wfHnVHOMcyK

This is one of the harder parts of the proposal to understand IMHO, but perhaps it will become easier with familiarity.

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[Ian Lance Taylor]

This definitely needs a better error message.

Ian

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[Brian Candler]

On Tuesday, 5 January 2021 at 20:27:57 UTC rog wrote:

You can do this with type-list interfaces: https://go2goplay.golang.org/p/wfHnVHOMcyK

This is one of the harder parts of the proposal to understand IMHO, but perhaps it will become easier with familiarity.

Thank you!