Appending to pointer slice - go

Question

Go is my first programming language and I am trying to learn about pointers by writing a program that organizes information based on taxonomies. I'm having some trouble understanding how to append to a pointer slice.
type List struct {
Taxonomies []Taxonomy
}
func (l *List) Add(t Taxonomy) {
var exists bool
var existing *Taxonomy
for _, taxonomy := range l.Taxonomies {
if taxonomy.Name == t.Name {
exists = true
existing = &taxonomy
}
}
if exists {
for _, term := range t.Data {
termExists := false
for _, existingTerm := range existing.Data {
if existingTerm.Name == term.Name {
termExists = true
break
}
}
if termExists {
continue
}
(*existing).Data = append((*existing).Data, term)
}
} else {
l.Taxonomies = append(l.Taxonomies, t)
}
}
type Taxonomy struct {
Name string
Data []Term
}
type Term struct {
Name, Link string
}
I think the problem is toward the bottom, this line:
(*existing).Data = append((*existing).Data, term)
By following the code in a debugger, I can see that the taxonomy stored in the "existing" variable is being updated when the append occurs, but the data is not updated in the actual List.
Can anyone tell me where I am going wrong?

Answer 1

l.Taxonomies is a []Taxonomy, so the taxonomy value is going to be a copy of the element, and changes to that copy will not be reflected in the original List value.
You can iterate using the index to avoid copying the value
for i := range l.Taxonomies {
if l.Taxonomies[i].Name == t.Name {
exists = true
existing = &l.Taxonomies[i]
}
}
However that still leaves the possibility of copying the data passed to methods like Append. Instead it's probably better to use pointers throughout:
type List struct {
Taxonomies []*Taxonomy
}
func (l *List) Add(t *Taxonomy) {
...