groupBy predicates: unary, binary, or both?

[w0rp via Digitalmars-d]

I'm sorry to bump up an old thread, but whatever happened to 
this? I was just using 'groupby' in Python for a common data 
migration pattern I've developed...

# Build a dictionary of parent -> child relationships
# from a datbase query, for quick and dirty data migration.
# This runs in linear time.
{
     y[0], tuple(y[1])
     for y in
     groupby(
         some_query_func(
             "SELECT parent_id, ... "
             # ...
             "ORDER BY parent_id "
         ),
         lambda x: x["parent_id"]
     )
}

... and I was thinking "it would be nice to have this in D, too!"