This variable m is a map of string keys to int values:
var m map[string]int
Map types are reference types, like pointers or slices,
and so the value of m above is nil; it doesn't point to an initialized map.
A nil map behaves like an empty map when reading,
but attempts to write to a nil map will cause a runtime panic;
don't do that. To initialize a map, use the built in make function:
m = make(map[string]int)