Type does not implement interface with a interface method signature

Question

Situation

Bar has a method Get() with a signature *Foo
Foo implements 2 interfaces I_Venus1 and I_Mars1.
I_Venus2 has a method Get() with a signature I_Venus1
I_Mars2 has a method Get() with a signature I_Mars1.

type Foo struct {
}

type Bar struct {
}

func (*Bar) Get() *Foo {
    return nil
}

type I_Venus1 interface {
}

type I_Venus2 interface {
    Get() I_Venus1
}

type I_Mars1 interface {
}

type I_Mars2 interface {
    Get() I_Mars1
}

var _ I_Venus1 = &(Foo{}) // Foo meets I_Venus1 interface
var _ I_Mars1 = &(Foo{}) // Foo meets I_Mars1 interface

Problem

var _ I_Venus2 = &(Bar{}) // *Bar does not implement I_Venus2 (wrong type for method Get)
var _ I_Mars2 = &(Bar{})  // *Bar does not implement I_Mars2 (wrong type for method Get)

I do not understand what prevents the go compiler from deciding that Bar implements both I_Venus2 and I_Mars1 interfaces.

Note : this question is a variation on

Go Interface Method Returning Interface Doesn't Match Method Returning Concrete Type

Bar.Get() returns *Foo whereas I_Venus1.Get() returns I_Venus1 and I_Mars2.Get() returns I_Mars1. *Foo ≠ I_Venus1 ≠ I_Mars1 — Oluwafemi Sule
– Oluwafemi Sule, Commented Mar 2, 2024 at 10:18
@OluwafemiSule, I can see identifiers do not match. What I cannot understand is why, since the compiler decides that *Foo implements both interface, the compiler does not decide that Bar implements the interfaces with the method signature. It does not seem consistent to my eyes. — thomas
– thomas, Commented Mar 2, 2024 at 11:13
"I do not understand what prevents the go compiler from deciding that Bar implements both I_Venus2 and I_Mars1 interfaces." The language specification forbids the compiler to do so. — Volker
– Volker, Commented Mar 2, 2024 at 12:26

thomas · Accepted Answer · 2024-03-02 17:31:53Z

-2

@peter has provided the good answer by pointing to go's FAQ.

Programmers who want covariant result types are often trying to express a type hierarchy through interfaces. In Go it's more natural to have a clean separation between interface and implementation.

answered Mar 2, 2024 at 17:31

thomas

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1 Comment

Maulik Bhalani Over a year ago

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