Delete element from custom type (slice of pointers) in Go

I have to remove an element from custom type "MergedVars" (which is a slice of pointers) while running a method of that custom type:

type Merged struct {
	Key          string
	Value        int
	Old          *Merged
}

type MergedVars []*Merged


func (m MergedVars) String() string {
	var out string

	for _, v := range m {
		out += fmt.Sprintf("Key: `%s`, Value: `%d`, Old: `%v`\n", v.Key, v.Value, v.Old)
	}

	return out
}
	

func (m *MergedVars) Insert(key string, value int) {
	v := &Merged{
		Key:          key,
		Value:        value,
	}

	for k, mv := range *m {
		if mv.Key == key {
			v.Old = mv
			m.remove(k)
		}
	}

	*m = append(*m, v)
}


// This fails
func (m *MergedVars) remove(pos int) {
	// https://github.com/golang/go/wiki/SliceTricks
	m, m[len(m)-1] = append(m[:pos], m[pos+1:]...), nil
}

So running this...

m := MergedVars{}
m.Insert("foo", 1)
m.Insert("bar", 2)
m.Insert("foo", 3)

... should get me a slice of two elements where the element with the key "foo" has value 3 and its "old" field points to "foo": 1

See it (not) working at:

http://play.golang.org/p/0-cx8tdOZD

How do I have to implement the remove function proparly?

From you requirement, what I understand is that you need to replace strut whose key matches with new strut with updated value and point Old to the previous one getting replaced.
One way to do as follows, where you don't need to remove from slice.

Please note in this case your insertion order fro the new foo, 3 which is inserted after bar will not be maintained, and will be the replace the foo, 1 at position 0, before bar.

func (m *MergedVars) Insert(key string, value int) {
	v := &Merged{
		Key:   key,
		Value: value,
	}

	found := false
	for k, mv := range *m {
		if mv.Key == key {
			v.Old = mv
			m1 := *m
			m1[k] = v
			found = true
		}
	}

	if !found {
		*m = append(*m, v)
	}
}

http://play.golang.org/p/KINvnAP08Q