18 - program/list ambiguity with stream:each (0.5-dev)

[joshsh]

The following script illustrates a scenario in which list transparency
becomes an issue.
{{{
@redefine a twice1:
    a a both >> .
@redefine a twice2:
    (a a) each >> .
42 :twice1 >> .
42 :twice2 >> .
}}}
  The first program, :twice1, does what it appears to do: it produces the
argument 'a' in two stacks -- {(a), (a)}.  The second program, :twice2,
appears to do the same thing, however, it produces a strange result {(a),
(dup)}.  This is because the stream:each primitive treats the list (a a) as
opaque, merely iterating through the items in the list without first
applying the reduction algorithm, whereas the transformation rule which
turns :twice2 into a point-free program depends on all lists being
transparent: they're equivalent to the lists they represent, without being
identical to them.  (a dup >>) is equivalent to (a a), but contains
different elements when iterated through by stream:each.  Either
stream:each and other list primitives will need to be made to treat lists
transparently, or a different transformation strategy for programs with
named parameters will need to be thought out.

Original link: http://code.google.com/p/ripple/issues/detail?id=18