Using functional options instead of method

[hyousef (Hasan Yousef)]

I tried the below, and it worked well:

package main

import "fmt"

type T interface {
}

type hashMap struct {
	m    map[T]T
	k    []T
}

func (h *hashMap) From(m map[T]T) {
	h.m = m
	h.k = make([]T, len(m))
	i := 0
	for key := range m {
		h.k[i] = key
		i++
	}
}

func main() {
	inv := new(hashMap)
	inv.From(map[T]T{"first": 1})
	fmt.Printf("%v", inv)
}

The output was correct as expected:

&{map[first:1] [first]}

I’m looking for a way where I can write:

inv := new(hashMap).from(map[T]T{"first": 1})
// instead of:
//	inv := new(hashMap)
//	inv.From(map[T]T{"first": 1})

So, I re-wrote the code as below:

package main

import "fmt"

type T interface {
}

type hashMap struct {
	m    map[T]T
	k    []T
	from func(m map[T]T) hashMap    // <---- new
} 

func from(m map[T]T) hashMap {        // <----- change of func signature
	h := new(hashMap)
	h.m = m
	h.k = make([]T, len(m))
	i := 0
	for key := range m {
		h.k[i] = key
		i++
	}
	return *h
}

func main() {
	inv := new(hashMap).from(map[T]T{"first": 1})
	fmt.Printf("%v", inv)
}

But I got the below output:

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xc0000005 code=0x0 addr=0x0 pc=0x49c2c2]

goroutine 1 [running]:
main.main()
	d:/goplay/hashmap.go:55 +0xf2
exit status 2

Any idea how to get it done the way I’m looking for?