Manipulate struct field when using interface

Question

The title might be misleading, but to the point...

I have a single interface Expression:

type Expression interface {
    String() // skiped in implementation below
}

The interface is implemented by multiple structs, some of which implements same interface as a field value:

type IdentExpression struct {
    value string
}

type UnaryExpression struct {
    token string
    value Expression
}

func (a *UnaryExpression) Simplify() {
    var finalValue Expression
    switch a.value.(type) {
    case UnaryExpression:
        tmp := a.value.(UnaryExpression)
        switch tmp.value.(type) {
        case UnaryExpression:
            tmp = tmp.value.(UnaryExpression)
            finalValue = tmp.value
        }
    }

    a.value = finalValue
}

Given expression -(-(-(1))), UnaryExpression.Simplify() will simplify the expression to -(1). (play)

I would like to extend the interface with Simplify() method:

type Expression interface {
    Simplify()
    String() string
}

// ...

func (a IdentExpression) Simplify() {} // do nothing

Resulting code does not work (play):

main.go:29: impossible type switch case: a.value (type Expression) cannot have dynamic type UnaryExpression (missing Simplify method)

main.go:30: impossible type assertion:
UnaryExpression does not implement Expression (Simplify method has pointer receiver)
main.go:59: cannot use UnaryExpression literal (type UnaryExpression) as type Expression in field value:
UnaryExpression does not implement Expression (Simplify method has pointer receiver)
main.go:60: cannot use UnaryExpression literal (type UnaryExpression) as type Expression in field value:
UnaryExpression does not implement Expression (Simplify method has pointer receiver)

I have found this answer, which looks similar however I do not know how to apply it in my case.

remove the pointer reference in Simplify function. Your code should work. — Pharaoh
– Pharaoh, Commented Jul 22, 2017 at 17:45

Nick · Accepted Answer · 2017-07-22 17:51:36Z

4

The key here is that you are using a pointer receiver in your definition of Simplify() with respect to UnaryExpression:

func (a *UnaryExpression) Simplify()

The other methods you are implementing don't use a pointer receiver:

// One example
func (a IdentExpression) Simplify() {}

Typically, in Go, it is considered best practice to have all methods on the same type use the same type of receiver (i.e. if one method uses a pointer receiver, they all should. Likewise, if one method uses a non-pointer receiver, they all should for that particular type).

In this case, the code will compile if you remove the pointer receiver from the Simplify method of UnaryExpression. Hope this helps!

Edit: Here is a more comprehensive answer that explains exactly why this error happens, it's really a good read.

edited Jul 22, 2017 at 17:51

answered Jul 22, 2017 at 17:39

Nick

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1 Comment

Artur Rychlewicz Over a year ago

If I remove the pointer, the code compiles, but not works :) Still, your answer together with the answer you linked help me to find the issue - I needed to replace all my assertions to pointers (here's the fixed code)

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