Allow assign operators to work for arbitrary messages on the left side

[piotrklibert]

Hi,

while I was working on PyParsing port for Io, I hit a problem with assignment operators, namely that they require their left operand to be a simple name. This is enforced during "operator shuffling" and is implemented in IoMessage_opShuffle.c.

After some wrangling with the implementation, I managed to create a patch. I opened a PR with it here: #394

Additionally, I wrote a blog post about the whole thing, posted here: https://klibert.pl/posts/adding_destructuring_bind_to_io.html - I plan to post it to news.ycombinator.com and maybe some other sites next week, and I'd appreciate any hints on how to make the post - and, of course, the PR - better.

TL;DR: my patch makes this Io code possible and working:

Object destructure := method(
  # target [a, b] <- list(9,8) becomes: target a := 9; target b := 8
  msg := call argAt(0)
  lst := call evalArgAt(1)
  target := call target
  msg arguments foreach(i, x,
      target setSlot(x name, lst at(i))
  )
  target
)
# inform the parser about our new operator
OperatorTable addAssignOperator("<-", "destructure")


o := Object clone
o [wa, wb, x] <- list(3, 123)
o println
# prints:
#  Object_0x19bcb60:
#  wa               = 3
#  wb               = 123
#  x                = nil