unfold and flattening

[Henry Cejtin]

In  the  old  code,  what  is  the +? infix function?  Also what is the array
function, which only takes a size argument and not an  element  to  fill  the
resulting  array  with?   Also  I'm  surprised that you use the basis curried
List.foldl.

Also you optimize the case where the list has a single  element  in  the  old
code  and  you  optimize  the  case where the list has no elements in the new
code.  Why the change?

Actually, is the singleton optimization legal?  For immutable  objects  sure,
but for mutable ones it seems clearly incorrect.

It  is  a  bit  ugly either way.  The real problem is that you would like the
control flow to be controlled by the  generator  of  the  elements,  not  the
consumer.  Then you could avoid the extra test per loop.

More  seriously,  optimizing  away the tuple construction is really important
especially if you use fold/unfold a lot.  Then you end up tupling things  all
the time.