We cannot add methods to non-local types, that is, types not defined in our package.

We can however create a new type that inherits from the type we want to add the method too:

1type MyInt int

Now, MyInt is a local type we can add methods to.

1func (i MyInt) Twice() MyInt {
2    return i * 2
3}

Careful that Go considers int and MyInt two distinct types. You can’t pass an int value where a MyInt is expected, and vice versa.

This means we have to convert ints to MyInts using type conversion:

1input := MyInt(9)

Not every type conversion is possible.

References

• go-struct-wrappers
• John Arundel, For the Love of Go

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