[go-nuts] Is there a better way to write this?

['Aaron Spangler' via golang-nuts]

I seem to be regularly working with maps of stringsets and I have
found myself writing the same pattern over and over and I ended up
having my own utility function.  More importantly the syntax seems
clunky when a key doesn't exist in the map.  Is there a better way (go
idiomatically way) to write this?

import  "google3/third_party/golang/stringset/stringset"

var urlMap = make(map[string]*stringset.Set)

func collectStringsetMap(m map[string]*stringset.Set, key string,
value ...string) *stringset.Set {
  s, ok := m[key]
  if !ok {
    newset := stringset.New()  // Why can't I just use &(stringset.New()) ?
    s = &newset
    m[key] = s
  }
  s.Add(value...)
  return s
}

If there is a way to clean up the code, let me know.

Also why can map's just take a default function to construct items
that are not yet present?
Something like:

var urlMap = make(map[string]*stringset.Set, ...func =
defaultStringsetConstructor...)

func defaultStringsetConstructor(key string) *stringset.Set {
  n := stringset.New()
  return &n
}

Or perhaps some trickery in the map lookup?

 s, ok := m[key, defaultStringsetConstructor]

At the end of the day you want the map to save any new items that were
not already in the map, but I don't see any less clunky ways of doing
this.  (What am I missing?)

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