[PEAK] "Coherence" - a Trellis-like language w/error handling

[P.J. Eby]

I ran across this yesterday, via reddit:

   http://coherence-lang.org/

The paper is very much worth reading, although I am still digesting 
all the ramifications of it.  Basically, the author has created a 
Trellis-like programming language, but with various additional 
twists.  In particular, it supports "micro-transactions" at a finer 
grain than the Trellis, as well as what might be called "structural 
transactions" for error handling.

There is a notion in this language of "fences" and "latches" which 
can wrap an object such that if an error occurs as a result of 
changes passing through the "fence", then all changes inside the 
fence are rolled back to the corresponding latches where those 
changes were made.  However, the boundary isn't a call stack like 
try/except, it's the attributes or cells where the changes are made.

In effect, it's like the old idea of having an "editor" or "buffer" 
object that holds changes to be applied all-at-once to another 
object, except that instead of using validation functions on the 
editor/buffer, this actually works by changing the target object, but 
rolling back all the changes if an error occurred as a result of any 
of the changes.

In addition, it has the notion of "hypothetical execution", meaning 
that you can do something like, "suppose I set this and this and 
this...  what would happen?"  In other words, a mini-transaction that 
gets rolled back after you get your information.

There's also "causal error handling", which basically means that you 
can get the equivalent of tracebacks between rules, when something 
makes something else blow up.

Anyway, these features aren't going to show up in Trellis any time 
soon, but it's interesting to see how someone else has solved similar 
problems, and I definitely want to study this more.